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A war fought from 1914 to 1918 between the Allies, notably Britain, France, Russia, and Italy (which entered in 1915), and the Central Powers: Germany, Austria-Hungary, Bulgaria, and the Ottoman Empire. The war was sparked by the assassination in 1914 of the heir to the throne of Austria (see Sarajevo) (see also Sarajevo). Prolonged stalemates, trench warfare, and immense casualties on both sides marked the fighting. The United States sought to remain neutral but was outraged by the sinking of the Lusitania by a German submarine in 1915 and by Germany's decision in 1916 to start unrestricted submarine warfare. In 1917, the United States entered the war on the side of the Allies and helped to tip the balance in their favor. In full retreat on its western front, Germany asked for an armistice, or truce, which was granted on November 11, 1918. By the terms of the Treaty of Versailles, signed in 1919, Germany had to make extensive concessions to the Allies and pay large penalties. The government leaders of World War I included Georges Clemenceau of France, David Lloyd George of Britain, Kaiser Wilhelm II of Germany, and Woodrow Wilson of the United States. World War I was known as the Great War, or the World War, until World War II broke out. (See map, next page.)