Japan had ruled the Korean peninsula between 1910 and 1945. When Japan surrendered in August 15, 1945, the 38th parallel was established as the boundary between Soviet and American occupation zones. This parallel divided the Korean peninsula roughly in the middle. In 1948, this parallel became the boundary between the Democratic People's Republic of Korea (North Korea) and the Republic of Korea (South Korea), both of which claim to be the government of the whole of Korea.

Explaining the choice of the 38th Parallel, US Colonels Dean Rusk observed, "even though it was further north than could be realistically reached by US forces, in the event of Soviet disagreement ... we felt it important to include the capital of Korea in the area of responsibility of American troops". He noted that he was "faced with the scarcity of US forces immediately available, and time and space factors, which would make it difficult to reach very far north, before Soviet troops could enter the area". As Rusk's comments indicate, the US doubted whether the Soviet government would agree to this. Soviet leader Joseph Stalin, however, maintained his wartime policy of co-operation, and on 16 August the Red Army halted at the 38th Parallel for three weeks to await the arrival of US forces in the south.

On 7 September 1945, General Douglas MacArthur issued Proclamation No. 1 to the people of Korea, announcing U.S. military control over Korea south of the 38th parallel and establishing English as the official language during military control. MacArthur ended up being in charge of southern Korea from 1945 to 1948 due to the lack of clear orders or initiative from Washington, D.C.

Closeup of the Korean Demilitarized Zone that surrounds the Military Demarcation Line. © Rishabh Tatiraju

Residents of Jeju opposed to the division of Korea had protested and had been on a general strike since 1947 against elections scheduled by the United Nations Temporary Commission on Korea (UNTCOK) to be held only in the territory controlled by the United States Army Military Government in Korea. The Workers' Party of South Korea (WPSK) and its supporters launched an insurgency in April 1948, attacking the police, and Northwest Youth League members stationed on Jeju mobilized to violently suppress the protests. The First Republic of Korea under President Syngman Rhee escalated the suppression of the uprising from August 1948, declaring martial law in November and beginning an "eradication campaign" against rebel forces in the rural areas of Jeju in March 1949, defeating them within two months. Many rebel veterans and suspected sympathizers were later killed upon the outbreak of the Korean War in June 1950, and the existence of the Jeju uprising was officially censored and repressed in South Korea for several decades.

The Jeju uprising was notable for its extreme violence; between 14,000 and 30,000 people (10 percent of Jeju's population) were killed, and 40,000 fled to Japan. Atrocities and war crimes were committed by both sides, but historians have noted that the methods used by the South Korean government to suppress protesters and rebels were especially cruel, with violence against civilians by pro-government forces contributing to the Yeosu-Suncheon rebellion in South Jeolla during the conflict.

In 2006, almost 60 years after the Jeju uprising, the South Korean government apologized for its role in the killings and promised reparations. In 2019, the South Korean police and defense ministry apologized for the first time over the massacres.

US Lieutenant General John R. Hodge was appointed as military governor. He directly controlled South Korea as head of the United States Army Military Government in Korea (USAMGIK 1945–48). In December 1945, Korea was administered by a US-Soviet Union Joint Commission, as agreed at the Moscow Conference, with the aim of granting independence after a five-year trusteeship. The idea was not popular among Koreans and riots broke out. To contain them, the USAMGIK banned strikes on 8 December 1945 and outlawed the PRK Revolutionary Government and the PRK People's Committees on 12 December 1945. Following further large-scale civilian unrest, the USAMGIK declared martial law.

Citing the inability of the Joint Commission to make progress, the US government decided to hold an election under United Nations auspices with the aim of creating an independent Korea. The Soviet authorities and the Korean Communists refused to co-operate on the grounds it would not be fair, and many South Korean politicians boycotted it. A general election was held in the South on 10 May 1948. North Korea held parliamentary elections three months later on 25 August.

The resultant South Korean government promulgated a national political constitution on 17 July 1948, and elected Syngman Rhee as President on 20 July 1948. This election is generally considered to have been manipulated by the Rhee regime. The Republic of Korea (South Korea) was established on 15 August 1948. In the Soviet Korean Zone of Occupation, the Soviet Union agreed to the establishment of a communist government led by Kim Il-sung. The Soviet Union withdrew its forces from Korea in 1948, and US troops withdrew in 1949.

The Mungyeong Massacre was a massacre conducted by 2nd and 3rd platoon, 7th company, 3rd battalion, 25th Infantry Regiment, 3rd Infantry Division of the South Korean Army on 24 December 1949 of 86 to 88 unarmed citizens in Mungyeong, North Gyeongsang district of South Korea, all of whom were civilians and a majority of whom were children and elderly people. The victims included 32 children. The victims were massacred because they were suspected communist supporters or collaborators. However, the South Korean government blamed the crime on communist guerrillas for decades.

On 26 June 2006, the Truth and Reconciliation Commission of South Korea concluded that the massacre was committed by the South Korean Army. However, a South Korean local court decided that charging the South Korean government with the massacre was barred by statute of limitations, as the five-year prescription ended in December 1954. On 10 February 2009, the South Korean high court also dismissed the victim's family complaint. In June 2011, the Supreme Court of Korea decided that the South Korean government should compensate victims of the inhumane crimes it had committed regardless of the deadline to make the claim.

By 1949, South Korean and US military actions had reduced the active number of indigenous communist guerrillas in the South from 5,000 to 1,000. However, Kim Il-sung believed that widespread uprisings had weakened the South Korean military and that a North Korean invasion would be welcomed by much of the South Korean population. Kim began seeking Stalin's support for an invasion in March 1949, traveling to Moscow to attempt to persuade him. Stalin initially did not think the time was right for a war in Korea. PLA forces were still embroiled in the Chinese Civil War, while US forces remained stationed in South Korea.

By spring 1950, he believed that the strategic situation had changed: PLA forces under Mao Zedong had secured final victory in China, US forces had withdrawn from Korea, and the Soviets had detonated their first nuclear bomb, breaking the US atomic monopoly. As the US had not directly intervened to stop the communist victory in China, Stalin calculated that they would be even less willing to fight in Korea, which had much less strategic significance. The Soviets had also cracked the codes used by the US to communicate with their embassy in Moscow, and reading these dispatches convinced Stalin that Korea did not have the importance to the US that would warrant a nuclear confrontation. Stalin began a more aggressive strategy in Asia based on these developments, including promising economic and military aid to China through the Sino-Soviet Treaty of Friendship, Alliance, and Mutual Assistance. In April 1950, Stalin gave Kim permission to attack the government in the South under the condition that Mao would agree to send reinforcements if needed. For Kim, this was the fulfillment of his goal to unite Korea after its division by foreign powers. Stalin made it clear that Soviet forces would not openly engage in combat, to avoid a direct war with the US.

Kim met with Mao in May 1950. Mao was concerned the US would intervene but agreed to support the North Korean invasion. China desperately needed the economic and military aid promised by the Soviets. However, Mao sent more ethnic Korean PLA veterans to Korea and promised to move an army closer to the Korean border. Once Mao's commitment was secured, preparations for war accelerated.

At dawn on Sunday, 25 June 1950, the KPA crossed the 38th Parallel behind artillery fire. The KPA justified its assault with the claim that ROK troops attacked first and that the KPA were aiming to arrest and execute the "bandit traitor Syngman Rhee". Fighting began on the strategic Ongjin Peninsula in the west (Battle of Ongjin). There were initial South Korean claims that the 17th Regiment captured the city of Haeju, and this sequence of events has led some scholars to argue that the South Koreans fired first.

Map of the North Korean Invasion, 25-28 June 1950. © Roy E. Appleman

Whoever fired the first shots in Ongjin, within an hour, KPA forces attacked all along the 38th Parallel. The KPA had a combined arms force including tanks supported by heavy artillery. The ROK had no tanks, anti-tank weapons or heavy artillery to stop such an attack. In addition, the South Koreans committed their forces in a piecemeal fashion and these were routed in a few days.

