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Under Kaiser Wilhelm II, Germany sought a political and imperial role consonant with its industrial strength, challenging Britain's world supremacy and threatening France, which still resented the loss in 1871 of Alsace-Lorraine. Austria wanted to curb an expanding Serbia (after 1912) and the threat it posed to its own Slavic lands. Russia feared Austrian and German political and economic aims in the Balkans and Turkey. An accelerated arms race resulted. The German standing army rose to more than 2 million men by 1914. The Russian and the French armies numbered more than a million, while the Austrian and the British armies were close to a million. Dozens of enormous battleships were built by the powers after 1906.

The assassination of Austrian Archduke Ferdinand by a Serbian on June 28, 1914 was the pretext for war. The system of alliances made the conflict Europe-wide; Germany's invasion of Belgium to outflank France forced Britain to enter the war. Patriotic fervor spread to all classes in virtually all countries.

German forces were stopped in France in one month. The rival armies dug trench networks. Artillery and improved machine guns prevented either side from making any lasting advance, despite repeated assaults (600,000 died at Verdun between February to July 1916). The poison gas used by Germany in 1915 proved ineffective. The entrance of more than one million American troops tipped the balance after mid-1917, forcing German to sue for peace in 1918. The formal armistice was signed on November 11, 1918.

In the East, Russian armies were thrown back at the battle of Tannenberg on August 20, 1914. Thereafter, the war grew increasingly unpopular in Russia. An allied attempt to relieve Russia through Turkey failed. The Russian Revolution of 1917 abolished the czarist regime and the new Bolshevik government signed the capitulatory Treaty of Brest-Litovsk in March 1918. Italy entered the war on the allied side in May 1915, but was pushed back by October 1917. A renewed offensive in October and November 1918 forced Austria to surrender.

The British Navy successfully blockaded Germany, which responded with submarine U-boat attacks. Unrestricted submarine warfare against neutrals after January 1917 helped bring the United States into the war. Other battlefields included Palestine and Mesopotamia, both of which Britain wrested from the Turkish Empire in 1917. Most of the colonies Germany held in Africa and the Pacific fell to Britain, France, Australia, Japan, and South Africa.

From 1916 on, the civilian populations and economies of both sides were mobilized to an unprecedented degree. Hardships especially intensified among fighting nations in 1917. More than 10 million soldiers died in the war.

At the Paris Peace Conference in 1919, concluded by the Treaty of Versailles, and in subsequent negotiations and local conflicts, the map of Europe was redrawn with a nod to U.S. President Woodrow Wilson's principle of self-determination. The Austro-Hungarian empire was split up, and much of its territory was given to Yugoslavia (formerly Serbia), Romania, Italy, and the newly independent states of Poland, and Czechoslovakia. Germany lost territory in the West and East, while Finland and the Baltic States were detached from Russia. Turkey lost nearly all of its Arab land to British sponsored Arab States or to direct French and British rule. Belgium's sovereignty was recognized.

A huge burden of reparations and partial demilitarization were imposed on Germany. President Wilson obtained approval for a League of Nations, but the U.S. Senate refused to allow the U.S. to join.