World War I

For much of the 19th century, the major European powers maintained a tenuous balance of power among themselves, known as the Concert of Europe. After 1848, this was challenged by a variety of factors, including Britain's withdrawal into so-called splendid isolation, the decline of the Ottoman Empire and the rise of Prussia under Otto von Bismarck. The 1866 Austro-Prussian War established Prussian hegemony in Germany, while victory in the 1870–1871 Franco-Prussian War allowed Bismarck to consolidate the German states into a German Empire under Prussian leadership.

After 1871, the creation of a unified Reich, supported by French indemnity payments and the annexation of Alsace-Lorraine, led to a huge increase in German industrial strength. Backed by Wilhelm II, Admiral Alfred von Tirpitz sought to exploit this to build a Kaiserliche Marine, or Imperial German Navy, able to compete with the British Royal Navy for world naval supremacy. He was greatly influenced by US naval strategist Alfred Thayer Mahan, who argued possession of a blue-water navy was vital for global power projection.

The years before 1914 were marked by a series of crises in the Balkans as other powers sought to benefit from Ottoman decline. While Pan-Slavic and Orthodox Russia considered itself the protector of Serbia and other Slav states, they preferred the strategically vital Bosporus straits be controlled by a weak Ottoman government, rather than an ambitious Slav power like Bulgaria. Since Russia had its own ambitions in Eastern Turkey and their clients had over-lapping claims in the Balkans, balancing them divided Russian policy makers and added to regional instability.

The Great Powers sought to re-assert control through the 1913 Treaty of London, which created an independent Albania, while enlarging the territories of Bulgaria, Serbia, Montenegro and Greece. However, disputes between the victors sparked the 33-day Second Balkan War, when Bulgaria attacked Serbia and Greece on 16 June 1913; it was defeated, losing most of Macedonia to Serbia and Greece, and Southern Dobruja to Romania. The result was that even countries which benefited from the Balkan Wars, such as Serbia and Greece, felt cheated of their "rightful gains", while for Austria it demonstrated the apparent indifference with which other powers viewed their concerns, including Germany. This complex mix of resentment, nationalism and insecurity helps explain why the pre-1914 Balkans became known as the "powder keg of Europe".

Archduke Franz Ferdinand of Austria, heir presumptive to the Austro-Hungarian throne, and his wife, Sophie, Duchess of Hohenberg, were assassinated on 28 June 1914 by Bosnian Serb student Gavrilo Princip, shot at close range while being driven through Sarajevo, the provincial capital of Bosnia-Herzegovina, formally annexed by Austria-Hungary in 1908.

The political objective of the assassination was to free Bosnia and Herzegovina of Austria-Hungarian rule and establish a common South Slav ("Yugoslav") state. The assassination precipitated the July Crisis which led to Austria-Hungary declaring war on Serbia and the start of World War I.

The German invasion of Belgium was a military campaign which began on 4 August 1914. Earlier, on 24 July, the Belgian government had announced that if war came it would uphold its neutrality. The Belgian government mobilised its armed forces on 31 July and a state of heightened alert (Kriegsgefahr) was proclaimed in Germany. On 2 August, the German government sent an ultimatum to Belgium, demanding passage through the country and German forces invaded Luxembourg. Two days later, the Belgian government refused the demands and the British government guaranteed military support to Belgium. The German government declared war on Belgium on 4 August; German troops crossed the border and began the Battle of Liège.

German military operations in Belgium were intended to bring the 1st, 2nd and 3rd Armies into positions in Belgium from which they could invade France, which, after the fall of Liège on 7 August, led to sieges of Belgian fortresses along the river Meuse at Namur and the surrender of the last forts (16–17 August). The government abandoned the capital, Brussels, on 17 August and after fighting on the Gete river, the Belgian field army withdrew westwards to the National Redoubt at Antwerp on 19 August. Brussels was occupied the following day and the siege of Namur began on 21 August.

After the Battle of Mons and the Battle of Charleroi, the bulk of the German armies marched south into France, leaving small forces to garrison Brussels and the Belgian railways. The III Reserve Corps advanced to the fortified zone around Antwerp and a division of the IV Reserve Corps took over in Brussels. The Belgian field army made several sorties from Antwerp in late August and September to harass German communications and to assist the French and the British Expeditionary Force (BEF), by keeping German troops in Belgium. German troop withdrawals to reinforce the main armies in France were postponed to repulse a Belgian sortie from 9 to 13 September and a German corps in transit was retained in Belgium for several days. Belgian resistance and German fear of francs-tireurs, led the Germans to implement a policy of terror (schrecklichkeit) against Belgian civilians soon after the invasion, in which massacres, executions, hostage-taking and the burning of towns and villages took place and became known as the Rape of Belgium.