On 27 June, Rhee evacuated from Seoul with some of the government. On 28 June, at 2 am, the ROK blew up the Hangang Bridge across the Han River in an attempt to stop the KPA. The bridge was detonated while 4,000 refugees were crossing it and hundreds were killed. Destroying the bridge also trapped many ROK units north of the Han River. In spite of such desperate measures, Seoul fell that same day during the First Battle of Seoul. A number of South Korean National Assemblymen remained in Seoul when it fell, and forty-eight subsequently pledged allegiance to the North.

On 25 June 1950, the United Nations Security Council unanimously condemned the North Korean invasion of South Korea, with UN Security Council Resolution 82. The Soviet Union, a veto-wielding power, had boycotted the Council meetings since January 1950, protesting Taiwan's occupation of China's permanent seat in the UN Security Council. After debating the matter, the Security Council, on 27 June 1950, published Resolution 83 recommending member states provide military assistance to the Republic of Korea. On 27 June President Truman ordered US air and sea forces to help South Korea.

The United Nations Security Council Resolution 84 was adopted on July 7, 1950. Having determined that the invasion of South Korea by forces from North Korea constituted a breach of the peace, the Council recommended that the members of the United Nations furnish such assistance to the South Korean state as may be necessary to repel the attack and restore peace and security to the area. The Council further recommended that all members providing military forces and other assistance to The Republic make these forces and assistance available to a unified command under the United States of America.

The Seoul National University Hospital massacre was a massacre of 700 to 900 doctors, nurses, inpatient civilians and wounded soldiers by the Korean People's Army (KPA) on 28 June 1950 at the Seoul National University Hospital, Seoul district of South Korea. During the First Battle of Seoul, the KPA wiped out one platoon which guarded Seoul National University Hospital on 28 June 1950. They massacred medical personnel, inpatients and wounded soldiers. The Korean People's Army shot or buried the people alive. The civilian victims alone numbered 900. According to South Korean Ministry of National Defense, the victims included 100 wounded South Korean soldiers.

Air forces of the United Nations Command carried out an extensive bombing campaign against North Korea from 1950 to 1953 during the Korean War. It was the first major bombing campaign for the United States Air Force (USAF) since its inception in 1947 from the United States Army Air Forces (USAAF). During the campaign, conventional weapons such as explosives, incendiary bombs, and napalm destroyed nearly all of the country's cities and towns, including an estimated 85 percent of its buildings.

A total of 635,000 tons of bombs, including 32,557 tons of napalm, were dropped on Korea. By comparison, the United States dropped 1.6 million tons in the European theater and 500,000 tons in the Pacific theater during all of World War II (including 160,000 on Japan). North Korea ranks alongside Cambodia (500,000 tons), Laos (2 million tons), and South Vietnam (4 million tons) as among the most heavily-bombed countries in history.

The Bodo League massacre was a massacre and war crime against communists and suspected sympathizers (many of whom were civilians who had no connection with communism or communists) that occurred in the summer of 1950 during the Korean War. Estimates of the death toll vary. Historians and experts on the Korean War estimate that the full total ranges from at least 60,000–110,000 (Kim Dong-choon) to 200,000 (Park Myung-lim).

The massacre was falsely blamed on the communists led by Kim Il-sung by the South Korean government. The South Korean government made efforts to conceal the massacre for four decades. Survivors were forbidden by the government from revealing it, under suspicion of being communist sympathizers; public revelation carried with it the threat of torture and death. During the 1990s and onwards, several corpses were excavated from mass graves, resulting in public awareness of the massacre. Half a century later, the South Korean Truth and Reconciliation Commission investigated what happened in the political violence largely kept hidden from history, unlike the publicized North Korean executions of South Korean right-wingers.

The Battle of Osan was the first engagement between the United States and North Korea during the Korean War. On July 5, 1950, Task Force Smith, an American task force of 540 infantry supported by an artillery battery, was moved to Osan, south of Seoul, the capital of South Korea, and was ordered to fight as a rearguard to delay the advancing North Korean forces while more US troops arrived to form a stronger defensive line to the south. The task force lacked both anti-tank guns and effective infantry anti-tank weapons and had been equipped with obsolete 2.36-inch (60 mm) rocket launchers and a few 57 mm recoilless rifles. Aside from a limited number of HEAT shells for the unit's 105 mm howitzers, crew-served weapons that could defeat T-34/85 tanks from the Soviet Union had not yet been distributed to the US Army forces in Korea.

Task Force Smith near Osan, 5 July 1950. © US Army

A North Korean tank column equipped with ex-Soviet T-34/85 tanks overran the task force in the first encounter and continued its advance south. After the North Korean tank column had breached US lines, the task force opened fire on a force of some 5,000 North Korean infantry that were approaching its position, which held up their advance. North Korean troops eventually flanked and overwhelmed the US positions, and the rest of the task force retreated in disorder.

In the summer of 1950, the Korean People's Army (KPA) launched a relentless drive southward, overwhelming the Republic of Korea (ROK) and United Nations (UN) forces. By August, the KPA had steadily forced the ROK Army and the Eighth United States Army to retreat, pressing deep into the Korean Peninsula. Equipped with experienced leadership and robust Soviet-supplied weaponry, the KPA exploited the inadequacies of their opponents, who were short on anti-tank weapons, artillery, and armor. 

Drive South. © HistoryMaps

As they advanced, the KPA not only targeted military resistance but also carried out purges against South Korea's intellectual and administrative elite. Civil servants, scholars, and other perceived adversaries were systematically eliminated in the areas they occupied, intensifying the impact of their conquest beyond the battlefield.

By September, the situation for UN forces had grown desperate. They were confined to the southeastern corner of Korea, near the port city of Pusan, behind a defensive line known as the Pusan Perimeter. Stretching 230 kilometers (140 miles) and partially marked by the natural barrier of the Naktong River, this shrinking territory represented just 10% of Korea’s landmass. Despite the odds, this perimeter became the focal point of a determined UN stand against the KPA's seemingly unstoppable advance.

The No Gun Ri massacre occurred on July 26–29, 1950, early in the Korean War, when an undetermined number of South Korean refugees were killed in a U.S. air attack and by small- and heavy-weapons fire of the American 7th Cavalry Regiment at a railroad bridge near the village of Nogeun-ri, 100 miles (160 km) southeast of Seoul. In 2005, a South Korean government inquest certified the names of 163 dead or missing and 55 wounded, and added that many other victims' names were not reported. The No Gun Ri Peace Foundation estimated in 2011 that 250–300 were killed, mostly women and children.

The incident was little-known outside Korea until publication of an Associated Press (AP) story in 1999 in which 7th Cavalry veterans corroborated survivors' accounts. The AP also uncovered declassified U.S. Army orders to fire on approaching civilians because of reports of North Korean infiltration of refugee groups. In 2001, the U.S. Army conducted an investigation and, after previously rejecting survivors' claims, acknowledged the killings, but described the three-day event as "an unfortunate tragedy inherent to war and not a deliberate killing". The Army rejected survivors' demands for an apology and compensation, and United States President Bill Clinton issued a statement of regret, adding the next day that "things happened which were wrong".

South Korean investigators disagreed with the U.S. report, saying they believed that 7th Cavalry troops were ordered to fire on the refugees. The survivors' group called the U.S. report a "whitewash". The AP later discovered additional archival documents showing that U.S. commanders ordered troops to "shoot" and "fire on" civilians at the war front during this period; these declassified documents had been found but not disclosed by the Pentagon investigators. Among the undisclosed documents was a letter from the U.S. ambassador in South Korea stating that the U.S. military had adopted a theater-wide policy of firing on approaching refugee groups. Despite demands, the U.S. investigation was not reopened. Prompted by the exposure of No Gun Ri, survivors of similar alleged incidents from 1950–51 filed reports with the Seoul government. In 2008, an investigative commission said more than 200 cases of alleged large-scale killings by the U.S. military had been registered, mostly air attacks.

