Mexican-American War

Mexico obtained independence from the Spanish Empire with the Treaty of Córdoba in 1821 after a decade of conflict between the royal army and insurgents for independence, with no foreign intervention. The conflict ruined the silver-mining districts of Zacatecas and Guanajuato. Mexico began as a sovereign nation with its future financial stability from its main export destroyed. Mexico briefly experimented with monarchy, but became a republic in 1824. This government was characterized by instability, and it was ill-prepared for a major international conflict when war broke out with the U.S. in 1846. Mexico had successfully resisted Spanish attempts to reconquer its former colony in the 1820s and resisted the French in the so-called Pastry War of 1838 but the secessionists' success in Texas and the Yucatán against the centralist government of Mexico showed its political weakness as the government changed hands multiple times. The Mexican military and the Catholic Church in Mexico, both privileged institutions with conservative political views, were stronger politically than the Mexican state.

The United States' 1803 Louisiana Purchase resulted in an undefined border between Spanish colonial territories and the U.S. Some of the boundary issues between the U.S. and Spain were resolved with the Adams-Onís Treaty of 1818. With the Industrial Revolution across the Atlantic increasing the demand for cotton for textile factories, there was a large external market of a valuable commodity produced by enslaved African-American labor in the southern states. This demand helped fuel expansion into northern Mexico. Northerners in the U.S. sought to develop the country's existing resources and expand the industrial sector without expanding the nation's territory. The existing balance of sectional interests would be disrupted by the expansion of slavery into new territory. The Democratic Party, to which President Polk belonged, in particular strongly supported expansion.

In 1800, Spain's colonial province of Texas (Tejas) had few inhabitants, with only about 7,000 non-native settlers. The Spanish crown developed a policy of colonization to more effectively control the territory. After independence, the Mexican government implemented the policy, granting Moses Austin, a banker from Missouri, a large tract of land in Texas. Austin died before he could bring his plan of recruiting American settlers for the land to fruition, but his son, Stephen F. Austin, brought over 300 American families into Texas. This started the steady trend of migration from the United States into the Texas frontier. Austin's colony was the most successful of several colonies authorized by the Mexican government. The Mexican government intended the new settlers to act as a buffer between the Tejano residents and the Comanches, but the non-Hispanic colonists tended to settle in areas with decent farmland and trade connections with Louisiana rather than farther west where they would have been an effective buffer against the Natives.

In 1829, because of the large influx of American immigrants, the non-Hispanic outnumbered native Spanish speakers in Texas. President Vicente Guerrero, a hero of Mexican independence, moved to gain more control over Texas and its influx of non-Hispanic colonists from the southern U.S. and discourage further immigration by abolishing slavery in Mexico. The Mexican government also decided to reinstate the property tax and increase tariffs on shipped American goods. The settlers and many Mexican businessmen in the region rejected the demands, which led to Mexico closing Texas to additional immigration, which continued from the United States into Texas illegally.

In 1834, Mexican conservatives seized the political initiative, and General Antonio López de Santa Anna became the centralist president of Mexico. The conservative-dominated Congress abandoned the federal system, replacing it with a unitary central government that removed power from the states. Leaving politics to those in Mexico City, General Santa Anna led the Mexican army to quash the semi-independence of Texas. He had done that in Coahuila (in 1824, Mexico had merged Texas and Coahuila into the enormous state of Coahuila y Tejas). Austin called Texians to arms and they declared independence from Mexico in 1836. After Santa Anna defeated the Texians in the Battle of the Alamo, he was defeated by the Texian Army commanded by General Sam Houston and was captured at the Battle of San Jacinto. In exchange for his life Santa Anna signed a treaty with Texas President David Burnet ending the war and recognizing Texian independence. The treaty was not ratified by the Mexican Congress as it had been signed by a captive under duress. Although Mexico refused to recognize Texian independence, Texas consolidated its status as an independent republic and received official recognition from Britain, France, and the United States, which all advised Mexico not to try to reconquer the new nation. Most Texians wanted to join the United States, but the annexation of Texas was contentious in the U.S. Congress, where Whigs and Abolitionists were largely opposed.: 150–155 In 1845, Texas agreed to the offer of annexation by the U.S. Congress and became the 28th state on December 29, 1845, which set the stage for the conflict with Mexico.

