World War I, originally widely called the World War or the Great War until 1939 and later known as the First World War or World War I, was a major war centered in Europe that began on 28 July 1914 and lasted until 11 November 1918. It involved all the world's great powers in two opposing alliances, the Allies and the Central Powers, which reorganized and expanded as more nations entered the conflict. More than 70 million military personnel, including 60 million Europeans, were mobilized, and more than 9 million combatants were killed in one of the largest and sixth-deadliest conflicts in world history. Long-term causes included the imperialistic foreign policies of Europe's great powers, and the proximate trigger was the assassination on 28 June 1914 of Archduke Franz Ferdinand of Austria in Sarajevo, Bosnia and Herzegovina, which led to a Habsburg ultimatum against the Kingdom of Serbia. Within weeks the major powers were at war, and through their colonies the conflict spread around the world. The fighting opened with the Austro-Hungarian invasion of Serbia, followed by the German invasion of Belgium, Luxembourg and France and a Russian attack against Germany. After the German march on Paris was halted, the Western Front became a static war of attrition with trench lines that changed little until 1917, while in the East the Russian army fought Austro-Hungarian forces but was forced back by the German army. Additional fronts opened as the Ottoman Empire joined in 1914, Italy and Bulgaria in 1915, and Romania in 1916. The Russian Empire collapsed in March 1917, and Russia left the war after the October Revolution later that year. Following a German offensive along the Western Front in 1918, United States forces entered the trenches and the Allies drove back the German armies in successful offensives. Germany, facing revolutionary unrest, agreed to a cease-fire on 11 November 1918, later known as Armistice Day, and the war ended in victory for the Allies. The home fronts were also tumultuous as participants mobilized manpower and economic resources for total war. By the end of the war, the German, Russian, Austro-Hungarian and Ottoman empires had ceased to exist, central Europe's map was redrawn into several smaller states, and the League of Nations was formed in hopes of preventing another such conflict. The war's nationalism, imperial breakups, the effects of Germany's defeat and problems with the Treaty of Versailles are generally agreed to have contributed to World War II.