The Battle of the Pusan Perimeter was one of the first major engagements of the Korean War. An army of 140,000 UN troops, having been pushed to the brink of defeat, were rallied to make a final stand against the invading Korean People's Army (KPA), 98,000 men strong.

UN forces, having been repeatedly defeated by the advancing KPA, were forced back to the "Pusan Perimeter", a 140-mile (230 km) defensive line around an area on the southeastern tip of South Korea that included the port of Busan. The UN troops, consisting mostly of forces from the Republic of Korea Army (ROKA), United States, and United Kingdom, mounted a last stand around the perimeter, fighting off repeated KPA attacks for six weeks as they were engaged around the cities of Taegu, Masan, and Pohang and the Naktong River. The massive KPA assaults were unsuccessful in forcing the UN troops back further from the perimeter, despite two major pushes in August and September.

Map of the Naktong Defense lines, September 1950. © United States Army Center of Military History

North Korean troops, hampered by supply shortages and massive losses, continually staged attacks on UN forces in an attempt to penetrate the perimeter and collapse the line. The UN forces, however, used the port to amass an overwhelming advantage in troops, equipment, and logistics. Tank battalions deployed to Korea directly from the US mainland from the port of San Francisco to the port of Pusan, the largest Korean port. By late August, the Pusan Perimeter had some 500 medium tanks battle-ready. In early September 1950, UN forces outnumbered the KPA 180,000 to 100,000 soldiers.

The United States Air Force (USAF) interrupted KPA logistics with 40 daily ground support sorties that destroyed 32 bridges, halting most daytime road and rail traffic. KPA forces were forced to hide in tunnels by day and move only at night. To deny materiel to the KPA, the USAF destroyed logistics depots, petroleum refineries, and harbors, while the US Navy air forces attacked transport hubs. Consequently, the over-extended KPA could not be supplied throughout the south. 

The Great Naktong Offensive was the North Korean Korean People's Army (KPA)'s unsuccessful final bid to break the Pusan Perimeter established by the UN forces. By August, the UN troops had been forced into the 140-mile (230 km) Pusan Perimeter on the southeast tip of the Korean peninsula. For the first time, the UN troops formed a continuous line which the KPA could neither flank nor overwhelm with superior numbers. KPA offensives on the perimeter were stalled and by the end of August all momentum was lost. Seeing the danger in a prolonged conflict along the perimeter, the KPA sought a massive offensive for September to collapse the UN line.

The KPA subsequently planned a simultaneous offensive for their entire army along five axes of the perimeter; and on September 1 intense fighting erupted around the cities of Masan, Kyongju, Taegu, Yongch'on and the Naktong Bulge. What followed was two weeks of extremely brutal fighting as the two sides vied to control the routes into Pusan. Initially successful in some areas, the KPA were unable to hold their gains against the numerically and technologically superior UN force. The KPA, again stalled at the failure of this offensive, was outflanked by the Inchon landings on 15 September and on 16 September the UN forces began their breakout from the Pusan Perimeter.

The Battle of Incheon was an amphibious invasion and a battle of the Korean War that resulted in a decisive victory and strategic reversal in favor of the United Nations Command (UN). The operation involved some 75,000 troops and 261 naval vessels and led to the recapture of the South Korean capital of Seoul two weeks later.

The battle began on 15 September 1950 and ended on 19 September. Through a surprise amphibious assault far from the Pusan Perimeter that UN and Republic of Korea Army (ROK) forces were desperately defending, the largely undefended city of Incheon was secured after being bombed by UN forces. The battle ended a string of victories by the North Korean Korean People's Army (KPA). The subsequent UN recapture of Seoul partially severed the KPA's supply lines in South Korea. The battle was followed by a rapid collapse of the KPA; within a month of the Incheon landing, the UN forces had taken 135,000 KPA troops prisoner.

The Pusan Perimeter Offensive marked a critical turning point during the Korean War. Beginning on 16 September 1950, the United Nations (UN) forces, primarily composed of American and South Korean troops, launched a decisive counteroffensive against the North Korean People's Army (KPA). After weeks of fierce defense against relentless North Korean assaults, the UN forces broke the KPA cordon surrounding the southeastern tip of the Korean Peninsula and initiated a northward advance. This operation was synchronized with the bold Inchon landing, which trapped North Korean forces between two UN offensives.

The Last Stand at Pusan

By August 1950, the KPA had pushed the UN forces into a narrow defensive line known as the Pusan Perimeter, encompassing a 140-mile (230-km) arc around the port city of Pusan. Despite repeated KPA offensives, UN forces, bolstered by reinforcements, air support, and superior logistics, managed to hold the line. The perimeter’s defense was critical, as losing Pusan would have cut off the UN's last foothold in Korea.

Planning the Counteroffensive

The Pusan Perimeter Offensive was meticulously planned to coincide with the Inchon landing on 15 September 1950. General Walton Walker's Eighth Army, stationed within the perimeter, would launch an assault to break through the KPA encirclement and link up with X Corps, which was advancing inland from Inchon. The offensive aimed to exploit the disarray caused by the Inchon landing and cut off retreating North Korean forces.

The Eighth Army was tasked with advancing along key axes toward Taejon and Suwon, engaging KPA forces and preventing their escape. The US I Corps in the center would spearhead the breakout, supported by other UN divisions, including the South Korean ROK Army on the eastern flank and additional US divisions on the southern flank.

The Offensive Unfolds

Pusan Perimeter breakout 16-22 September 1950. © United States Army

Breaking the Naktong Line

On 16 September, the UN launched coordinated attacks along the Naktong River. Despite poor weather and strong initial resistance, the tide began to turn by 18 September. Key actions included:

• US 2nd Infantry Division successfully crossed the Naktong near Changnyong, securing high ground and forcing the KPA to retreat.
• US 5th Regimental Combat Team (RCT) broke through near Waegwan, capturing critical positions and driving back the KPA 3rd Division.
• South Korean forces on the eastern front recaptured Pohang and pushed northward, effectively dismantling KPA positions in the region.

Encircling the North Koreans

The 1st Cavalry Division advanced along the Taegu-Waegwan corridor, while South Korean units maneuvered to cut off the KPA retreat north of Taegu. By 21 September, the UN forces had encircled significant elements of the KPA 1st, 3rd, and 13th Divisions, which were trapped in the mountains north of Taegu.

Coordinated Pursuit

As the KPA's defensive lines collapsed, UN forces rapidly advanced. The US 24th Infantry Division crossed the Naktong on 19 September and pushed toward Kumch'on, inflicting heavy casualties and capturing equipment. By 22 September, all major crossings of the Naktong were secured, and the UN forces began a general pursuit of the retreating North Koreans.

Collapse of the KPA and Aftermath

The Pusan Perimeter Offensive, combined with the Inchon landing, shattered the KPA’s grip on South Korea. The coordinated attacks demoralized and decimated the KPA forces, with many units reduced to a fraction of their original strength. By 23 September, the North Korean army was in full retreat, abandoning vast quantities of equipment and leaving thousands of dead and captured behind.

The success of the offensive allowed the UN forces to transition from defense to a rapid advance northward. By late September, they recaptured Seoul, linking the Pusan Perimeter with the Inchon front. The campaign marked a significant shift in momentum, transforming the Korean War into an offensive operation for the UN coalition.

On 25 September, Seoul was recaptured by UN forces. US air raids caused heavy damage to the KPA, destroying most of its tanks and much of its artillery. KPA troops in the south, instead of effectively withdrawing north, rapidly disintegrated, leaving Pyongyang vulnerable. During the general retreat only 25,000 to 30,000 KPA soldiers managed to reach the KPA lines. On 27 September, Stalin convened an emergency session of the Politburo, in which he condemned the incompetence of the KPA command and held Soviet military advisers responsible for the defeat.

