World War II (often abbreviated to WWII or WW2), also known as the Second World War, was a global war that lasted from 1939 to 1945, although related conflicts began earlier. It involved the vast majority of the world's countries—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 over 30 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.
World War II was the deadliest conflict in human history, marked by 50 million to 85 million fatalities, most of which were civilians in the Soviet Union and China. It included massacres, the deliberate genocide of the Holocaust, strategic bombing, starvation, disease and the first use of nuclear weapons in history.