The Togoland Campaign (6–26 August 1914) was a French and British invasion of the German colony of Togoland in West Africa, which began the West African Campaign of the First World War. German colonial forces withdrew from the capital Lomé and the coastal province to fight delaying actions on the route north to Kamina, where the Kamina Funkstation (wireless transmitter) linked the government in Berlin to Togoland, the Atlantic and South America.

The main British and French force from the neighbouring colonies of Gold Coast and Dahomey advanced from the coast up the road and railway, as smaller forces converged on Kamina from the north. The German defenders were able to delay the invaders for several days at the Affair of Agbeluvoe (affair, an action or engagement not of sufficient magnitude to be called a battle) and the Affair of Khra but surrendered the colony on 26 August 1914. In 1916, Togoland was partitioned by the victors and in July 1922, British Togoland and French Togoland were established as League of Nations mandates.

The Battle of the Frontiers comprised battles fought along the eastern frontier of France and in southern Belgium, shortly after the outbreak of the First World War. The battles resolved the military strategies of the French Chief of Staff General Joseph Joffre with Plan XVII and an offensive interpretation of the German Aufmarsch II deployment plan by Helmuth von Moltke the Younger, the German concentration on the right (northern) flank, to wheel through Belgium and attack the French in the rear.

The German advance was delayed by the movement of the French Fifth Army (General Charles Lanrezac) towards the north-west to intercept them and the presence of the British Expeditionary Force (BEF) on the French left. The Franco-British troops were driven back by the Germans, who were able to invade northern France. French and British rearguard actions delayed the Germans, allowing the French time to transfer forces on the eastern frontier to the west to defend Paris, culminating in the First Battle of the Marne.

The Atlantic U-boat campaign of World War I was the prolonged naval conflict between German submarines and the Allied navies in Atlantic waters—the seas around the British Isles, the North Sea and the coast of France.

Initially the U-boat campaign was directed against the British Grand Fleet. Later U-boat fleet action was extended to include action against the trade routes of the Allied powers. This campaign was highly destructive, and resulted in the loss of nearly half of Britain's merchant marine fleet during the course of the war. To counter the German submarines, the Allies moved shipping into convoys guarded by destroyers, blockades such as the Dover Barrage and minefields were laid, and aircraft patrols monitored the U-boat bases.

The U-boat campaign was not able to cut off supplies before the US entered the war in 1917 and in later 1918, the U-boat bases were abandoned in the face of the Allied advance. The tactical successes and failures of the Atlantic U-boat Campaign would later be used as a set of available tactics in World War II in a similar U-boat war against the British Empire.

The Battle of Tannenberg, also known as the Second Battle of Tannenberg, was fought between Russia and Germany between 26 and 30 August 1914, the first month of World War I. The battle resulted in the almost complete destruction of the Russian Second Army and the suicide of its commanding general, Alexander Samsonov. A series of follow-up battles (First Masurian Lakes) destroyed most of the First Army as well and kept the Russians off balance until the spring of 1915.

The battle is particularly notable for fast rail movements by the German Eighth Army, enabling them to concentrate against each of the two Russian armies in turn, first delaying the First Army and then destroying the Second before once again turning on the First days later. It is also notable for the failure of the Russians to encode their radio messages, broadcasting their daily marching orders in the clear, which allowed the Germans to make their movements with the confidence they would not be flanked.

The almost miraculous outcome brought considerable prestige to Field Marshal Paul von Hindenburg and his rising staff-officer Erich Ludendorff. Although the battle actually took place near Allenstein (Olsztyn), Hindenburg named it after Tannenberg, 30 km (19 mi) to the west, in order to avenge the Teutonic Knights' defeat at the First Battle of Tannenberg 500 years earlier.

The siege of Tsingtao (or Tsingtau) was the attack on the German port of Tsingtao (now Qingdao) in China during World War I by Japan and the United Kingdom. The siege was waged against the German Empire between 27 August and 7 November 1914. The siege was the first encounter between Japanese and German forces, the first Anglo-Japanese operation of the war, and the only major land battle in the Asian and Pacific theatre during World War I.

The First Battle of the Marne was a battle of the First World War fought from 5 to 12 September 1914. It was fought in a collection of skirmishes around the Marne River Valley. It resulted in an Entente victory against the German armies in the west. The battle was the culmination of the Retreat from Mons and pursuit of the Franco–British armies which followed the Battle of the Frontiers in August and reached the eastern outskirts of Paris.

Field Marshal Sir John French, commander of the British Expeditionary Force (BEF), began to plan for a full British retreat to port cities on the English Channel for an immediate evacuation. The military governor of Paris, Joseph Simon Gallieni, wanted the Franco–British units to counter-attack the Germans along the Marne River and halt the German advance. Entente reserves would restore the ranks and attack the German flanks. On 5 September, the counter-offensive by six French armies and the British Expeditionary Force (BEF) began.