By the Treaties of Velasco made after Texans captured General Santa Ana after the Battle of San Jacinto, the southern border of Texas was placed at the "Rio Grande del Norte." The Texans claimed this placed the southern border at the modern Rio Grande. The Mexican government disputed this placement on two grounds: first, it rejected the idea of Texas independence; and second, it claimed that the Rio Grande in the treaty was actually the Nueces River, since the current Rio Grande has always been called "Rio Bravo" in Mexico. The latter claim belied the full name of the river in Mexico, however: "Rio Bravo del Norte." The ill-fated Texan Santa Fe Expedition of 1841 attempted to realize the claim to New Mexican territory east of the Rio Grande, but its members were captured by the Mexican Army and imprisoned. Reference to the Rio Grande boundary of Texas was omitted from the U.S. Congress's annexation resolution to help secure passage after the annexation treaty failed in the Senate. President Polk claimed the Rio Grande boundary, and when Mexico sent forces over the Rio Grande, this provoked a dispute. In July 1845, Polk sent General Zachary Taylor to Texas, and by October, Taylor commanded 3,500 Americans on the Nueces River, ready to take by force the disputed land. Polk wanted to protect the border and also coveted for the U.S. the continent clear to the Pacific Ocean.

President Polk ordered General Taylor and his forces south to the Rio Grande. Taylor ignored Mexican demands to withdraw to the Nueces. He constructed a makeshift fort (later known as Fort Brown/Fort Texas) on the banks of the Rio Grande opposite the city of Matamoros, Tamaulipas. The Mexican forces prepared for war. On April 25, 1846, a 2,000-man Mexican cavalry detachment attacked a 70-man U.S. patrol commanded by Captain Seth Thornton, which had been sent into the contested territory north of the Rio Grande and south of the Nueces River. In the Thornton Affair, the Mexican cavalry routed the patrol, killing 11 American soldiers and capturing 52.

A few days after the Thornton Affair, the Siege of Fort Texas began on May 3, 1846. Mexican artillery at Matamoros opened fire on Fort Texas, which replied with its own guns. The bombardment continued for 160 hours and expanded as Mexican forces gradually surrounded the fort. Thirteen U.S. soldiers were injured during the bombardment, and two were killed. Among the dead was Jacob Brown, after whom the fort was later named.

On May 8, 1846, Zachary Taylor and 2,400 troops arrived to relieve the fort. However, General Arista rushed north with a force of 3,400 and intercepted him about 5 miles (8 km) north of the Rio Grande River, near modern-day Brownsville, Texas. The U.S. Army employed "flying artillery", their term for horse artillery, a mobile light artillery mounted on horse carriages with the entire crew riding horses into battle. The fast-firing artillery and highly mobile fire support had a devastating effect on the Mexican army. In contrast to the "flying artillery" of the Americans, the Mexican cannons at the Battle of Palo Alto had lower-quality gunpowder that fired at velocities slow enough to make it possible for American soldiers to dodge artillery rounds. The Mexicans replied with cavalry skirmishes and their own artillery. The U.S. flying artillery somewhat demoralized the Mexican side, and seeking terrain more to their advantage, the Mexicans retreated to the far side of a dry riverbed (resaca) during the night and prepared for the next battle. It provided a natural fortification, but during the retreat, Mexican troops were scattered, making communication difficult.

During the Battle of Resaca de la Palma on May 9, 1846, the two sides engaged in fierce hand-to-hand combat. The U.S. Cavalry managed to capture the Mexican artillery, causing the Mexican side to retreat—a retreat that turned into a rout. Fighting on unfamiliar terrain, his troops fleeing in retreat, Arista found it impossible to rally his forces. Mexican casualties were significant, and the Mexicans were forced to abandon their artillery and baggage. Fort Brown inflicted additional casualties as the withdrawing troops passed by the fort, and additional Mexican soldiers drowned trying to swim across the Rio Grande. Taylor crossed the Rio Grande and began his series of battles in Mexican territory.

Polk received word of the Thornton Affair, which, added to the Mexican government's rejection of Slidell, Polk believed, constituted a casus belli. His message to Congress on May 11, 1846, claimed that "Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon American soil."

The U.S. Congress approved the declaration of war on May 13, 1846, after a few hours of debate, with southern Democrats in strong support. Sixty-seven Whigs voted against the war on a key slavery amendment, but on the final passage only 14 Whigs voted no, including Rep. John Quincy Adams. Later, a freshman Whig Congressman from Illinois, Abraham Lincoln, challenged Polk's assertion that American blood had been shed on American soil, calling it "a bold falsification of history". Regarding the beginning of the war, Ulysses S. Grant, who had opposed the war but served as an army lieutenant in Taylor's army, claims in his Personal Memoirs (1885) that the main goal of the U.S. Army's advance from Nueces River to the Rio Grande was to provoke the outbreak of war without attacking first, to debilitate any political opposition to the war.

