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.

A world war is an international conflict that involves most or all of the world's major powers. Conventionally, the term is reserved for two major international conflicts that occurred during the first half of the 20th century, World War I (1914–1918) and World War II (1939–1945), although some historians have also characterised other global conflicts as world wars, such as the Nine Years' War, the War of the Spanish Succession, the Seven Years' War, the French Revolutionary and Napoleonic Wars, the Cold War, and the War on Terror.