The UN offensive into North Korea in late 1950 marked a turning point in the Korean War, as United Nations forces, having pushed the North Korean army (KPA) out of South Korea, pursued them across the 38th parallel. Following the dramatic Inchon landing in September and the breakout from the Pusan Perimeter, the KPA was in disarray. The UN aimed to destroy the remnants of the North Korean military and unify the Korean Peninsula under South Korean control.

The offensive began on 30 September when ROK forces crossed the 38th parallel on the east coast, with the rest of the UN forces following shortly after. By October, the rapid advance saw UN troops capturing Pyongyang, the North Korean capital, on 19 October and continuing toward the Yalu River, the border with China. The advance was marked by overwhelming UN air and logistical superiority, which left the KPA unable to regroup or resist effectively.

Capture of Pyongyang 15-19 October 1950. © US Army

As UN forces neared the Yalu River, China intervened to prevent a perceived threat to its border. The first Chinese attacks occurred in late October, temporarily halting the UN advance. After regrouping, the UN launched another offensive on 24 November, hoping to break Chinese resistance and secure a quick victory. However, the Chinese Second Phase Offensive, starting on 25 November, caught the UN by surprise, forcing a chaotic retreat and reversing much of the progress made during the offensive.

The decision to cross the 38th parallel and pursue the KPA into North Korea remains controversial. While it initially seemed to promise a swift end to the war, it provoked Chinese intervention, escalating the conflict and prolonging the fighting. The sudden reversal in late November marked the beginning of a long and bloody stalemate.

By the end of 1950, the ambitious UN campaign to unify Korea had not only failed but also led to a precarious situation for UN forces. The conflict’s focus shifted to consolidating positions and resisting further Chinese offensives, marking a significant and costly escalation in the war.

The Namyangju massacre was a mass killing conducted by South Korean police and local militia forces between October 1950 and early 1951 in Namyangju, Gyeonggi-do district of South Korea. More than 460 people were summarily executed, including at least 23 children under the age of 10. After the victory of the Second Battle of Seoul, South Korean authorities arrested and summarily executed several individuals along with their families on suspicion of sympathizing with North Korea. During the massacre, South Korean Police conducted the Goyang Geumjeong Cave massacre in Goyang near Namyangju.On 22 May 2008, the Truth and Reconciliation Commission demanded that the South Korean government apologize for the massacre and support a memorial service for the victims.

The Battle of Unsan unfolded from October 25 to November 4, 1950, marking one of the earliest major clashes between the United Nations Command (UNC) and Chinese forces during the Korean War. Set near Unsan in North Korea, the battle was part of China’s First Phase Campaign, which aimed to disrupt the advancing UNC forces. The conflict demonstrated the surprise, strategy, and determination of the People's Volunteer Army (PVA) while exposing vulnerabilities in the UNC's coordination and preparedness.

By October 1950, the Korean People's Army (KPA) was in full retreat. UNC forces had broken out of the Pusan Perimeter, recaptured Seoul, and pursued the KPA deep into North Korea, nearing the Chinese border. As part of the UNC's push, the ROK 1st Infantry Division was ordered to secure the area around Unsan, a critical access point to the Yalu River. At the same time, Chinese forces under Mao Zedong covertly entered Korea to prevent North Korean collapse, despite their own logistical and strategic limitations.

The PVA sought to exploit the overextended UN lines, focusing their attacks on South Korean forces. The goal of the First Phase Campaign was to weaken UNC forces through limited engagements while minimizing direct confrontation with US units. By October 24, the ROK 1st Infantry Division had taken Unsan but was left vulnerable, with its flanks exposed and UNC units spread thin.

Map of Unsan Engagement. © US Army

The battle began on October 25 when the ROK 1st Infantry Division encountered strong resistance north of Unsan. The PVA 120th Division, employing heavy artillery and surprise tactics, checked the ROK advance and revealed the presence of Chinese forces. Over the next few days, the PVA encircled the ROK division, cutting off its retreat routes. Though the South Koreans managed to temporarily reopen the road south with American support, their advance stalled, and the fighting reached a stalemate.

Warnings from ROK commanders about the growing Chinese threat were dismissed amid overconfidence in an imminent UNC victory. On October 29, the US 8th Cavalry Regiment was deployed to relieve the ROK forces at Unsan, unaware of the full strength of the PVA in the area.

On November 1, the PVA launched a coordinated assault to destroy the UNSC forces at Unsan. The 39th Corps attacked in a pincer movement, targeting the US 8th Cavalry Regiment and the remnants of the ROK 15th Infantry Regiment. Chinese forces penetrated gaps in the American lines, surrounding and isolating battalions.

The ROK 15th Infantry Regiment was overwhelmed and destroyed, leaving the US 8th Cavalry's 1st and 2nd Battalions exposed. Efforts to withdraw were hampered by Chinese roadblocks and ambushes, forcing units to escape on foot, abandoning vehicles and heavy equipment. The 3rd Battalion of the 8th Cavalry was initially spared but came under heavy assault on the night of November 1 when Chinese commandos infiltrated their camp. Despite attempts by the 5th Cavalry Regiment to rescue the battalion, the survivors only managed to break out after days of intense fighting, with fewer than 200 men reaching safety.

The Battle of Unsan was a costly defeat for the UNC. The US 8th Cavalry Regiment lost most of its strength, suffering heavy casualties and abandoning significant amounts of equipment. The ROK 1st Infantry Division was also severely weakened, and the ROK 6th Infantry Division was destroyed in concurrent fighting at Onjong. In total, the Chinese claimed a significant victory, though at the cost of over 10,000 casualties during the First Phase Campaign.

The Chinese success at Unsan emboldened their leadership and provided critical insights into US tactics, shaping their strategy for subsequent campaigns. For the UN, the Chinese withdrawal after the battle fueled a false sense of security, leading to the ill-fated Home-by-Christmas Offensive. This miscalculation set the stage for devastating defeats at the Ch'ongch'on River and Chosin Reservoir later in the year. The Battle of Unsan underscored the challenges of underestimating an adversary and the risks of overextending a force during wartime.

The Battle of Onjong unfolded from October 25 to 29, 1950, as one of the first significant encounters between Chinese and South Korean forces during the Korean War. It marked the main thrust of China’s First Phase Campaign, which aimed to stop the United Nations Command (UNC) advance toward the Yalu River. The battle resulted in the near destruction of the ROK II Corps, leaving the US Eighth Army’s right flank exposed and marking the beginning of a shift in momentum in the war.

By October 1950, the UNC had driven the North Korean People’s Army into retreat. After the breakout from the Pusan Perimeter, the Inchon landing, and subsequent advances, the UNC pursued the remnants of the North Korean forces toward the Yalu River, the border with China. Confident in imminent victory, the UNC deployed forces deep into North Korea, with the ROK II Corps leading the advance.

Unbeknownst to the UNC, the People's Volunteer Army (PVA) of China was secretly entering the conflict. Chairman Mao Zedong, determined to prevent a North Korean collapse, ordered the PVA to launch a bridgehead-building operation that would ambush and destroy isolated UNC units. The ROK II Corps, advancing along the Taebaek Mountains, became the main target of this First Phase Campaign.

Map of Chinese intervention in the west between October 25 to November 1, 1950. © US Army

The village of Onjong, located northeast of Unsan, was a critical crossroads for access to the Yalu River. On October 24, the ROK 6th Infantry Division captured Onjong and began advancing in separate directions—one regiment moved north toward Kojang, while another prepared to head northwest toward Pukchin. The scattered deployment left the ROK vulnerable, and UNC commanders underestimated the possibility of organized resistance following the collapse of the North Korean forces.

Meanwhile, the PVA 40th Corps under Commander Peng Dehuai deployed to intercept the advancing ROK forces. On the night of October 24, Chinese units set up ambush positions on ridges overlooking the Onjong-Pukchin road and prepared to strike.

