World War II (WWII or WW2), also known as the Second World War, was a global war that lasted from 1939 to 1945, though related conflicts began earlier. It involved the vast majority of the world's nations-including all of the great powers-eventually forming two opposing military alliances: the Allies and the Axis.
It was the most widespread war in history, and directly involved more than 100 million people, from more than 30 different countries. 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. Marked by mass deaths of civilians, including the Holocaust, the Three Alls Policy, the strategic bombing of enemy industrial and/or population centers, and the first use of nuclear weapons in combat, it resulted in an estimated 50 million to 85 million fatalities. These made World War II the deadliest conflict in human history.