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.

The First World War saw the Entente Powers, led by France, Russia, the British Empire, and later Italy (from 1915) and the United States (from 1917), defeat the Central Powers, led by the German, Austro-Hungarian, Bulgarian and Ottoman Empires. Russia withdrew from the war after the revolution in 1917.

Fighting took place throughout Europe, the Middle East, Africa, the Pacific, and parts of Asia. One of the deadliest wars in history, it resulted in an estimated 9 million soldiers dead and 23 million wounded, plus up to 8 million civilian deaths from numerous causes including genocide.

For four years, from 1914 to 1918, World War I raged across Europe's western and eastern fronts after growing tensions and then the assassination of Archduke Franz Ferdinand of Austria ignited the war.