World War II (WWII or WW2), also known as the Second
World War, was a global war that was underway by 1939
and ended in 1945. 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, with more than 100 million
people serving in military units.