World War II was the most extensive and costly armed conflict in the history of the world, involving the great majority of the world’s nations, being fought simultaneously in several major theaters, and costing approximately 50 million lives. The war was fought mainly between an alliance of the British Commonwealth, France, the United States, the Soviet Union, and China—collectively known as the Allies; and the Axis Powers, an alliance between Germany, Italy, and Japan. Most of the fighting occurred in the Atlantic theater in and around Europe, and in the Pacific theater in the Pacific and East Asia.

The war was significant in that it was the first war in which air power was a significant factor. Indeed, the first combat operation in World War II was a German bombing attack against Poland, while the last combat operation was the American atom bomb attack against Nagasaki. The war also saw the re-emergence of the United States from its isolationism, the destruction and rebuilding of Germany and Japan into major industrial powers, and the emergence of the United States and the Soviet Union as global superpowers. The war also directly led to the United Nations, which was founded by the victorious Allies in order to prevent such a large and destructive conflict from ever again happening.