Also called the Great War, World War I was a global war originating in Europe that lasted from 1914 to 1918. The world’s great powers assembled in two opposing alliances: the Allies and the Central Powers. Because of technological and industrial sophistication, combat was horribly gruesome and Europe was stuck in a tactical stalemate for much of the conflict. The U.S. entered the war in 1917 on the side of the Allies and helped to defeat the Central powers. Overall, WWI resulted in over 41 million casualties, created major political changes, and left unresolved rivalries which ultimately contributed to the start of the Second World War.