World War II or the
Second World War was a global conflict between two alliances: the Allies and the Axis powers. Nearly all of the world's countries, including all of the great powers, participated in the conflict, and many invested all available economic, industrial, and scientific capabilities in pursuit of total war, blurring the distinction between civilian and military resources. Aircraft played a major role, enabling the strategic bombing of population centres and delivery of the only two nuclear weapons ever used in war. It was by far the deadliest conflict in history, resulting in 70–85 million fatalities. Millions died due to genocides, including the Holocaust, as well as starvation, massacres, and disease. In the wake of Axis defeat, Germany, Austria, and Japan were occupied, and war crime tribunals were conducted against German and Japanese leaders.