In Mexico, although President Paredes issued a manifesto on May 23, 1846, and a declaration of a defensive war on April 23, the Mexican Congress officially declared war on July 7, 1846.

After the declaration of war on May 13, 1846, United States Army General Stephen W. Kearny moved southwest from Fort Leavenworth, Kansas, in June 1846 with about 1,700 men in his Army of the West. Kearny's orders were to secure the territories Nuevo México and Alta California.

In Santa Fe, Governor Manuel Armijo wanted to avoid battle, but on August 9, Colonel Diego Archuleta and militia officers Manuel Chaves and Miguel Pino forced him to muster a defense. Armijo set up a position in Apache Canyon, a narrow pass about 10 miles (16 km) southeast of the city. However, on August 14, before the American army was even in view, he decided not to fight. The New Mexican army retreated to Santa Fe, and Armijo fled to Chihuahua.

Kearny and his troops encountered no Mexican forces when they arrived on August 15. Kearny and his force entered Santa Fe and claimed the New Mexico Territory for the United States without a shot fired. Kearny declared himself the military governor of the New Mexico Territory on August 18 and established a civilian government. American officers drew up a temporary legal system for the territory called the Kearny Code.

Word of Congress' declaration of war reached California by August 1846. American consul Thomas O. Larkin, stationed in Monterey, worked successfully during the events in that vicinity to avoid bloodshed between Americans and the Mexican military garrison commanded by General José Castro, the senior military officer in California.

Captain John C. Frémont, leading a U.S. Army topographical expedition to survey the Great Basin, entered Sacramento Valley in December 1845. Frémont's party was at Upper Klamath Lake in the Oregon Territory when it received word that war between Mexico and the U.S. was imminent; the party then returned to California.

Mexico had issued a proclamation that unnaturalized foreigners were no longer permitted to have land in California and were subject to expulsion. With rumors swirling that General Castro was massing an army against them, American settlers in the Sacramento Valley banded together to meet the threat. On June 14, 1846, 34 American settlers seized control of the undefended Mexican government outpost of Sonoma to forestall Castro's plans. One settler created the Bear Flag and raised it over Sonoma Plaza. Within a week, 70 more volunteers joined the rebels' force, which grew to nearly 300 in early July. This event, led by William B. Ide, became known as the Bear Flag Revolt.

Commodore John D. Sloat, commander of the U.S. Navy's Pacific Squadron, near Mazatlan, Mexico, had received orders to seize San Francisco Bay and blockade California ports when he was positive that war had begun. Sloat set sail for Monterey, reaching it on July 1. On 5 July, Sloat received a message from Capt. John B. Montgomery of the Portsmouth in San Francisco Bay reporting the events of the Bear Flag Revolt in Sonoma and its open support by Brevet Capt. John C. Frémont. In a message to Montgomery, Sloat relayed his decision to seize Monterey and ordered the commander to take possession of Yerba Buena (San Francisco), adding, "I am very anxious to know if Captain Frémont will cooperate with us." On July 9, 70 sailors and Marines landed at Yerba Buena and raised the American flag. Later that day in Sonoma, the Bear Flag was lowered, and the American flag was raised in its place.

Mexico's defeats at Palo Alto and Resaca de la Palma set the stage for the return of Santa Anna, who at the outbreak of the war, was in exile in Cuba. He wrote to the government in Mexico City, stating he did not want to return to the presidency, but he would like to come out of exile in Cuba to use his military experience to reclaim Texas for Mexico. President Farías was driven to desperation. He accepted the offer and allowed Santa Anna to return. Unbeknownst to Farías, Santa Anna had secretly been dealing with U.S. representatives to discuss a sale of all contested territory to the U.S. at a reasonable price, on the condition that he be allowed back in Mexico through the U.S. naval blockades. Polk sent his own representative to Cuba, Alexander Slidell MacKenzie, to negotiate directly with Santa Anna. The negotiations were secret and there are no written records of the meetings, but there was some understanding that came out of the meetings. Polk asked Congress for $2 million to be used in negotiating a treaty with Mexico. The U.S. allowed Santa Anna to return to Mexico, lifting the Gulf Coast naval blockade. However, in Mexico, Santa Anna denied all knowledge of meeting with the U.S. representative or any offers or transactions. Rather than being Polk's ally, he pocketed any money given him and began to plan the defense of Mexico. The Americans were dismayed, including General Scott, as this was an unexpected result. "Santa Anna gloated over his enemies' naïveté: 'The United States was deceived in believing that I would be capable of betraying my mother country.'" Santa Anna avoided getting involved in politics, dedicating himself to Mexico's military defense. While politicians attempted to reset the governing framework to a federal republic, Santa Anna left for the front to retake lost northern territory. Although Santa Anna was elected president in 1846, he refused to govern, leaving that to his vice president, while he sought to engage with Taylor's forces. With the restored federal republic, some states refused to support the national military campaign led by Santa Anna, who had fought with them directly in the previous decade. Santa Anna urged Vice President Gómez Farías to act as a dictator to get the men and materiel needed for the war. Gómez Farías forced a loan from the Catholic Church, but the funds were not available in time to support Santa Anna's army.

