World War I began in 1914 and involved many of the world's leading military and industrial powers. The United States entered the war in 1917 after years of economic assistance to the Allied powers and an incident in which a German submarine sank an American passenger ship. The U.S. mobilized its economy and propaganda to support the war effort and push for American intervention against the Central Powers. After over three years of costly war, an armistice in 1918 brought the conflict to an end.