<P> World War I (often abbreviated as WWI or WW1), 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 (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 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 . </P> <P> 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 Russian Empire, the French Third Republic, and the United Kingdom of Great Britain and Ireland) versus the Central Powers of Germany and Austria - Hungary . Although Italy was 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 . </P> <P> The trigger for the war was the assassination of Archduke Franz Ferdinand of Austria, heir to the throne of Austria - Hungary, by Yugoslav nationalist Gavrilo Princip in Sarajevo on 28 June 1914 . This set off a diplomatic crisis when Austria - Hungary delivered an ultimatum to the Kingdom of Serbia and, as a result, entangled international alliances formed over the previous decades were invoked . Within weeks the major powers were at war, and the conflict soon spread around the world . </P> <P> Russia was the first to order a partial mobilisation of its armies on 24--25 July, and when on 28 July Austria - Hungary declared war on Serbia, Russia declared general mobilisation on 30 July . Germany presented an ultimatum to Russia to demobilise, and when this was refused, declared war on Russia on 1 August . Being outnumbered on the Eastern Front, Russia urged its Triple Entente ally France to open up a second front in the west . </P>

Why did the first world war broke out