The Pacific Coast Campaign refers to United States naval operations against targets along Mexico's Pacific Coast during the Mexican–American War. The objective of the campaign was to secure the Baja Peninsula of Mexico, and to blockade/capture west-coast ports of Mexico—especially Mazatlan, a major port-of-entry for imported supplies. The resistance of Mexican forces to the north in the Los Angeles area and the lack of ships, soldiers and logistical support prevented an early occupation of the peninsula and the west-coast Mexican seaports. The U.S. Navy attempted blockades of the ports three times before being able to successfully blockade and/or occupy them.

Following an easy initial occupation and the capitulation of La Paz by Governor Col. Francisco Palacios Miranda, loyalist residents met, declared Miranda a traitor, and rose in revolt. Under a new governor, Mauricio Castro Cota, and then under the leadership of Manuel Pineda Munoz (who defended Mulege from American landings), the loyalists attempted to expel the Americans from La Paz and San José del Cabo. Pineda was eventually captured and the Mexican army under Cota finally defeated at Todos Santos but only after the Treaty of Guadalupe Hidalgo that ended the war returned captured regions south of San Diego to Mexico.

Following the Battle of Resaca de la Palma, General Zachary Taylor with a force of United States Regulars, Volunteers and Texas Rangers crossed the Rio Grande on 18 May, while in early June, Mariano Arista turned over command of what remained of his army to Francisco Mejia, who led them to Monterrey. On 8 June, United States Secretary of War William L. Marcy ordered Taylor to continue command of operations in northern Mexico, suggested taking Monterrey, and defined his objective to "dispose the enemy to desire an end to the war."

In early July, General Tomas Requena garrisoned Monterrey with 1,800 men, with the remnants of Arista's army and additional forces from Mexico City arriving by the end of August such that the Mexican forces totaled 7,303 men. General Pedro de Ampudia received orders from Antonio López de Santa Anna to retreat further to the city of Saltillo, where Ampudia was to establish a defensive line, but Ampudia disagreed, sensing glory if he could stop Taylor's advance. Ampudia's forces included a largely Irish-American volunteers called San Patricios (or the Saint Patrick's Battalion).

In the Battle of Monterrey, Taylor's forces were outnumbered four to one, but managed to defeat the Mexican army in a day-long battle. The hard-fought urban combat led to heavy casualties on both sides. The battle ended with both sides negotiating a two-month armistice and the Mexican forces being allowed to make an orderly evacuation in return for the surrender of the city. The U.S. victory set the stage for future U.S. successes in the war, and it helped to secure California for the United States.

The invading army occupied the city and remained until June 18, 1848. As soon as the occupation occurred, the U.S. Army committed several executions of civilians and several women were raped. The newspaper, citing military sources reported more than fifty civilians killed in Monterrey in a single event. Similar acts of violence occurred in other surrounding occupied towns such as Marín, Apodaca as well as other towns between the Rio Grande and Monterrey. In most cases those attacks were perpetrated by the Texas Rangers. Several American volunteers condemned the attacks, and blamed the Texas Rangers for committing hate crimes on civilians allegedly for revenge of the former Mexican campaigns in Texas. Taylor admitted the atrocities committed by his men, but took no action to punish them.

Following the Battle of Monterey, the Americans held northern California but General José María Castro and Governor Pío Pico planned resistance in the south around the Los Angeles area. Commodore Robert F. Stockton arrived at Monterey Bay aboard the Congress on July 15 and took over command from John D. Sloat. Stockton accepted the Bear Flag revolutionaries, under the command of Major John C. Frémont, as the California Battalion. Stockton then garrisoned Sonoma, San Juan Bautista, Santa Clara, and Sutter's Fort. Stockton's plan for dealing with Castro was to have Commander Samuel Francis Du Pont carry Fremont's men in the Cyane to San Diego to block any movement southwards, while Stockton would land a force at San Pedro which would move overland against Castro. Fremont arrived at San Diego on July 29 and reached San Pedro on August 6 aboard the Congress.

