World War II or the Second World War, often abbreviated as WWII or WW2, was a global war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all the great powers—forming two opposing military alliances: the Allies and the Axis powers. In a state of total war, directly involving more than 100 million personnel from more than 30 countries, the major participants threw their entire economic, industrial, and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. Aircraft played a major role in the conflict, including the strategic bombing of population centres, and, with the development of nuclear weapons, the only two uses of such in war. World War II was the deadliest conflict in human history, resulting in 70 to 85 million fatalities, with more civilians than military personnel killed. Tens of millions of people died due to genocides, starvation, massacres, and disease. In the wake of the war, Germany and Japan were occupied, and war crimes tribunals were conducted against German and Japanese leaders.