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World War I (WWI) or the First World War, formerly called the Great War, was a major war centred on Europe that began in the summer of 1914 and lasted until November 1918. It involved all of the world's great powers,[4] assembled in two opposing alliances: the Allies (centred around the Triple Entente) and the Central Powers.[5] More than 70 million military personnel, including 60 million Europeans, were mobilised in one of the largest wars in history.[6][7] More than 9 million combatants were killed, largely because of great technological advances in firepower without corresponding advances in mobility. It was the second deadliest conflict in Western history.[8]

The assassination on 28 June 1914 of Archduke Franz Ferdinand of Austria, the heir to the throne of Austria-Hungary, was the proximate trigger of the war. Long-term causes, such as imperialistic foreign policies of the great powers of Europe, such as the German Empire, the Austro-Hungarian Empire, the Ottoman Empire, the Russian Empire, the British Empire, France, and Italy, played a major role. Ferdinand's assassination by a Yugoslav nationalist resulted in a Habsburg ultimatum against the Kingdom of Serbia.[9][10] Several alliances formed over the past decades were invoked, so within weeks the major powers were at war; via their colonies, the conflict soon spread around the world.

On 28 July, the conflict opened with the Austro-Hungarian invasion of Serbia,[11][12] followed by the German invasion of Belgium, Luxembourg and France; and a Russian attack against Germany. After the German march on Paris was brought to a halt, the Western Front settled into a static battle of attrition with a trench line that changed little until 1917. In the East, the Russian army successfully fought against the Austro-Hungarian forces but was forced back by the German army. Additional fronts opened after the Ottoman Empire joined the war in 1914, Italy and Bulgaria in 1915 and Romania in 1916. The Russian Empire collapsed in 1917, and Russia left the war after the October Revolution later that year. After a 1918 German offensive along the western front, United States forces entered the trenches and the Allies drove back the German armies in a series of successful offensives. Germany agreed to a cease-fire on 11 November 1918, later known as Armistice Day.

By the war's end, four major imperial powers—the German, Russian, Austro-Hungarian and Ottoman Empires—had been militarily and politically defeated. The latter two ceased to exist.[13] The revolutionised Soviet Union emerged from the Russian Empire, while the map of central Europe was completely redrawn into several smaller states.[14] The League of Nations was formed in the hope of preventing another such conflict. The European nationalism spawned by the war and the breakup of empires, and the repercussions of Germany's defeat and the Treaty of Versailles led to the beginning of World War II in 1939.