By 9 September, the success of the Franco–British counteroffensive left the German 1st and 2nd Armies at risk of encirclement, and they were ordered to retreat to the Aisne River. The retreating armies were pursued by the French and British. The German armies ceased their retreat after 40 mi (65 km) on a line north of the Aisne River, where they dug in on the heights and fought the First Battle of the Aisne.

The German retreat from 9 to 13 September marked the end of the attempt to defeat France by crushing the French armies with an invasion from the north through Belgium and in the south over the common border. Both sides commenced reciprocal operations to envelop the northern flank of their opponent, in what became known as the Race to the Sea which culminated in the First Battle of Ypres.

The Race to the Sea took place from about 17 September – 19 October 1914 during the First World War, after the Battle of the Frontiers (7 August – 13 September) and the German advance into France. The invasion had been stopped at the First Battle of the Marne (5–12 September) and was followed by the First Battle of the Aisne (13–28 September), a Franco-British counter-offensive. The term describes reciprocal attempts by the Franco-British and German armies to envelop the northern flank of the opposing army through the provinces of Picardy, Artois and Flanders, rather than an attempt to advance northwards to the sea. The "race" ended on the North Sea coast of Belgium around 19 October, when the last open area from Diksmuide to the North Sea was occupied by Belgian troops who had retreated after the Siege of Antwerp (28 September – 10 October). The outflanking attempts had resulted in a number of encounter battles but neither side was able to gain a decisive victory.

After the opposing forces had reached the North Sea, both tried to conduct offensives leading to the mutually costly and indecisive Battle of the Yser from 16 October to 2 November and the First Battle of Ypres from 19 October to 22 November.

Over the winter lull, the French army established the theoretical basis of offensive trench warfare, originating many of the methods which became standard for the rest of the war. Infiltration tactics, in which dispersed formations of infantry were followed by nettoyeurs de tranchée (trench cleaners), to capture by-passed strong points were promulgated. Artillery observation from aircraft and creeping barrages, were first used systematically in the Second Battle of Artois from 9 May to 18 June 1915. Falkenhayn issued memoranda on 7 and 25 January 1915, to govern defensive battle on the Western Front, in which the existing front line was to be fortified and to be held indefinitely with small numbers of troops, to enable more divisions to be sent to the Eastern Front. New defences were to be built behind the front line to contain a breakthrough until the position was restored by counter-attacks. The Westheer began the huge task of building field fortifications, which were not complete until the autumn of 1915.

The First Battle of Ypres was a battle of the First World War, fought on the Western Front around Ypres, in West Flanders, Belgium. The battle was part of the First Battle of Flanders, in which German, French, Belgian armies and the British Expeditionary Force (BEF) fought from Arras in France to Nieuwpoort (Nieuport) on the Belgian coast, from 10 October to mid-November. The battles at Ypres began at the end of the Race to the Sea, reciprocal attempts by the German and Franco-British armies to advance past the northern flank of their opponents. North of Ypres, the fighting continued in the Battle of the Yser (16–31 October), between the German 4th Army, the Belgian army and French marines.

The fighting has been divided into five stages, an encounter battle from 19 to 21 October, the Battle of Langemarck from 21 to 24 October, the battles at La Bassée and Armentières to 2 November, coincident with more Allied attacks at Ypres and the Battle of Gheluvelt (29–31 October), a fourth phase with the last big German offensive, which culminated at the Battle of Nonne Bosschen on 11 November, then local operations which faded out in late November. Brigadier-General James Edmonds, the British official historian, wrote in the History of the Great War, that the II Corps battle at La Bassée could be taken as separate but that the battles from Armentières to Messines and Ypres, were better understood as one battle in two parts, an offensive by III Corps and the Cavalry Corps from 12 to 18 October against which the Germans retired and an offensive by the German 6th Army and 4th Army from 19 October to 2 November, which from 30 October, took place mainly north of the Lys, when the battles of Armentières and Messines merged with the Battles of Ypres.

Warfare between mass armies, equipped with the weapons of the Industrial Revolution and its later developments, proved to be indecisive, because field fortifications neutralised many classes of offensive weapon. The defensive firepower of artillery and machine guns dominated the battlefield and the ability of the armies to supply themselves and replace casualties prolonged battles for weeks. Thirty-four German divisions fought in the Flanders battles, against twelve French, nine British and six Belgian divisions, along with marines and dismounted cavalry. Over the winter, Falkenhayn reconsidered Germany strategy because Vernichtungsstrategie and the imposition of a dictated peace on France and Russia had exceeded German resources. Falkenhayn devised a new strategy to detach either Russia or France from the Allied coalition through diplomacy as well as military action. A strategy of attrition (Ermattungsstrategie) would make the cost of the war too great for the Allies, until one dropped out and made a separate peace. The remaining belligerents would have to negotiate or face the Germans concentrated on the remaining front, which would be sufficient for Germany to inflict a decisive defeat.

