World War I (WWI or WW1), also known as the First World War, was a global war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918. From the time of its occurrence until the approach of World War II in 1939, it was called simply the World War or the Great War, and thereafter the First World War or World War I.[5][6][7] In America it was initially called theEuropean War.[8] More than 9 million combatants were killed; a casualty rate exacerbated by the belligerents' technological and industrial sophistication, and tactical stalemate. It was the fifth-deadliest conflict in world history, paving the way for major political changes, including revolutions in many of the nations involved.[9]
The war drew in all the world's economic great powers,[10] which were assembled in two opposing alliances: the Allies (based on theTriple Entente of the United Kingdom, 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.[11] These alliances were both reorganised and expanded as more nations entered the war: Italy, Japan and the United States joined the Allies, and the Ottoman Empire andBulgaria the Central Powers. Ultimately, more than 70 million military personnel, including 60 million Europeans, were mobilised in one of the largest wars in history.[12][13]

Although a resurgence of imperialism was an underlying cause, the immediate trigger for war was the 28 June 1914 assassination of Archduke Franz Ferdinand of Austria, heir to the throne of Austria-Hungary, by Yugoslav nationalist Gavrilo Princip in Sarajevo. This set off a diplomatic crisis when Austria-Hungary delivered an ultimatum to the Kingdom of