On October 25, the ROK 2nd Infantry Regiment advanced northwest from Onjong. Eight miles into their movement, they encountered a devastating ambush by Chinese forces. Positioned on high ground, the PVA unleashed heavy fire on the regiment's flanks and front. The ROK 3rd Battalion collapsed under the assault, abandoning vehicles and artillery as they fled back toward Onjong.

Attempts by the ROK to reinforce the position were futile. The Chinese forces revealed their presence to prisoners, who reported nearly 10,000 Chinese soldiers in the area. By the night of October 26, the PVA 118th and 120th Divisions launched a massive attack on Onjong, overwhelming the ROK defenders within 30 minutes. The shattered ROK 2nd Infantry Regiment effectively ceased to exist, and its survivors retreated southward in disarray.

In the aftermath of the initial rout, the ROK command attempted to regroup. Two regiments—the ROK 19th Infantry Regiment and the ROK 10th Infantry Regiment—were sent to recapture Onjong. Simultaneously, the ROK 7th Infantry Regiment, the last intact unit of the ROK 6th Infantry Division, began a withdrawal southward. However, Chinese forces laid another trap, encircling the ROK 7th Infantry Regiment and ambushing the approaching reinforcements.

On the night of October 28, the PVA annihilated the ROK 10th and 19th Infantry Regiments in a well-coordinated attack. The ROK 7th Infantry Regiment, isolated and running low on supplies, was ambushed by the PVA 118th Division on October 29. After a brief but intense fight, the regiment dispersed, leaving its survivors scattered in the hills.

The Battle of Onjong devastated the ROK II Corps. The ROK 6th Infantry Division and the ROK 10th Infantry Regiment were effectively destroyed, with thousands of casualties and the loss of most of their equipment. The PVA achieved its objective of crippling the UNC’s right flank, leaving the Eighth Army vulnerable to further attacks.

This vulnerability was immediately exploited at the Battle of Unsan, where the PVA encircled and defeated additional UNC forces. The cumulative effect of these battles forced the Eighth Army to retreat to the Ch'ongch'on River, marking a turning point in the war. Although the Chinese First Phase Campaign ended on November 5 due to logistical challenges, it succeeded in halting the UNC advance and shifting the momentum of the conflict.

The defeat exposed the overextension and poor coordination of UNC forces, particularly among ROK units. It also revealed the capability and resolve of the Chinese forces, whose intervention in the war was now undeniable. The success at Onjong laid the groundwork for the PVA’s Second Phase Offensive, which would culminate in the decisive battles at the Ch'ongch'on River and Chosin Reservoir later that year.

On 30 June 1950, five days after the outbreak of the war, Zhou Enlai, premier of the PRC and vice-chairman of the Central Military Committee of the CCP (CMCC), decided to send a group of Chinese military intelligence personnel to North Korea to establish better communications with Kim II-Sung as well as to collect first-hand materials on the fighting. One week later,it was decided that the Thirteenth Army Corps under the Fourth Field Army of the People's Liberation Army (PLA), one of the best trained and equipped units in China, would be immediately transformed into the Northeastern Border Defense Army (NEBDA) to prepare for "an intervention in the Korean War if necessary".

On 20 August 1950, Premier Zhou Enlai informed the UN that "Korea is China's neighbor... The Chinese people cannot but be concerned about a solution of the Korean question". Thus, through neutral-country diplomats, China warned that in safeguarding Chinese national security, they would intervene against the UN Command in Korea. On 1 October 1950, the day that UN troops crossed the 38th Parallel, the Soviet ambassador forwarded a telegram from Stalin to Mao and Zhou requesting that China send five to six divisions into Korea, and Kim Il-sung sent frantic appeals to Mao for Chinese military intervention. 

On 18 October 1950, Zhou met with Mao Zedong, Peng Dehuai and Gao Gang, and the group ordered two hundred thousand PVA troops to enter North Korea, which they did on 19 October. UN aerial reconnaissance had difficulty sighting PVA units in daytime, because their march and bivouac discipline minimized aerial detection. The PVA marched "dark-to-dark" (19:00–03:00), and aerial camouflage (concealing soldiers, pack animals, and equipment) was deployed by 05:30. Meanwhile, daylight advance parties scouted for the next bivouac site. During daylight activity or marching, soldiers were to remain motionless if an aircraft appeared, until it flew away; PVA officers were under order to shoot security violators. Such battlefield discipline allowed a three-division army to march the 460 km (286 mi) from An-tung, Manchuria, to the combat zone in some 19 days. Another division night-marched a circuitous mountain route, averaging 29 km (18 mi) daily for 18 days. After secretly crossing the Yalu River on 19 October, the PVA 13th Army Group launched the First Phase Offensive on 25 October, attacking the advancing UN forces near the Sino-Korean border. This military decision made solely by China changed the attitude of the Soviet Union. Twelve days after PVA troops entered the war, Stalin allowed the Soviet Air Force to provide air cover and supported more aid to China.

On 5 November 1950, the US Joint Chiefs of Staff issued orders for the retaliatory atomic bombing of Manchurian PRC military bases, if either their armies crossed into Korea or if PRC or KPA bombers attacked Korea from there. President Truman ordered the transfer of nine Mark 4 nuclear bombs "to the Air Force's Ninth Bomb Group, the designated carrier of the weapons signed an order to use them against Chinese and Korean targets", which he never transmitted. Truman and Eisenhower both had military experience and viewed nuclear weapons as potentially usable components of their military.

As PVA forces pushed back the UN forces from the Yalu River, Truman stated during a 30 November 1950 press conference that using nuclear weapons was "always [under] active consideration", with control under the local military commander. The Indian ambassador, K. Madhava Panikkar, reports "that Truman announced he was thinking of using the atom bomb in Korea. 

The Second Phase Offensive was an offensive by the Chinese People's Volunteer Army (PVA) against UN forces. The two major engagements of the campaign were the Battle of the Ch'ongch'on River in the western part of North Korea and the Battle of Chosin Reservoir in the eastern part of North Korea.

Casualties were heavy on both sides. The battles were fought in temperatures as low as −30 °C (−22 °F) and casualties from frostbite may have exceeded those from battle wounds. U.S. intelligence and air reconnaissance failed to detect the large numbers of Chinese soldiers present in North Korea. Thus, the UN units, the Eighth United States Army on the west and the X Corps on the east, kicked off the "Home-by-Christmas" offensive on 24 November with "unwarranted confidence...believing that they comfortably outnumbered enemy forces." The Chinese attacks came as a surprise. The Home-by-Christmas offensive, with the objective of conquering all of North Korea and ending the war, was quickly abandoned in light of the massive Chinese assault. The Second Phase Offensive forced all UN forces to go on the defensive and retreat. China had recaptured nearly all of North Korea by the end of the offensive.

The Battle of the Ch’ongch’on River (November 25 – December 2, 1950) was a pivotal engagement in the Korean War, marking the collapse of the United Nations' (UN) "Home-by-Christmas Offensive" and the resurgence of Communist forces. The battle unfolded as General Douglas MacArthur sought to expel the Chinese People's Volunteer Army (PVA) from Korea following earlier successes. In response, PVA Commander Peng Dehuai launched the Second Phase Campaign, aiming to outflank and rout the advancing UN forces.

Map of Battle of Ch'ongch'on River, November 25 - 28, 1950. © US Army

The Chinese began with a surprise night assault on November 25, devastating the UN right flank held by Republic of Korea (ROK) II Corps. ROK forces, lacking adequate firepower and defending difficult terrain, were quickly overwhelmed. Chinese troops exploited gaps in the lines, encircling and destroying ROK units at Tokchon and Yongdong-ni. To the west, PVA forces targeted U.S. IX Corps, inflicting heavy losses on the 2nd Infantry Division at Kujang-dong. By November 27, the UN front was collapsing, forcing a broad retreat southward.