The Christmas truce (German: Weihnachtsfrieden; French: Trêve de Noël; Dutch: Kerstbestand) was a series of widespread unofficial ceasefires along the Western Front of the First World War around Christmas 1914.

The truce occurred five months after hostilities had begun. Lulls occurred in the fighting as armies ran out of men and munitions and commanders reconsidered their strategies following the stalemate of the Race to the Sea and the indecisive result of the First Battle of Ypres. In the week leading up to 25 December, French, German and British soldiers crossed trenches to exchange seasonal greetings and talk. In some areas, men from both sides ventured into no man's land on Christmas Eve and Christmas Day to mingle and exchange food and souvenirs. There were joint burial ceremonies and prisoner swaps, while several meetings ended in carol-singing. Men played games of football with one another, creating one of the most memorable images of the truce. Hostilities continued in some sectors, while in others the sides settled on little more than arrangements to recover bodies.

The following year, a few units arranged ceasefires but the truces were not nearly as widespread as in 1914; this was, in part, due to strongly worded orders from commanders, prohibiting truces. Soldiers were no longer amenable to truce by 1916; the war had become increasingly bitter after the human losses suffered during the battles of 1915.

The Sinai and Palestine campaign of the Middle Eastern theatre of World War I was fought by the Arab Revolt and the British Empire, against the Ottoman Empire and its Imperial German allies. It started with an Ottoman attempt at raiding the Suez Canal in 1915, and ended with the Armistice of Mudros in 1918, leading to the cession of Ottoman Syria.

The campaign was generally not well known or understood during the war. In Britain, the public thought of it as a minor operation, a waste of precious resources which would be better spent on the Western Front, while the peoples of India were more interested in the Mesopotamian campaign and the occupation of Baghdad. Australia did not have a war correspondent in the area until Captain Frank Hurley, the first Australian Official Photographer, arrived in August 1917 after visiting the Western Front. Henry Gullett, the first Official War Correspondent, arrived in November 1917.

The long-lasting effect of this campaign was the Partitioning of the Ottoman Empire, when France won the mandate for Syria and Lebanon, while the British Empire won the mandates for Mesopotamia and Palestine. The Republic of Turkey came into existence in 1923 after the Turkish War of Independence ended the Ottoman Empire. The European mandates ended with the formation of the Kingdom of Iraq in 1932, the Lebanese Republic in 1943, the State of Israel in 1948, and the Hashemite Kingdom of Transjordan and Syrian Arab Republic in 1946.

The Gallipoli campaign was a military campaign in the First World War that took place on the Gallipoli peninsula (Gelibolu in modern Turkey), from 17 February 1915 to 9 January 1916. The Entente powers, Britain, France and Russia, sought to weaken the Ottoman Empire, one of the Central Powers, by taking control of the Ottoman straits. This would expose the Ottoman capital at Constantinople to bombardment by Allied battleships and cut it off from the Asian part of the empire. With Turkey defeated, the Suez Canal would be safe and a year-round Allied supply route could be opened through the Black Sea to warm-water ports in Russia.

The attempt by the Allied fleet to force a passage through the Dardanelles in February 1915 failed and was followed by an amphibious landing on the Gallipoli peninsula in April 1915. In January 1916, after eight months' fighting, with approximately 250,000 casualties on each side, the land campaign was abandoned and the invasion force withdrawn. It was a costly campaign for the Entente powers and the Ottoman Empire, as well as for the sponsors of the expedition, especially the First Lord of the Admiralty (1911–1915), Winston Churchill. The campaign was considered a great Ottoman victory. In Turkey, it is regarded as a defining moment in the history of the state, a final surge in the defence of the motherland as the Ottoman Empire retreated. The struggle formed the basis for the Turkish War of Independence and the declaration of the Republic of Turkey eight years later, with Mustafa Kemal Atatürk, who rose to prominence as a commander at Gallipoli, as founder and president.

The RMS Lusitania was a UK-registered ocean liner that was torpedoed by an Imperial German Navy U-boat during the First World War on 7 May 1915, about 11 nautical miles (20 kilometres) off the Old Head of Kinsale, Ireland. The attack took place in the declared maritime war-zone around the UK, shortly after unrestricted submarine warfare against the ships of the United Kingdom had been announced by Germany following the Allied powers' implementation of a naval blockade against it and the other Central Powers. The passengers had been warned before departing New York of the danger of voyaging into the area in a British ship.

The Cunard liner was attacked by U-20 commanded by Kapitänleutnant Walther Schwieger. After the single torpedo struck, a second explosion occurred inside the ship, which then sank in only 18 minutes.: 429 761 people survived out of the 1,266 passengers and 696 crew aboard, and 123 of the casualties were American citizens. The sinking turned public opinion in many countries against Germany. It also contributed to the American entry into the War two years later; images of the stricken liner were used heavily in US propaganda and military recruiting campaigns.

