World War I (WWI or WW1), also known as the First World War, or the Great War, was a global war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918. More than 70 million military personnel, including 60 million Europeans, were mobilised in one of the largest wars in history. Over 9 million combatants and 7 million civilians died as a result of the war (including the victims of a number of genocides), a casualty rate exacerbated by the belligerents' technological and industrial sophistication, and the tactical stalemate caused by trench warfare, a grueling form of warfare in which the defender held the advantage. It was one of the deadliest conflicts in history, and paved the way for major political changes, including revolutions in many of the nations involved.
The war drew in all the world’s economic great powers, assembled in two opposing alliances: the Allies (based on the Triple Entente of the United Kingdom/British Empire, France and the Russian Empire) and the Central Powers of Germany and Austria-Hungary. Although Italy had also been a member of the Triple Alliance alongside Germany and Austria-Hungary, it did not join the Central Powers, as Austria-Hungary had taken the offensive against the terms of the alliance. These alliances were reorganised and expanded as more nations entered the war: Italy, Japan and the United States joined the Allies, while the Ottoman Empire and Bulgaria joined the Central Powers.