On August 13, 1846, Stockton led his column into town, followed by Fremont's force a half-hour later. On August 14, the remnants of the Californio army surrendered. On September 23, twenty men under the command of Cerbulo Varela exchanged shots with the Americans at Government House, which ignited Los Angeles. On September 24, 150 Californios, organized under José María Flores, a Mexican Officer who remained in California, at Castro's old camp at La Mesa. Gillespie's forces were effectively besieged, while Gillespie sent Juan "Flaco" Brown to Commodore Stockton for help. Gillespie's men retreated to Fort Hill on September 28, but without water, they surrendered the next day. Terms called for Gillespie's men to leave Los Angeles, which they did on September 30, 1846, and boarded the American merchant vessel Vandalia. Flores quickly cleared the remaining American forces in southern California.

Commodore Matthew C. Perry led a detachment of seven vessels along the northern coast of Tabasco state. Perry arrived at the Tabasco River (now known as the Grijalva River) on October 22, 1846, and seized the town Port of Frontera along with two of their ships. Leaving a small garrison, he advanced with his troops towards the town of San Juan Bautista (Villahermosa today). Perry arrived in the city of San Juan Bautista on October 25, seizing five Mexican vessels. Colonel Juan Bautista Traconis, Tabasco Departmental commander at that time, set up barricades inside the buildings. Perry realized that the bombing of the city would be the only option to drive out the Mexican Army, and to avoid damage to the merchants of the city, withdrew its forces preparing them for the next day.

On the morning of October 26, as Perry's fleet prepared to start the attack on the city, the Mexican forces began firing at the American fleet. The U.S. bombing began to yield the square, so that the fire continued until evening. Before taking the square, Perry decided to leave and return to the port of Frontera, where he established a naval blockade to prevent supplies of food and military supplies from reaching the state capital.

The Battle of San Pasqual, also spelled San Pascual, was a military encounter that occurred during the Mexican–American War in what is now the San Pasqual Valley community of the city of San Diego, California. The series of military skirmishes ended with both sides claiming victory, and the victor of the battle is still debated. On December 6 and December 7, 1846, General Stephen W. Kearny's US Army of the West, along with a small detachment of the California Battalion led by a Marine Lieutenant, engaged a small contingent of Californios and their Presidial Lancers Los Galgos (The Greyhounds), led by Major Andrés Pico. After U.S. reinforcements arrived, Kearny's troops were able to reach San Diego.

The Battle of Río San Gabriel, fought on 8 January 1847, was a decisive action of the California campaign of the Mexican–American War and occurred at a ford of the San Gabriel River, at what are today parts of the cities of Whittier, Pico Rivera and Montebello, about ten miles south-east of downtown Los Angeles.

On January 12, Frémont and two of Pico's officers agreed to terms for a surrender. Articles of Capitulation were signed on January 13 by Frémont, Andrés Pico and six others at a ranch at Cahuenga Pass (modern-day North Hollywood). This became known as the Treaty of Cahuenga, which marked the end of armed resistance in California.

The Battle of La Mesa was the final battle of the California Campaign during the Mexican–American War, occurring on January 9, 1847, in present-day Vernon, California, the day after the Battle of Rio San Gabriel. The battle was a victory for the United States Army under Commodore Robert F. Stockton and General Stephen Watts Kearny.

The battle was the last armed resistance to the American conquest of California, and General José María Flores returned to Mexico afterward. Three days after the battle, on January 12, the last significant group of residents surrendered to U.S. forces. The conquest and annexation of Alta California was settled with the signing of the Treaty of Cahuenga by U.S. Army Lieutenant-Colonel John C. Frémont and Mexican General Andrés Pico on January 13, 1847.

When Kearny departed with his forces for California, he left Colonel Sterling Price in command of U.S. forces in New Mexico. He appointed Charles Bent as New Mexico's first territorial governor. An issue more significant than the galling daily insults was that many New Mexican citizens feared that their land titles, issued by the Mexican government, would not be recognized by the United States. They worried that American sympathizers would prosper at their expense. Following Kearny's departure, dissenters in Santa Fe plotted a Christmas uprising. When the plans were discovered by the US authorities, the dissenters postponed the uprising. They attracted numerous Native American allies, including Puebloan peoples, who also wanted to push the Americans from the territory.