The Great Retreat was a strategic withdrawal on the Eastern Front of World War I in 1915. The Imperial Russian Army gave up the salient in Galicia and Poland. The Russians' critically under-equipped and (at the points of engagement) outnumbered forces suffered great losses in the Central Powers' July–September summer offensive operations, this leading to the Stavka ordering a withdrawal to shorten the front lines and avoid the potential encirclement of large Russian forces in the salient. While the withdrawal itself was relatively well conducted, it was a severe blow to Russian morale.

The Battle of Verdun was fought from 21 February to 18 December 1916 on the Western Front in France. The battle was the longest of the First World War and took place on the hills north of Verdun-sur-Meuse. The German 5th Army attacked the defences of the Fortified Region of Verdun (RFV, Région Fortifiée de Verdun) and those of the French Second Army on the right (east) bank of the Meuse. Using the experience of the Second Battle of Champagne in 1915, the Germans planned to capture the Meuse Heights, an excellent defensive position, with good observation for artillery-fire on Verdun. The Germans hoped that the French would commit their strategic reserve to recapture the position and suffer catastrophic losses at little cost to the Germans.

The Battle of Jutland was a naval battle fought between Britain's Royal Navy Grand Fleet, under Admiral Sir John Jellicoe, and the Imperial German Navy's High Seas Fleet, under Vice-Admiral Reinhard Scheer, during the First World War. The battle unfolded in extensive manoeuvring and three main engagements (the battlecruiser action, the fleet action and the night action), from 31 May to 1 June 1916, off the North Sea coast of Denmark's Jutland Peninsula. It was the largest naval battle and the only full-scale clash of battleships in that war. Jutland was the third fleet action between steel battleships, following the Battle of the Yellow Sea in 1904 and the decisive Battle of Tsushima in 1905, during the Russo-Japanese War. Jutland was the last major battle in world history fought primarily by battleships.

The Arab Revolt was a military uprising of Arab forces against the Ottoman Empire in the Middle Eastern theatre of World War I. On the basis of the McMahon–Hussein Correspondence, an agreement between the British government and Hussein bin Ali, Sharif of Mecca, the revolt was officially initiated at Mecca on June 10, 1916. The aim of the revolt was to create a single unified and independent Arab state stretching from Aleppo in Syria to Aden in Yemen, which the British had promised to recognize.

The Sharifian Army led by Hussein and the Hashemites, with military backing from the British Egyptian Expeditionary Force, successfully fought and expelled the Ottoman military presence from much of the Hejaz and Transjordan. The rebellion eventually took Damascus and set up the Arab Kingdom of Syria, a short-lived monarchy led by Faisal, a son of Hussein.

Following the Sykes–Picot Agreement, the Middle East was later partitioned by the British and French into mandate territories rather than a unified Arab state, and the British reneged on their promise to support a unified independent Arab state.

The Battle of the Somme, also known as the Somme offensive, was a battle of the First World War fought by the armies of the British Empire and French Third Republic against the German Empire. It took place between 1 July and 18 November 1916 on both sides of the upper reaches of the Somme, a river in France. The battle was intended to hasten a victory for the Allies. More than three million men fought in the battle and one million men were wounded or killed, making it one of the deadliest battles in human history.

The French and British had committed themselves to an offensive on the Somme during the Chantilly Conference in December 1915. The Allies agreed upon a strategy of combined offensives against the Central Powers in 1916 by the French, Russian, British and Italian armies, with the Somme offensive as the Franco-British contribution. Initial plans called for the French army to undertake the main part of the Somme offensive, supported on the northern flank by the Fourth Army of the British Expeditionary Force (BEF). When the Imperial German Army began the Battle of Verdun on the Meuse on 21 February 1916, French commanders diverted many of the divisions intended for the Somme and the "supporting" attack by the British became the principal effort. The British troops on the Somme comprised a mixture of the remains of the pre-war army, the Territorial Force and Kitchener's Army, a force of wartime volunteers.

At the end of the battle, British and French forces had penetrated 6 mi (10 km) into German-occupied territory along the majority of the front, their largest territorial gain since the First Battle of the Marne in 1914. The operational objectives of the Anglo-French armies were unfulfilled, as they failed to capture Péronne and Bapaume, where the German armies maintained their positions over the winter. British attacks in the Ancre valley resumed in January 1917 and forced the Germans into local withdrawals to reserve lines in February before the scheduled retirement by about 25 mi (40 km) in Operation Alberich to the Siegfriedstellung (Hindenburg Line) in March 1917. Debate continues over the necessity, significance and effect of the battle.

