The United States declared war on Germany on April 6, 1917, and entered the Great War (which was not called World War I until after the start of World War II in 1939). World War I was fought between 1914 and 1918, with thirty countries formally involved. The Central Powers included Germany, Austria-Hungary, Bulgaria, the Ottoman Empire, and Italy (part of the Triple Alliance, but Italy declared neutrality in 1914). The Allied Powers, also known as the Entente Powers, included the United Kingdom, France, Russia (who left in 1917 due to civil war), Italy (joined in 1915), Romania, Canada, Japan, and the United States (joined in 1917). More than sixteen million people were killed during the conflict, which primarily took place in central Europe where trench warfare dominated military strategy. WWI also served as a political lesson on the dangers of secret alliances that embroiled the European powers following the assassination of the heir to the Austro-Hungarian Empire by Serbian nationalists.