Provisional governor Charles Bent and several other Americans were killed by the rebels. In two short campaigns, United States troops and militia crushed the rebellion of the Hispano and Pueblo people. The New Mexicans, seeking better representation, regrouped and fought three more engagements, but after being defeated, they abandoned open warfare. Hatred of New Mexicans for the occupying American army combined with the oft-exercised rebelliousness of Taos residents against authority imposed on them from elsewhere were causes of the revolt. In the aftermath of the revolt the Americans executed at least 28 rebels. The Treaty of Guadalupe Hidalgo in 1850 guaranteed the property rights of New Mexico's Hispanic and American Indian residents.

On February 22, 1847, having heard of this weakness from the written orders found on an ambushed U.S. scout, Santa Anna seized the initiative and marched Mexico's entire army north to fight Taylor with 20,000 men, hoping to win a smashing victory before Scott could invade from the sea. The two armies met and fought the largest battle of the war at the Battle of Buena Vista. Taylor, with 4,600 men, had entrenched at a mountain pass called La Angostura, or "the narrows", several miles south of Buena Vista ranch. Santa Anna, having little logistics to supply his army, suffered desertions all the long march north and arrived with only 15,000 men in a tired state.

Having demanded and been refused the surrender of the U.S. Army, Santa Anna's army attacked the next morning, using a ruse in the battle with the U.S forces. Santa Anna flanked the U.S. positions by sending his cavalry and some of his infantry up the steep terrain that made up one side of the pass, while a division of infantry attacked frontally to distract and draw out the U.S. forces along the road leading to Buena Vista. Furious fighting ensued, during which the U.S. troops were nearly routed, but managed to cling to their entrenched position, thanks to the Mississippi Rifles, a volunteer regiment led by Jefferson Davis, who formed them into a defensive V formation. The Mexicans had nearly broken the American lines at several points, but their infantry columns, navigating the narrow pass, suffered heavily from the American horse artillery, which fired point-blank canister shots to break up the attacks.

Initial reports of the battle, as well as propaganda from the Santanistas, credited the victory to the Mexicans, much to the joy of the Mexican populace, but rather than attack the next day and finish the battle, Santa Anna retreated, losing men along the way, having heard word of rebellion and upheaval in Mexico City. Taylor was left in control of part of northern Mexico, and Santa Anna later faced criticism for his withdrawal. Mexican and American military historians alike agree that the U.S. Army could likely have been defeated if Santa Anna had fought the battle to its finish.

After the battles of Monterrey and Buena Vista, much of Zachary Taylor's Army of Occupation was transferred to the command of Major General Winfield Scott in support of the upcoming campaign. Polk had decided that the way to bring the war to an end was to invade the Mexican heartland from the coast. Mexican military intelligence knew in advance of U.S. plans to attack Veracruz, but internal government turmoil left them powerless to send crucial reinforcements before the American assault commenced. On March 9, 1847, Scott performed the first major amphibious landing in U.S. history in preparation for a siege. A group of 12,000 volunteer and regular soldiers successfully offloaded supplies, weapons, and horses near the walled city using specially designed landing crafts. Included in the invading force were several future generals: Robert E. Lee, George Meade, Ulysses S. Grant, James Longstreet, and Thomas "Stonewall" Jackson.

Veracruz was defended by Mexican General Juan Morales with 3,400 men. Mortars and naval guns under Commodore Matthew C. Perry were used to reduce the city walls and harass defenders. The bombardment on March 24, 1847, opened in the walls of Veracruz a thirty-foot gap. The defenders in the city replied with their own artillery, but the extended barrage broke the will of the Mexicans, who faced a numerically superior force, and they surrendered the city after 12 days under siege. U.S. troops suffered 80 casualties, while the Mexicans had around 180 killed and wounded, with hundreds of civilians killed. During the siege, the U.S. soldiers began to fall victim to yellow fever.

Santa Anna allowed Scott's army to march inland, counting on yellow fever and other tropical diseases to take their toll before Santa Anna chose a place to engage the enemy. Mexico had used this tactic before, including when Spain attempted to reconquer Mexico in 1829. Disease could be a decisive factor in the war. Santa Anna was from Veracruz, so he was on his home territory, knew the terrain, and had a network of allies. He could draw on local resources to feed his hungry army and gain intelligence on the enemy's movements. From his experience in the northern battles on open terrain, Santa Anna sought to negate the U.S. Army's primary advantage, its use of artillery.