The Zimmermann Telegram was a secret diplomatic communication issued from the German Foreign Office in January 1917 that proposed a military alliance between Germany and Mexico if the United States entered World War I against Germany. Mexico would recover Texas, Arizona, and New Mexico. The telegram was intercepted and decoded by British intelligence. Revelation of the contents enraged Americans, especially after German Foreign Secretary Arthur Zimmermann publicly admitted on March 3 that the telegram was genuine. It helped to generate support for the American declaration of war on Germany in April. The decryption was described as the most significant intelligence triumph for Britain during World War I, and one of the earliest occasions on which a piece of signal intelligence influenced world events.

The United States entered into World War I in April 1917, more than two and a half years after the war began in Europe. Apart from an Anglophile element urging early support for the British and an anti-Tsarist element sympathizing with Germany's war against Russia, American public opinion had generally reflected a desire to stay out of the war: the sentiment for neutrality was particularly strong among Irish Americans, German Americans, and Scandinavian Americans, as well as among church leaders and women in general. On the other hand, even before World War I had broken out, American opinion had been overall more negative toward Germany than toward any other country in Europe. Over time, especially after reports of German atrocities in Belgium in 1914 and following the sinking of the passenger liner RMS Lusitania in 1915, Americans increasingly came to see Germany as the aggressor in Europe.

While the country was at peace, American banks made huge loans to the Entente powers, which were used mainly to buy munitions, raw materials, and food from across the Atlantic. Although Woodrow Wilson made minimal preparations for a land war before 1917, he did authorize a ship-building program for the United States Navy. The president was narrowly re-elected in 1916 on an anti-war platform.

Germany also made a secret offer to help Mexico regain territories lost in the Mexican–American War in an encoded telegram known as the Zimmermann Telegram, which was intercepted by British intelligence. Publication of that communique outraged Americans just as German submarines started sinking American merchant ships in the North Atlantic. Wilson then asked Congress for "a war to end all wars" that would "make the world safe for democracy", and Congress voted to declare war on Germany on April 6, 1917. U.S. troops began major combat operations on the Western Front under General John J. Pershing in the summer of 1918.

The 1917 French Army mutinies took place amongst French Army troops on the Western Front in Northern France during World War I. They started just after the unsuccessful and costly Second Battle of the Aisne, the main action in the Nivelle Offensive in April 1917. The new French commander of the armies in France, General Robert Nivelle had promised a decisive victory over the Germans in 48 hours; morale in French armies rose to a great height and the shock of failure soured their mood overnight.

The mutinies and associated disruptions involved, to various degrees, nearly half of the French infantry divisions stationed on the Western Front. The term "mutiny" does not precisely describe events; soldiers remained in trenches and were willing to defend but refused orders to attack. Nivelle was sacked and replaced by General Philippe Pétain, who restored morale by talking to the men, promising no more suicidal attacks, providing rest and leave for exhausted units and moderating discipline. He held 3,400 courts martial in which 554 mutineers were sentenced to death and 26 were executed.

The catalyst for the mutinies was the extreme optimism and dashed hopes of the Nivelle Offensive, pacifism (stimulated by the Russian Revolution and the trade union movement) and disappointment at the non-arrival of American troops. French soldiers on the front had unrealistically been expecting US troops to arrive within days of the U.S. declaration of war. The mutinies were kept secret from the Germans and their full extent was not revealed until decades later. The German failure to detect the mutinies has been described as one of the most serious intelligence failures of the war.

The Third Battle of Ypres, also known as the Battle of Passchendaele, was a campaign of the First World War, fought by the Allies against the German Empire. The battle took place on the Western Front, from July to November 1917, for control of the ridges south and east of the Belgian city of Ypres in West Flanders, as part of a strategy decided by the Allies at conferences in November 1916 and May 1917. Passchendaele lies on the last ridge east of Ypres, 5 mi (8.0 km) from Roulers (now Roeselare), a junction of the Bruges-(Brugge)-to-Kortrijk railway. The station at Roulers was on the main supply route of the German 4th Army. Once Passchendaele Ridge had been captured, the Allied advance was to continue to a line from Thourout (now Torhout) to Couckelaere (Koekelare).

The Battle of Caporetto (also known as the Twelfth Battle of the Isonzo, the Battle of Kobarid or the Battle of Karfreit) was a battle on the Italian front of World War I.

The battle was fought between the Kingdom of Italy and the Central Powers and took place from 24 October to 19 November 1917, near the town of Kobarid (now in north-western Slovenia, then part of the Austrian Littoral). The battle was named after the Italian name of the town (also known as Karfreit in German).

Austro-Hungarian forces, reinforced by German units, were able to break into the Italian front line and rout the Italian forces opposing them. The battle was a demonstration of the effectiveness of the use of stormtroopers and the infiltration tactics developed in part by Oskar von Hutier. The use of poison gas by the Germans also played a key role in the collapse of the Italian Second Army.

