World War I 1914-18 also called First World War, or Great War, an international conflict that in 1914-18 embroiled most of the nations of Europe along with Russia, the United States, the Middle East, and other regions. The war pitted the Central Powers--mainly Germany, Austria-Hungary, and Turkey--against the Allies--mainly France, Great Britain, Russia, Italy, Japan, and, from 1917, the United States. It ended with the defeat of the Central Powers.

By 1910 the major nations of Europe had aligned themselves into two potentially hostile alliances, with Germany and Austria in one and France, Great Britain, and Russia in the other. When a Serbian nationalist assassinated Archduke Francis Ferdinand of Austria at Sarajevo on June 28, 1914, a chain of threats, ultimatums, and mobilizations was set in motion that resulted in a general war between these two alliances by mid-August.

Germany had long been prepared to fight a land war on two fronts--i.e., against France on the west and against Russia on the east. In the west its armies outflanked France's main defensive forces and swept westward through Belgium, thereby bringing Great Britain into the war by treaty obligation. The German armies then turned south toward Paris. The French, reinforced by a British Expeditionary Force, managed to stabilize their defensive lines by November along the Aisne River, thereby saving Paris, but this meant that the rest of the war in that theatre was fought on French territory. Because of the tremendous firepower of modern artillery and machine guns, the war quickly evolved into one of attrition fought from lines of trenches. Frontal infantry assaults typically gained ground only by yards, and these attacks, whether successful or not, were enormously costly in human life. A deadlock soon ensued on the Western Front that could not be broken even by the enormous battles of the Somme and Verdun (both 1916) or by the massive German offensives of early 1918.

In the east an early Russian offensive in 1914 drove deep into East Prussia, German Poland, and Galicia, but the Russians were stopped by German and Austrian forces by the end of the year, and, in a startling German offensive begun in May 1915, they were thrown back into their own territory. Though it mounted several more offensives and suffered enormous casualties, the Russian army proved unable either to break through the German defensive lines or to take any German territory.

Other fronts in the war were to a greater or lesser extent peripheral to the main theatres but were nonetheless bloody. They included Gallipoli and the Dardanelles, where Britain unsuccessfully attempted to invade Turkey proper; the Caucasus and Persia, where Russia and Turkey fought; Mesopotamia and Egypt, where British forces (and, in Egypt, the Arabs organized by T.E. Lawrence) fought the Turks; and the Isonzo valley northwest of Trieste, where Italian and Austrian troops fought a long series of costly battles.

At sea only Germany and Great Britain had substantial fleets. Britain attempted, with considerable success, to blockade Germany and cut off its maritime access to food and raw materials from overseas. In response Germany turned to one of its newest weapons, the submarine, to interrupt the maritime supply lines of the British Isles. Germany's policy of unrestricted submarine warfare, however, which led to the sinking of much neutral shipping, ultimately persuaded the United States to enter the war against Germany in 1917. The major naval engagement of the war--indeed, one of the largest naval battles in history--was the inconclusive Battle of Jutland fought between the British Grand Fleet and the German High Seas Fleet in May 1916.

Russia's poor performance in the war and its grievous losses inspired widespread domestic discontent that led to the overthrow of the Russian monarchy in early 1917 and to the Bolshevik Revolution in November of that year. At the Bolshevik leader Lenin's order, Russia unilaterally ceased hostilities on November 26 and a month later signed a formal armistice with Germany, thus withdrawing from participation in the war. The release of German forces in the east for service on the deadlocked Western Front, however, was offset by the arrival of U.S. troops in France. Used tentatively at first, the rapidly reinforced American forces--1,200,000 by September 1918--soon proved their worth.

By autumn 1918 the position of the Central Powers had deteriorated rapidly. The Austro-Hungarian Empire, shaken by military defeats and by nationalist uprisings encouraged by the Russian Revolution, virtually disintegrated during October. Germany's great offensives on the Western Front during April-July failed, and the Allied forces then began a steady advance that recovered almost all of German-occupied France and part of Belgium by October 1918. German military and civilian morale thereupon collapsed, and amid widespread political unrest the German kaiser William II abdicated on November 9. Two days later an Armistice was signed between Germany and the Allies at Rethondes, Fr., thus ending World War I.