World War I, also known as the First World War or the Great War, was a global war originating in Europe that lasted from 28 July 1914 to 11 November 1918. Contemporaneously described as the "War to End All Wars", more than 70 million military personnel, including 60 million Europeans, were mobilised in one of the largest wars in history. Over nine million combatants and seven million civilians died as a result of the war, a casualty rate exacerbated by the belligerents' technological and industrial sophistication, and the tactical stalemate caused by gruelling trench warfare. It was one of the deadliest conflicts in history and precipitated major political change, including the Revolutions of 1917–1923 in many of the nations involved. Unresolved rivalries at the end of the conflict contributed to the start of the Second World War twenty-one years later.
The war drew in all the world's economic great powers, assembled in two opposing alliances: the Allies versus the Central Powers of Germany and Austria-Hungary.