The October Revolution, also known as the Bolshevik Revolution, was a revolution in Russia led by the Bolshevik Party of Vladimir Lenin that was a key moment in the larger Russian Revolution of 1917–1923. It was the second revolutionary change of government in Russia in 1917. It took place through an armed insurrection in Petrograd (now Saint Petersburg) on 7 November 1917. It was the precipitating event of the Russian Civil War.

Events came to a head in the fall as the Directorate, led by the left-wing Socialist Revolutionary Party, controlled the government. The left-wing Bolsheviks were deeply unhappy with the government, and began spreading calls for a military uprising. On 10 October 1917, the Petrograd Soviet, led by Trotsky, voted to back a military uprising. On 24 October, the government shut down numerous newspapers and closed the city of Petrograd in an attempt to forestall the revolution; minor armed skirmishes broke out. The next day a full scale uprising erupted as a fleet of Bolshevik sailors entered the harbor and tens of thousands of soldiers rose up in support of the Bolsheviks. Bolshevik Red Guards forces under the Military-Revolutionary Committee began the occupation of government buildings on 25 October 1917. The following day, the Winter Palace was captured. As the Revolution was not universally recognized, the country descended into the Russian Civil War, which would last until 1923 and ultimately lead to the creation of the Soviet Union in late 1922.

The Battle of Cambrai was a British attack in the First World War, followed by the biggest German counter-attack against the British Expeditionary Force (BEF) since 1914. The town of Cambrai, in the département of Nord, was an important supply centre for the German Siegfriedstellung (known to the British as the Hindenburg Line) and capture of the town and the nearby Bourlon Ridge would threaten the rear of the German line to the north. Major General Henry Tudor, Commander, Royal Artillery (CRA), of the 9th (Scottish) Division, advocated the use of new artillery-infantry tactics on his sector of the front. During preparations, J. F. C. Fuller, a staff officer with the Tank Corps, looked for places to use tanks for raids. General Julian Byng, commander of the Third Army, decided to combine both plans. The French and British armies had used tanks en masse earlier in 1917, although to considerably less effect.

After a big British success on the first day, mechanical unreliability, German artillery and infantry defences exposed the frailties of the Mark IV tank. On the second day, only about half of the tanks were operational and British progress was limited. In the History of the Great War, the British official historian Wilfrid Miles and modern scholars do not place exclusive credit for the first day on tanks but discuss the concurrent evolution of artillery, infantry and tank methods. Numerous developments since 1915 matured at Cambrai, such as predicted artillery fire, sound ranging, infantry infiltration tactics, infantry-tank co-ordination and close air support. The techniques of industrial warfare continued to develop and played a vital part during the Hundred Days Offensive in 1918, along with replacement of the Mark IV tank with improved types. The rapid reinforcement and defence of Bourlon Ridge by the Germans, as well as their counter-attack, were also notable achievements, which gave the Germans hope that an offensive strategy could end the war before American mobilisation became overwhelming.

On 15 December 1917, an armistice was signed between the Russian Soviet Federative Socialist Republic on the one side and the Austro-Hungarian Empire, the Kingdom of Bulgaria, the German Empire and the Ottoman Empire—the Central Powers—on the other. The armistice took effect two days later, on 17 December. By this agreement Russia de facto exited World War I, although fighting would briefly resume before the Treaty of Brest-Litovsk was signed on 3 March 1918, and Russia made peace.

The German spring offensive, or Kaiserschlacht ("Kaiser's Battle"), also known as the Ludendorff offensive, was a series of German attacks along the Western Front during the First World War, beginning on 21 March 1918. Following American entry into the war in April 1917, the Germans had realised that their only remaining chance of victory was to defeat the Allies before the United States could ship soldiers across the Atlantic and fully deploy its resources. The German Army had gained a temporary advantage in numbers as nearly 50 divisions had been freed by the Russian defeat and withdrawal from the war with the Treaty of Brest-Litovsk.

There were four German offensives, codenamed Michael, Georgette, Gneisenau, and Blücher-Yorck. Michael was the main attack, which was intended to break through the Allied lines, outflank the British forces (which held the front from the Somme River to the English Channel) and defeat the British Army. Once that was achieved, it was hoped that the French would seek armistice terms. The other offensives were subsidiary to Michael and were designed to divert Allied forces from the main offensive effort on the Somme. No clear objective was established before the start of the offensives and once the operations were underway, the targets of the attacks were constantly changed according to the battlefield (tactical) situation.

Once they began advancing, the Germans struggled to maintain the momentum, partly due to logistical issues. The fast-moving stormtrooper units could not carry enough food and ammunition to sustain themselves for long, and the army could not move in supplies and reinforcements fast enough to assist them.

The German Army made the deepest advances either side had made on the Western Front since 1914. They re-took much ground that they had lost in 1916–17 and took some ground that they had not yet controlled. Despite these apparent successes, they suffered heavy casualties in return for land that was of little strategic value and hard to defend. The offensive failed to deliver a blow that could save Germany from defeat, which has led some historians to describe it as a pyrrhic victory.

