Almost every country in the world participated in World War II. Most were neutral at the beginning, but only a relative few nations remained neutral to the end. The Second World War pitted two alliances against each other, the Axis powers and the Allied powers. It is estimated that 74 million people died, with estimates ranging from 40 million to 90 million dead. The leading Axis powers were Nazi Germany, the Empire of Japan and the Kingdom of Italy; while the British Empire, the United States, the Soviet Union and China were the "Big Four" Allied powers.

World War II or the Second World War was a global conflict between two major alliances: the Allies and the Axis powers. The vast majority of the world's countries, including all the great powers, fought as part of these military alliances. Many participating countries invested all available economic, industrial, and scientific capabilities into the 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.