Santa Anna chose Cerro Gordo as the place to engage the U.S. troops, calculating the terrain would offer the maximum advantage for the Mexican forces. Scott marched westward on April 2, 1847, toward Mexico City with 8,500 initially healthy troops, while Santa Anna set up a defensive position in a canyon around the main road and prepared fortifications. Santa Anna had entrenched with what the U.S. Army believed were 12,000 troops but in fact was around 9,000. He had artillery trained on the road where he expected Scott to appear. However, Scott had sent 2,600 mounted dragoons ahead, and they reached the pass on April 12. The Mexican artillery prematurely fired on them and therefore revealed their positions, beginning the skirmish.

Instead of taking the main road, Scott's troops trekked through the rough terrain to the north, setting up his artillery on the high ground and quietly flanking the Mexicans. Although by then aware of the positions of U.S. troops, Santa Anna and his troops were unprepared for the onslaught that followed. In the battle fought on April 18, the Mexican army was routed. The U.S. Army suffered 400 casualties, while the Mexicans suffered over 1,000 casualties with 3,000 taken prisoner. The U.S. Army had expected a quick collapse of the Mexican forces. Santa Anna, however, was determined to fight to the end, and Mexican soldiers continued to regroup after battles to fight yet again.

On June 13, 1847, Commodore Perry assembled the Mosquito Fleet and began moving towards the Grijalva River, towing 47 boats that carried a landing force of 1,173. On June 15, 12 miles (19 km) below San Juan Bautista, the fleet ran through an ambush with little difficulty. Again at an "S" curve in the river known as the "Devil's Bend", Perry encountered Mexican fire from a river fortification known as the Colmena redoubt, but the fleet's heavy naval guns quickly dispersed the Mexican force.

On June 16, Perry arrived at San Juan Bautista and commenced bombing the city. The attack included two ships that sailed past the fort and began shelling it from the rear. David D. Porter led 60 sailors ashore and seized the fort, raising the American flag over the works. Perry and the landing force arrived and took control of the city around 14:00.

With guerrillas harassing his line of communications back to Veracruz, Scott decided not to weaken his army to defend Puebla but, leaving only a garrison at Puebla to protect the sick and injured recovering there, advanced on Mexico City on August 7 with his remaining force. The capital was laid open in a series of battles around the right flank of the city defenses, the Battle of Contreras and Battle of Churubusco. After Churubusco, fighting halted for an armistice and peace negotiations, which broke down on September 6, 1847. With the subsequent battles of Molino del Rey and of Chapultepec, and the storming of the city gates, the capital was occupied. Scott became military governor of occupied Mexico City. His victories in this campaign made him an American national hero.

The Battle of Chapultepec in September 1847 was a siege on the castle of Chapultepec, built on a hill in Mexico City in the colonial era. At this time, this castle was a renowned military school in the capital. After the battle, which ended in a victory for the U.S., the legend of "Los Niños Héroes" was born. Although not confirmed by historians, six military cadets between the ages of 13 and 17 stayed in the school instead of evacuating. They decided to stay and fight for Mexico. These Niños Héroes (boy heroes) became icons in Mexico's patriotic pantheon. Rather than surrender to the U.S. Army, some military cadets leaped from the castle walls. A cadet named Juan Escutia wrapped himself in the Mexican flag and jumped to his death.

In late September 1847, Santa Anna made one last attempt to defeat the U.S. Army, by cutting them off from the coast. General Joaquín Rea began the Siege of Puebla, soon joined by Santa Anna. Scott had left some 2,400 soldiers in Puebla, of whom around 400 were fit. After the fall of Mexico City, Santa Anna hoped to rally Puebla's civilian population against the U.S. soldiers under siege and subject to guerrilla attacks. Before the Mexican army could wipe out the Americans in Puebla, more troops landed in Veracruz under the command of Brigadier General Joseph Lane. At Puebla, they sacked the town. Santa Anna was not able to provision his troops, who effectively dissolved as a fighting force to forage for food. Puebla was relieved by Lane on October 12, following his defeat of Santa Anna at the Battle of Huamantla on October 9. The battle was Santa Anna's last. Following the defeat, the new Mexican government led by Manuel de la Peña y Peña asked Santa Anna to turn over command of the army to General José Joaquín de Herrera.

Following the capture of the capital, the Mexican government moved to the temporary capital at Querétaro. In Mexico City, U.S. forces became an army of occupation and subject to stealth attacks from the urban population. Conventional warfare gave way to guerrilla warfare by Mexicans defending their homeland. They inflicted significant casualties on the U.S. Army, particularly on soldiers slow to keep up.