The Hundred Days Offensive (8 August to 11 November 1918) was a series of massive Allied offensives which ended the First World War. Beginning with the Battle of Amiens (8–12 August) on the Western Front, the Allies pushed the Central Powers back, undoing their gains from the German spring offensive. The Germans retreated to the Hindenburg Line, but the Allies broke through the line with a series of victories, starting with the Battle of St Quentin Canal on 29 September. The offensive, together with a revolution breaking out in Germany, led to the Armistice of 11 November 1918 which ended the war with an Allied victory. The term "Hundred Days Offensive" does not refer to a battle or strategy, but rather the rapid series of Allied victories against which the German Army had no reply.

The Battle of Megiddo was fought between 19 and 25 September 1918, on the Plain of Sharon, in front of Tulkarm, Tabsor and Arara in the Judean Hills as well as on the Esdralon Plain at Nazareth, Afulah, Beisan, Jenin and Samakh. Its name, which has been described as "perhaps misleading" since very limited fighting took place near Tel Megiddo, was chosen by Allenby for its biblical and symbolic resonance.

The battle was the final Allied offensive of the Sinai and Palestine Campaign of the First World War. The contending forces were the Allied Egyptian Expeditionary Force, of three corps including one of mounted troops, and the Ottoman Yildirim Army Group which numbered three armies, each the strength of barely an Allied corps.

These battles resulted in many tens of thousands of prisoners and many miles of territory being captured by the Allies. Following the battles, Daraa was captured on 27 September, Damascus on 1 October and operations at Haritan, north of Aleppo, were still in progress when the Armistice of Mudros was signed ending hostilities between the Allies and Ottomans.

The operations of General Edmund Allenby, the British commander of the Egyptian Expeditionary Force, achieved decisive results at comparatively little cost, in contrast to many offensives during the First World War. Allenby achieved this through the use of creeping barrages to cover set-piece infantry attacks to break a state of trench warfare and then use his mobile forces (cavalry, armoured cars and aircraft) to encircle the Ottoman armies' positions in the Judean Hills, cutting off their lines of retreat.

The Armistice of 11 November 1918 was the armistice signed at Le Francport near Compiègne that ended fighting on land, sea and air in World War I between the Allies and their last remaining opponent, Germany. Previous armistices had been agreed with Bulgaria, the Ottoman Empire and Austria-Hungary. It was concluded after the German government sent a message to American president Woodrow Wilson to negotiate terms on the basis of a recent speech of his and the earlier declared "Fourteen Points", which later became the basis of the German surrender at the Paris Peace Conference, which took place the following year.

The actual terms, which were largely written by Foch, included the cessation of hostilities on the Western Front, the withdrawal of German forces from west of the Rhine, Allied occupation of the Rhineland and bridgeheads further east, the preservation of infrastructure, the surrender of aircraft, warships, and military materiel, the release of Allied prisoners of war and interned civilians, eventual reparations, no release of German prisoners and no relaxation of the naval blockade of Germany. The armistice was extended three times while negotiations continued on a peace treaty.

One of the most dramatic effects of the war was the expansion of governmental powers and responsibilities in Britain, France, the United States, and the Dominions of the British Empire. To harness all the power of their societies, governments created new ministries and powers. New taxes were levied and laws enacted, all designed to bolster the war effort; many have lasted to the present. Similarly, the war strained the abilities of some formerly large and bureaucratized governments, such as in Austria-Hungary and Germany.

Gross domestic product (GDP) increased for three Allies (Britain, Italy, and the United States), but decreased in France and Russia, in neutral Netherlands, and in the three main Central Powers. The shrinkage in GDP in Austria, Russia, France, and the Ottoman Empire ranged between 30% and 40%. In Austria, for example, most pigs were slaughtered, so at war's end there was no meat.

Macro- and micro-economic consequences devolved from the war. Families were altered by the departure of many men. With the death or absence of the primary wage earner, women were forced into the workforce in unprecedented numbers. At the same time, industry needed to replace the lost laborers sent to war. This aided the struggle for voting rights for women.

World War I further compounded the gender imbalance, adding to the phenomenon of surplus women. The deaths of nearly one million men during the war in Britain increased the gender gap by almost a million: from 670,000 to 1,700,000. The number of unmarried women seeking economic means grew dramatically. In addition, demobilisation and economic decline following the war caused high unemployment. The war increased female employment; however, the return of demobilized men displaced many from the workforce, as did the closure of many of the wartime factories.

The war contributed to the evolution of the wristwatch from women's jewelry to a practical everyday item, replacing the pocket watch, which requires a free hand to operate. Military funding of advancements in radio contributed to the post-war popularity of the medium.