World War II was a global war that lasted from 1939 to 1945 (with the U.S. joining in 1941). It involved all of the world's great powers- eventually forming two opposing military alliances: the Allies and the Axis. In a state of total war, the major participants threw their entire economic, industrial, and scientific capabilities behind the war effort, erasing the distinction between civilian and military resources. WWII was the deadliest conflict in human history. Over 60 million people were killed, most of which were civilians. It included massacres, the deliberate genocide of the Holocaust, strategic bombing, starvation, disease and the first use of nuclear weapons in history. After the Allied victory in 1945, WWII has dramatically altered the political alignment and social structure of our world to this very day.