The Great War of 1914–1919 was a nearly global conflagration that included all the major powers of Europe, their colonies, and overseas allies. The immediate provocation was a relatively minor incident—the assassination of the heir to the Austro-Hungarian Empire—but the causes were long-standing and much more complex. Pressure to seek war and resist compromise had been mounting in the late nineteenth and early twentieth centuries, fed by aggressive nationalism, ambitious militarism, and complex national alliances. The war, when it came, was not what anyone expected.
New kind of warfare. New technologies transformed the experience of war. Offensive battle plans stalled in the trenches, where soldiers were pounded by heavy artillery, trapped by machine-gun fire, and vulnerable to poisonous gas. Casualties were counted in the hundreds of thousands, and progress was measured in yards gained.
Total war. World War I engaged civilian populations to an unprecedented degree. On the home front, women took up the work abandoned by recruits. Governments took control of wartime production, and propaganda campaigns demonized the enemy and glorified the war effort. Civilians were also targets of war through aerial bombing and naval blockades.
The Russian Revolution. The revolution was triggered by the war but sprang from the long-standing failure of the tsarist government to meet the needs of the Russian people. For a while it seemed that a liberal democracy might emerge, but within months the Bolshevik Party under the direction of Lenin overthrew the provisional government.
Peace and unresolved questions. Armistice came in 1918, shortly after the United States entered the war. At the Paris Peace Conference, the victors, especially Britain and France, dictated harsh terms to the defeated Central Powers, dismantled their colonial empires, and imposed economic penalties. The bitterness engendered by the peace settlement virtually ensured that another conflict would follow.