Central to the battle was the "Gauntlet", a deadly six-mile stretch of road near Kunu-ri that became a killing field for the retreating U.S. 2nd Infantry Division. Trapped by Chinese roadblocks and ambushes, the division suffered catastrophic losses in personnel and equipment. The Turkish Brigade, sent to delay the PVA advance, was encircled and rendered combat ineffective, though their actions allowed some U.S. forces to withdraw. Despite air support, the retreat from Kunu-ri was marked by chaos and heavy casualties.

The aftermath was devastating for the UN forces. The U.S. 2nd Infantry Division lost over 4,000 men, while other units, including the Turkish Brigade, were severely weakened. Chinese losses were also substantial, with up to 45,000 casualties attributed to combat and harsh winter conditions. However, the PVA had decisively forced the UN to abandon North Korea, and by December 3, the Eighth Army began what was described as "the longest retreat in U.S. military history," falling back to the 38th Parallel.

The battle marked the peak of Chinese military success in Korea, decisively reversing UN gains and prolonging the war. While the Communists achieved their objectives, logistical strains prevented further exploitation of their victory. UN forces, under new leadership from General Matthew Ridgway, regrouped and would later launch counteroffensives, setting the stage for the stalemate that defined much of the Korean War.

On 27 November 1950, the Chinese force surprised the US X Corps commanded by Major General Edward Almond in the Chosin Reservoir area. A brutal 17-day battle in freezing weather soon followed. Between 27 November and 13 December, 30,000 UN troops (later nicknamed "The Chosin Few") under the field command of Major General Oliver P. Smith were encircled and attacked by about 120,000 Chinese troops under the command of Song Shilun, who had been ordered by Mao Zedong to destroy the UN forces. The UN forces were nevertheless able to break out of the encirclement and to make a fighting withdrawal to the port of Hungnam, inflicting heavy casualties on the Chinese. The retreat of the US Eighth Army from northwest Korea in the aftermath of the Battle of the Ch'ongch'on River and the evacuation of the X Corps from the port of Hungnam in northeast Korea marked the complete withdrawal of UN troops from North Korea.

The Third Battle of Seoul, fought from December 31, 1950, to January 7, 1951, marked a critical turning point in the Korean War. Known as the Chinese New Year's Offensive or the January-Fourth Retreat, this campaign saw the Chinese People’s Volunteer Army (PVA) launch a massive assault south of the 38th Parallel, culminating in the capture of Seoul. Despite their success, the PVA's advances ultimately galvanized UN forces, setting the stage for future counteroffensives.

Following the catastrophic UN defeats at the Ch'ongch'on River and Chosin Reservoir, Chinese forces crossed the 38th Parallel to press their advantage and force the withdrawal of UN troops from Korea. On December 31, the PVA attacked along the Imjin and Hantan Rivers, exploiting weaknesses in Republic of Korea (ROK) forces and breaching UN defenses. By January 1, PVA troops overwhelmed the ROK 1st and 6th Divisions, forcing a retreat, while simultaneous assaults near Chuncheon routed ROK III Corps. With UN defenses crumbling, Lieutenant General Matthew Ridgway ordered the evacuation of Seoul by January 4.

Map of the Chinese Third Phase Campaign. © US Army

The withdrawal unfolded under relentless Chinese pressure. In the west, US I Corps and British Commonwealth forces covered the retreat, engaging in fierce rearguard actions, including heavy fighting by the British 29th Infantry Brigade near Koyang. In the east, US IX Corps and ROK troops faced a similar onslaught, as the PVA sought to encircle retreating UN forces. Although Seoul fell to Chinese and North Korean troops on January 4, the UN forces managed an orderly retreat south of the Han River.

The Chinese victory at Seoul came at a steep cost. Months of continuous combat and logistical strain left the PVA exhausted and undersupplied. Meanwhile, the fall of the South Korean capital, though a blow to UN morale, solidified international support for continued operations in Korea. The prospect of a UN withdrawal was abandoned, and Ridgway began revitalizing his forces for a counteroffensive.

By late January, UN forces regrouped under Ridgway’s leadership and launched Operation Thunderbolt, pushing back against Chinese advances. Although the Third Battle of Seoul demonstrated the PVA’s operational strength, their inability to sustain their momentum ultimately marked the peak of Chinese success in the Korean War.

Operation Thunderbolt, launched on 25 January 1951, marked the first major counteroffensive under General Matthew Ridgway’s command of the U.S. Eighth Army during the Korean War. Following earlier Chinese offensives that had driven UN forces south of Seoul, the operation aimed to reclaim lost territory, inflict heavy casualties, and disrupt enemy plans. Ridgway’s cautious yet aggressive strategy involved reconnaissance in force, ensuring a measured advance while avoiding overextension.

Operation Thunderbolt (1951) map. © US Army

The offensive began with coordinated attacks by I Corps on the western front and IX Corps in the center. UN forces, including the 25th Infantry Division, 1st Cavalry Division, and the Turkish Brigade, pushed northward in multiple columns, encountering scattered Chinese and North Korean resistance. Early progress was deliberate, with troops methodically clearing roads and securing flanks while advancing toward the Han River. By the end of January, it was evident that enemy forces were retreating to regroup north of the river.

Operation Thunderbolt (1951) 1-11 February. © US Army

In early February, the operation expanded into a full-scale assault. I Corps captured critical positions near Seoul, including Kwanak Mountain, and secured the Kimpo Peninsula and Airfield, while IX Corps reached the Han River near Yangpyeong. Chinese forces mounted stubborn resistance in some areas, particularly around Yangpyeong, but failed to stop the UN advance. By 11 February, the UN had established a strong presence along the southern bank of the Han River, achieving the operation’s main objectives.

The success of Operation Thunderbolt demonstrated Ridgway’s emphasis on coordinated offensives and methodical advances. It forced the Chinese and North Koreans to abandon their positions south of the Han River and disrupted their ability to launch further offensives in the western sector. The operation inflicted significant enemy casualties while minimizing losses among UN forces.

Following this victory, Ridgway launched Operation Killer on 20 February 1951 to eliminate remaining enemy forces. Operation Thunderbolt marked a turning point in the war, shifting momentum back to the UN and laying the groundwork for future counteroffensives.

The Geochang massacre was a massacre conducted by the third battalion of the 9th regiment of the 11th Division of the South Korean Army between 9 February 1951 and 11 February 1951 of 719 unarmed citizens in Geochang, South Gyeongsang district of South Korea. The victims included 385 children. The 11th Division also conducted the Sancheong-Hamyang massacre two days earlier. The general commanding the division was Choe Deok-sin.

In June 2010, An Jeong-a, a researcher for the Truth and Reconciliation Commission, disclosed National Defense Ministry official documents on his thesis that the massacre had been done under official South Korean Army order in order to annihilate citizens living in the guerrilla influenced area. On September 9, 2010, An was fired for disclosing Geochang massacre documents. The National Defense Ministry accused An of disclosing the documents which he had been only permitted to view under the condition of nondisclosure.

The Battle of Hoengsong, fought from February 11 to 15, 1951, was a key engagement during the Korean War and a pivotal part of the Chinese People’s Volunteer Army (PVA) Fourth Phase Offensive. Following the UN’s gains during Operation Thunderbolt, the PVA launched a surprise counteroffensive in the west-central region of Korea. Their victory at Hoengsong inflicted severe losses on the United Nations forces, marking one of the most lopsided defeats suffered by US forces during the conflict.

The PVA's attack began with overwhelming force on the night of February 11. Targeting the Republic of Korea Army (ROK) 8th Infantry Division, Chinese troops rapidly penetrated its defenses, severing communications and breaking the unit’s cohesion. ROK forces were surrounded and annihilated in a series of encirclements, with most troops killed or captured. Supporting US artillery and armor, left exposed by the collapse of the ROK line, attempted to withdraw but encountered numerous PVA roadblocks. Many vehicles, weapons, and personnel were lost as the retreat disintegrated into chaos.

