World War II, or the Second World War (often abbreviated as WWII or WW2), was a global war that was underway by 1939 and ended in 1945. It involved a 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, with more than 100 million people serving in military units. In a state of "total war", the major participants placed their entire economic, industrial, and scientific capabilities at the service of the war effort, erasing the distinction between civilian and military resources.
This war was marked by significant events involving the mass death of civilians, including the Holocaust and the only use of nuclear weapons in warfare, and it resulted in 50 million to over 73 million fatalities. These deaths make World War II by far the deadliest conflict in all of human history