World War I

World War I was a military conflict that lasted from 1914 to 1918 and involved most of the world's great powers,^[1] assembled in two opposing alliances: the Allies (centred around the Triple Entente) and the Central Powers.^[2] More than 70 million military personnel, including 60 million Europeans, were mobilized in one of the largest wars in history.^[3][4] More than 15 million people were killed, making it one of the deadliest conflicts in history.^[5] This war (abbreviated as WW-I, WWI, or WW1) is also known as the First World War, the Great War, the World War (prior to the outbreak of the Second World War), and the War to End All Wars.

The assassination on 28 June 1914 of Archduke Franz Ferdinand of Austria, the heir to the throne of Austria-Hungary, is seen as the immediate trigger of the war, though long-term causes, such as imperialistic foreign policy, played a major role. Ferdinand's assassination at the hands of a Bosnian Serb nationalist Gavrilo Princip resulted in Habsburg ultimatum against the Kingdom of Serbia.^[6] Several alliances that had been formed over the past decades were invoked, so within weeks the major powers were at war; with all having colonies, the conflict soon spread around the world.

The conflict opened with the German invasion of Belgium, Luxembourg and France; the Austro-Hungarian invasion of Serbia and a Russian attack against Prussia. 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 were forced back by the German army. Additional fronts opened with the Ottoman Empire joining the war in 1914, Italy in 1915 and Romania in 1916. Imperial Russia left the war in 1917. After a 1918 German offensive along the western front, American forces entered the trenches and the German armies were driven back in a series of successful allied offensives. Germany surrendered on Armistice Day, November 11, 1918.

By the war's end, four major imperial powers—the German, Russian, Austro-Hungarian and Ottoman Empires—had been militarily and politically defeated, with the last two ceasing to exist.^[7] The revolutionized Soviet Union emerged from the Russian Empire, while the map of central Europe was completely redrawn into numerous smaller states.^[8] 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 of the Treaty of Versailles would eventually lead to the beginning of World War II in 1939.^[9]