Battle of Hoengsong, 11-13 February 1951. © US Army

Efforts to regroup were hindered by poor coordination and a slow chain of command. American units, including Support Force 21 and elements of the US 2nd Infantry Division, faced relentless Chinese assaults and ambushes along their retreat routes. Attempts to break through enemy roadblocks proved costly, with some formations suffering heavy casualties while others barely escaped encirclement. The PVA's well-coordinated attacks overwhelmed the defenders, driving them south toward Hoengsong in disarray.

By February 13, UN forces had abandoned Hoengsong, leaving it to the PVA. Casualties were staggering: the UN lost approximately 11,800 men, including 1,900 Americans, 100 Dutch, and thousands of South Koreans. The ROK 8th Division was rendered nearly combat-ineffective, losing most of its personnel and equipment. American and Dutch forces also lost significant artillery, vehicles, and weapons, marking a disastrous setback for the UN.

Despite the defeat at Hoengsong, the UN regrouped. Chinese forces attempted to follow up their success by attacking Chipyong-ni from February 13 to 15 but were repelled in a fierce defensive action. The PVA advance was further checked during the Third Battle of Wonju (February 13–18), halting their momentum and stabilizing the front. The Battle of Hoengsong highlighted the vulnerabilities in UN coordination and ROK readiness, but it also galvanized efforts to improve defensive strategies in the face of Chinese offensives.

The Battle of Chipyong-ni represents the "high-water mark" of the Chinese invasion of South Korea. The UN forces fought a short but desperate battle that broke the attack's momentum. The battle is sometimes known as the "Gettysburg of the Korean War": 5,600 South Korean, US, and French troops were surrounded on all sides by 25,000 PVA. UN forces had previously retreated in the face of large PVA/KPA forces instead of getting cut off, but this time they stood and fought, and won. Due to the ferocity of the Chinese attack and the heroism of the defenders, the battle has also been called "one of the greatest regimental defense actions in military history".

Operation Ripper, also known as the Fourth Battle of Seoul, was intended to destroy as much as possible of the Chinese People's Volunteer Army (PVA) and Korean People's Army (KPA) forces around Seoul and the towns of Hongch'on, 50 miles (80 km) east of Seoul, and Chuncheon, 15 miles (24 km) further north. The operation also aimed to bring UN troops to the 38th Parallel. It followed upon the heels of Operation Killer, an eight-day UN offensive that concluded February 28, to push PVA/KPA forces north of the Han River.

Operation Ripper was preceded by the largest artillery bombardment of the Korean War. In the middle, the US 25th Infantry Division quickly crossed the Han and established a bridgehead. Further to the east, IX Corps reached its first phase line on 11 March. Three days later the advance proceeded to the next phase line. During the night of 14–15 March, elements of the ROK 1st Infantry Division and the US 3rd Infantry Division liberated Seoul, marking the fourth and last time the capital changed hands since June 1950. The PVA/KPA forces were compelled to abandon it when the UN approach to the east of the city threatened them with encirclement.

Following the recapture of Seoul the PVA/KPA forces retreated northward, conducting skilful delaying actions that utilized the rugged, muddy terrain to maximum advantage, particularly in the mountainous US X Corps sector. Despite such obstacles, Operation Ripper pressed on throughout March. In the mountainous central region, US IX and US X Corps pushed forward methodically, IX Corps against light opposition and X Corps against staunch enemy defenses. Hongch'on was taken on the 15th and Chuncheon secured on the 22nd. The capture of Chuncheon was the last major ground objective of Operation Ripper.

Troops from the Chinese People's Volunteer Army (PVA) attacked United Nations Command (UN) positions on the lower Imjin River in an attempt to achieve a breakthrough and recapture the South Korean capital Seoul. The attack was part of the Chinese Spring Offensive, the aim of which was to regain the initiative on the battlefield after a series of successful UN counter-offensives in January–March 1951 had allowed UN forces to establish themselves beyond the 38th Parallel at the Kansas Line.

Communist Spring Offensive 1951 (situation 30 April 1951 and operations since 22 April 1951). © United States Military Academy

The section of the UN line where the battle took place was defended primarily by British forces of the 29th Infantry Brigade, consisting of three British and one Belgian infantry battalions supported by tanks and artillery. Despite facing a greatly numerically superior enemy, the brigade held its general positions for three days. When the units of the 29th Infantry Brigade were ultimately forced to fall back, their actions in the Battle of the Imjin River together with those of other UN forces, for example in the Battle of Kapyong, had blunted the impetus of the PVA offensive and allowed UN forces to retreat to prepared defensive positions north of Seoul, where the PVA were halted. It is often known as the "Battle that saved Seoul."

The Battle of Kapyong was fought between UN forces — primarily Canadian, Australian, and New Zealand—and the 118th Division of the Chinese People's Volunteer Army (PVA). The fighting occurred during the Chinese Spring Offensive and saw the 27th British Commonwealth Brigade establish blocking positions in the Kapyong Valley, on a key route south to the capital, Seoul. The two forward battalions—the 3rd Battalion, Royal Australian Regiment and 2nd Battalion, Princess Patricia's Canadian Light Infantry, both battalions consisting of about 700 men each—were supported by guns from the 16th Field Regiment of the Royal Regiment of New Zealand Artillery along with a company of US mortars and fifteen Sherman tanks. These forces occupied positions astride the valley with hastily developed defences. As thousands of soldiers from the Republic of Korea Army (ROK) began to withdraw through the valley, the PVA infiltrated the brigade position under the cover of darkness, and assaulted the Australians on Hill 504 during the evening and into the following day.

Although heavily outnumbered, the Australian and American tanks held their positions into the afternoon of April 24 before they were withdrawn from the battlefield to positions in the rear of the brigade headquarters, with both sides having suffered heavy casualties. The PVA then turned their attention to the surrounded Canadians on Hill 677, whose encirclement prevented any resupply or reinforcements from entering. The Canadian 2 PCCLI were ordered to make a last stand on Hill 677. During a fierce night battle on April 24/25 the Chinese forces were unable to dislodge the 2 PPCLI and sustained enormous losses. The next day the PVA withdrew back up the valley in order to regroup, and the Canadians were relieved late on April 26. The fighting helped blunt the PVA offensive and the actions of the Australians and Canadians at Kapyong were critical in preventing a breakthrough against the UN central front, the encirclement of US forces in Korea, and ultimately the capture of Seoul. The Canadian and Australian battalions bore the brunt of the assault and stopped an entire PVA division estimated at 10,000-20,000 in strength during the hard-fought defensive battle.

The UN May–June 1951 counteroffensive was launched in response to the Chinese spring offensive of April-May 1951. It was the final large-scale offensive of the war that saw significant territorial changes. By 19 May the second phase of the spring offensive, the Battle of the Soyang River, on the eastern section of the front, was losing momentum due to reinforcement of the UN forces, supply difficulties and mounting losses from UN air and artillery strikes. On 20 May the Chinese People's Volunteer Army (PVA) and Korean People's Army (KPA) began to withdraw after suffering heavy losses, simultaneously the UN launched their counteroffensive in the west and central portions of the front. On 24 May, once the PVA/KPA advance had been halted, the UN began a counteroffensive there also. In the west UN forces were unable to maintain contact with the PVA/KPA as they withdrew faster than the UN advance. In the central area the UN forces made contact with the PVA/KPA at chokepoints north of Chuncheon inflicting heavy losses. In the east UN forces had remained in contact with the PVA/KPA and progressively pushed them back north of the Soyang River.

By mid-June UN forces had reached Line Kansas approximately 2–6 miles (3.2–9.7 km) north of the 38th Parallel from which they had withdrawn at the start of the spring offensive and in some areas advanced to Line Wyoming further north. With the discussions for the start of ceasefire negotiations underway, the UN advance stopped on the Kansas-Wyoming Line which was fortified as the Main line of resistance and despite some limited attacks this would essentially remain the frontline throughout the next 2 years of stalemate.

