In 1914 a Europe primed for hostility erupted into World War I. One cause of the war was the nationalist aspirations among Austria's Slavic peoples. These aspirations were further enflamed by the Pan-Serbism of Serbia and Pan-Slavism of Russia, ideological aims that poisoned relations between these nations and Austria. Another cause was the German alliance system. Originally conceived by Bismarck to isolate France and maintain peace to Germany's benefit, the system deteriorated under William II and his ministers. Under these leaders, Germany backed Austria unconditionally and allowed a crucial treaty with Russia to lapse. Fear of Germany compelled Britain, France, and Russia to put aside their differences and, under French diplomatic leadership, form the Triple Entente to counter Germany's Triple Alliance. The Bosnian Crisis and Balkan War of 1912 exacerbated tensions between Austria, Serbia and Russia. After the assassination of Archduke Francis Ferdinand, Austria, with German support, forced a military confrontation with Serbia. Hoping to recover lost prestige, Russia backed Serbia.

Locked into opposing alliances and standing battle plans, the Great Powers quickly declared war on each other and mobilized. Stirred by nationalist passions and an aesthetic vision of violence, most Europeans welcomed the war. Germany attacked France via Belgium, drawing Britain into the war. After the initial bloody clashes, the fighting settled into the costly stalemate of trench warfare. In the east, Germany defeated Russia but not decisively enough to knock it from the war. Russia continued fighting until 1918 when, exhausted and wracked by revolution, it sued for peace. Meanwhile, Britain fought a disastrous campaign against the Ottoman Empire at Gallipoli; Italy was defeated by Austria and Germany at Caporetto; and the U.S. was drawn into the war by Germany's declaration of unrestrained submarine warfare. Germany's final offensive in 1918 failed to crush Allied resistance before the arrival of American troops. After the collapse of the Kaiser, the new German republic signed an armistice.

The resulting peace was shaped by Woodrow Wilson's liberal Fourteen Points and European-mainly French-resistance to them. The Peace of Paris redrew the map of Europe according to the principle of self-determination, but that principle was thwarted enough to leave many nationalists unsatisfied. For example, Czechoslovakia was formed from the breakup of Austria, but Sudeten Germans were prevented from unifying with Germany. Moreover, the Treaty of Versailles imposed economic and military conditions on Germany that left it resentful, its nationalist feelings primed for a spark to ignite them again.

World War I occasioned the Russian Revolution of 1917. Weakened by the war, the tsarist state collapsed in March. In its place rose a liberal provisional government that chose disastrously to continue the war. Unable to control social chaos and economic crisis, the government under Kerensky lost mass support. In November, the Bolsheviks under Lenin seized power. Manned by committed professional revolutionaries like Trotsky and Stalin, the Bolsheviks enacted Lenin's revision of Marxist doctrine. In January 1918, after failing to win a majority in an election, the Bolsheviks dissolved the Constituent Assembly. In March, Lenin accepted the Treaty of Brest-Litovsk with Germany, freeing the Communists to concentrate on rebuilding Russia.

World War I deeply scarred European consciousness. The war's mechanized efficiency destroyed confidence in scientific progress, and its brutality undermined faith in liberal Enlightenment values. The apparent failure of those values made Europeans all the more susceptible to the call of fascism in the 1920s and 1930s. Further, veterans accustomed to violence and yearning for wartime camaraderie offered a pool of willing recruits to leaders such as Mussolini and Hitler. Unable to recover from the effects of total war, Europe was fertile ground for totalitarianism and a second world war.