General Scott sent about a quarter of his strength to secure his line of communications to Veracruz from the Light Corps of General Rea and other Mexican guerrilla forces that had made stealth attacks since May. Mexican guerrillas often tortured and mutilated the bodies of the U.S. troops, as revenge and warning. Americans interpreted these acts not as Mexicans' defense of their patria, but as evidence of Mexicans' brutality as racial inferiors. For their part, U.S. soldiers took revenge on Mexicans for the attacks, whether or not they were individually suspected of guerrilla acts.

Scott viewed guerrilla attacks as contrary to the "laws of war" and threatened the property of populations that appeared to harbor the guerrillas. Captured guerrillas were to be shot, including helpless prisoners, with the reasoning that the Mexicans did the same. Historian Peter Guardino contends that the U.S. Army command was complicit in the attacks against Mexican civilians. By threatening the civilian populations' homes, property, and families with burning whole villages, looting, and raping women, the U.S. Army separated guerrillas from their base. "Guerrillas cost the Americans dearly, but indirectly cost Mexican civilians more."

Scott strengthened the garrison of Puebla and by November had added a 1,200-man garrison at Jalapa, established 750-man posts along the main route between the port of Veracruz and the capital, at the pass between Mexico City and Puebla at Rio Frio, at Perote and San Juan on the road between Jalapa and Puebla, and at Puente Nacional between Jalapa and Veracruz. He had also detailed an anti-guerrilla brigade under Lane to carry the war to the Light Corps and other guerrillas. He ordered that convoys would travel with at least 1,300-man escorts. Victories by Lane over the Light Corps at Atlixco (October 18, 1847), at Izúcar de Matamoros (November 23, 1847), and at Galaxara Pass (November 24, 1847) weakened General Rea's forces.

Later a raid against the guerrillas of Padre Jarauta at Zacualtipan (February 25, 1848) further reduced guerrilla raids on the American line of communications. After the two governments concluded a truce to await ratification of the peace treaty, on March 6, 1848, formal hostilities ceased. However, some bands continued in defiance of the Mexican government until the U.S. Army's evacuation in August. Some were suppressed by the Mexican Army or, like Padre Jarauta, executed.

The Treaty of Guadalupe Hidalgo, signed on February 2, 1848, by diplomat Nicholas Trist and Mexican plenipotentiary representatives Luis G. Cuevas, Bernardo Couto, and Miguel Atristain, ended the war. The treaty gave the U.S. undisputed control of Texas, established the U.S.-Mexican border along the Rio Grande, and ceded to the United States the present-day states of California, Nevada, and Utah, most of New Mexico, Arizona and Colorado, and parts of Texas, Oklahoma, Kansas, and Wyoming. In return, Mexico received $15 million ($470 million today) – less than half the amount the U.S. had attempted to offer Mexico for the land before the opening of hostilities – and the U.S. agreed to assume $3.25 million ($102 million today) in debts that the Mexican government owed to U.S. citizens. The area of domain acquired was given by the Federal Interagency Committee as 338,680,960 acres. The cost was $16,295,149 or approximately 5 cents per acre. The area amounted to one-third of Mexico's original territory from its 1821 independence. The treaty was ratified by the U.S. Senate by a vote of 38 to 14 on March 10 and by Mexico through a legislative vote of 51–34 and a Senate vote of 33–4, on May 19.

In much of the United States, victory and the acquisition of new land brought a surge of patriotism. Victory seemed to fulfill Democrats' belief in their country's Manifest Destiny. Although the Whigs had opposed the war, they made Zachary Taylor their presidential candidate in the election of 1848, praising his military performance while muting their criticism of the war. Many of the military leaders on both sides of the American Civil War of 1861–1865 had trained at the U.S. Military Academy at West Point and had fought as junior officers in Mexico.

For Mexico, the war had remained a painful historical event for the country, losing territory and highlighting the domestic political conflicts that were to continue for another 20 years. The Reform War between liberals and conservatives in 1857 was followed by the Second French Intervention, which set up the Second Mexican Empire. The war caused Mexico to enter "a period of self-examination ... as its leaders sought to identify and address the reasons that had led to such a debacle." In the immediate aftermath of the war, a group of Mexican writers including Ignacio Ramírez, Guillermo Prieto, José María Iglesias, and Francisco Urquidi compiled a self-serving assessment of the reasons for the war and Mexico's defeat, edited by Mexican army officer Ramón Alcaraz. Denying that Mexican claims to Texas had anything to do with the war, they instead wrote that for "the true origin of the war, it is sufficient to say that the insatiable ambition of the United States, favored by our weakness, caused it.