For the remainder of the war, the UN and the PVA/KPA fought but exchanged little territory, as the stalemate held. Large-scale bombing of North Korea continued, and protracted armistice negotiations began on 10 July 1951 at Kaesong, an ancient capital of Korea located in PVA/KPA held territory. On the Chinese side, Zhou Enlai directed peace talks, and Li Kenong and Qiao Guanghua headed the negotiation team. Combat continued while the belligerents negotiated; the goal of the UN forces was to recapture all of South Korea and to avoid losing territory. The PVA and the KPA attempted similar operations and later effected military and psychological operations in order to test the UN Command's resolve to continue the war. The two sides constantly traded artillery fire along the front, the UN forces possessing a large firepower advantage over the Chinese-led forces. For example, in the last three months of 1952 the UN fired 3,553,518 field gun shells and 2,569,941 mortar shells, while the Communists fired 377,782 field gun shells and 672,194 mortar shells: an overall 5.83:1 ratio in the UN's favor. The Communist insurgency, reinvigorated by North Korean support and scattered bands of KPA stragglers, also resurged in the south. In the autumn of 1951, Van Fleet ordered Major General Paik Sun-yup to break the back of guerrilla activity. From December 1951 to March 1952, ROK security forces claimed to have killed 11,090 partisans and sympathizers and captured 9,916 more.

United Nations forces met with North Korean and Chinese officials at Panmunjeom from 1951 to 1953 for truce talks. The talks dragged on for many months. The main point of contention during the talks was the question surrounding the prisoners of war. Moreover, South Korea was uncompromising in its demand for a unified state. On June 8, 1953, an agreement to the POW problem was reached.

Those prisoners who refused to return to their countries were allowed to live under a neutral supervising commission for three months. At the end of this period, those who still refused repatriation would be released. Among those who refused repatriation were 21 American and one British POWs, all but two of whom chose to defect to the People's Republic of China.

By the summer of 1951, the Korean War had reached a stalemate as peace negotiations began at Kaesong. The opposing armies faced each other across a line which ran from east to west, through the middle of the Korean peninsula, located in hills a few miles north of the 38th Parallel in the central Korean mountain range. United Nations and the North Korean Korean People's Army (KPA) and Chinese People's Volunteer Army (PVA) forces jockeyed for position along this line, clashing in several relatively small but intense and bloody battles. Bloody Ridge began as an attempt by UN forces to seize a ridge of hills which they believed were being used as observation posts to call in artillery fire on a UN supply road.

After withdrawing from Bloody Ridge, the Korean People's Army (KPA) set up new positions just 1,500 yards (1,400 m) away on a 7-mile (11 km) long hill mass. If anything, the defenses were even more formidable here than on Bloody Ridge. The Battle of Heartbreak Ridge was one of several major engagements in the hills of North Korea a few miles north of the 38th Parallel (the pre-war boundary between North and South Korea), near Chorwon.

In 1951, the US escalated closest to atomic warfare in Korea. Because China deployed new armies to the Sino-Korean frontier, ground crews at the Kadena Air Base, Okinawa, assembled atomic bombs for Korean warfare, "lacking only the essential pit nuclear cores". In October 1951, the United States effected Operation Hudson Harbor to establish a nuclear weapons capability. USAF B-29 bombers practiced individual bombing runs from Okinawa to North Korea (using dummy nuclear or conventional bombs), coordinated from Yokota Air Base in east-central Japan. Hudson Harbor tested "actual functioning of all activities which would be involved in an atomic strike, including weapons assembly and testing, leading, ground control of bomb aiming". The bombing run data indicated that atomic bombs would be tactically ineffective against massed infantry, because the "timely identification of large masses of enemy troops was extremely rare".

General Matthew Ridgway was authorized to use nuclear weapons if a major air attack originated from outside Korea. An envoy was sent to Hong Kong to deliver a warning to China. The message likely caused Chinese leaders to be more cautious about potential US use of nuclear weapons, but whether they learned about the B-29 deployment is unclear and the failure of the two major Chinese offensives that month likely was what caused them to shift to a defensive strategy in Korea. The B-29s returned to the United States in June.

The Battle of Hill Eerie refers to several Korean War engagements between the United Nations Command (UN) forces and the Chinese People's Volunteer Army (PVA) in 1952 at Hill Eerie, a military outpost about 10 miles (16 km) west of Ch'orwon. It was taken several times by both sides; each sabotaging the others' position.

The Battle of Old Baldy refers to a series of five engagements for Hill 266 in west-central Korea. They occurred over a period of 10 months in 1952–1953, though there was also vicious fighting both before and after these engagements.

Baekma-goji or White Horse was the crest of a 395-metre (1,296 ft) forested hill mass that extended in a northwest-to-southeast direction for about 2 miles (3.2 km), part of the area controlled by the U.S. IX Corps, and considered an important outpost hill with a good command over the Yokkok-chon Valley, dominating the western approaches to Cheorwon. Loss of the hill would force IX Corps to withdraw to the high ground south of the Yokkok-chon in the Cheorwon area, denying the IX Corps use of the Cheorwon road net and would open up the entire Cheorwon area to enemy attack and penetration.

During ten days of battle, the hill would change hands 24 times after repeated attacks and counterattacks for its possession. Afterwards, Baengma-goji looked like a threadbare white horse, thence its name of Baengma, meaning a white horse.

The Battle of Triangle Hill was a protracted military engagement during the Korean War. The main combatants were two United Nations (UN) infantry divisions, with additional support from the United States Air Force, against elements of the Chinese People's Volunteer Army (PVA) 15th and 12th Corps.The battle was part of UN attempts to gain control of the "Iron Triangle".

The immediate UN objective was Triangle Hill, a forested ridge of high ground 2 kilometers (1.2 mi) north of Gimhwa-eup. The hill was occupied by the veterans of the PVA's 15th Corps. Over the course of nearly a month, substantial US and Republic of Korea Army (ROK) forces made repeated attempts to capture Triangle Hill and the adjacent Sniper Ridge. Despite clear superiority in artillery and aircraft, the escalating UN casualties resulted in the attack being halted after 42 days of fighting, with PVA forces regaining their original positions.

The Battle of Pork Chop Hill comprises a pair of related Korean War infantry battles during April and July 1953. These were fought while the United Nations Command (UN) and the Chinese and North Koreans negotiated the Korean Armistice Agreement. The UN won the first battle but the Chinese won the second battle.

The Third Battle of the Hook took place between a United Nations Command (UN) force, consisting mostly of British troops, supported on their flanks by American and Turkish units against a predominantly Chinese force.

The Battle of Kumsong was one of the last battles of the Korean War. During the ceasefire negotiations seeking to end the Korean War, the United Nations Command (UNC) and Chinese and North Korean forces were unable to agree on the issue of prisoner repatriation. South Korean President Syngman Rhee, who refused to sign the armistice, released 27,000 North Korean prisoners who refused repatriation. This action caused an outrage among the Chinese and North Korean commands and threatened to derail the ongoing negotiations. As a result, the Chinese decided to launch an offensive aimed at the Kumsong salient. This would be the last large-scale Chinese offensive of the war, scoring a victory over the UN forces.

The Korean Armistice Agreement is an armistice that brought about a complete cessation of hostilities of the Korean War. It was signed by United States Army Lieutenant General William Harrison Jr. and General Mark W. Clark representing the United Nations Command (UNC), North Korea leader Kim Il-sung and General Nam Il representing the Korean People's Army (KPA), and Peng Dehuai representing the Chinese People's Volunteer Army (PVA). The armistice was signed on 27 July 1953, and was designed to "ensure a complete cessation of hostilities and of all acts of armed force in Korea until a final peaceful settlement is achieved."

South Korea never signed the Armistice Agreement, due to President Syngman Rhee’s refusal to accept having failed to unify Korea by force. China normalized relations and signed a peace treaty with South Korea in 1992.