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World War I (abbreviated as WW-I, WWI, or WW1), also known as the First World War, the Great War, and the War to End All Wars, was a global military conflict that embroiled most of the world's great powers, assembled in two opposing alliances: the Triple Entente and the Triple Alliance. Over 70 million military personnel were mobilized in one of the largest wars in history. The main combatants descended into a state of total war, pumping their entire scientific and industrial capabilities into the war effort. Over 15 million people were killed, making it one of the deadliest conflicts in history.
The immediate or proximate cause of war was the assassination on 28 June 1914 of Archduke Franz Ferdinand of Austria, heir to the Austro-Hungarian throne, by Gavrilo Princip, a Serbian nationalist. Austria-Hungary's resulting demands against the Kingdom of Serbia activated a sequence of alliances. Within weeks the major European powers were at war; their global empires meant that the conflict soon spread worldwide.
By the war's end, four major imperial powers—Germany, Russia, Austria-Hungary, and the Ottoman Empire—had been militarily and politically defeated, with the latter two ceasing to exist as autonomous entities. The revolutionized Soviet Union emerged from the Russian Empire, while the map of central Europe was completely redrawn into numerous smaller states. The League of Nations was formed in the hope of preventing another such conflict. The European nationalism spawned by the war, the repercussions of Germany's defeat, and the Treaty of Versailles would eventually lead to the beginning of World War II in 1939.