World War II was a global war that lasted from 1939 to 1945. It began as the joining of what had initially been two separate conflicts, with the first beginning in Asia in 1937 (the Second Sino-Japanese War) and the other beginning in Europe in 1939 (the German and Soviet invasion of Poland).

The war split the majority of the world's nations into two opposing military alliances: the Allies and the Axis. It involved the mobilization of over 100 million military personnel, making it the most widespread war in history, and placed the participants in a state of 'total war', which erased the distinction between civil and military resources and resulted in the complete activation of a nation's economic, industrial, and scientific capabilities for the purposes of the war effort. Over 70 million people, the majority of them civilians, were killed, making it the deadliest conflict in human history.

The Allies won the war, and as a result, the Soviet Union and the United States emerged as the world's leading superpowers. This set the stage for the Cold War, which lasted for the next 45 years. The United Nations was formed in the hope of preventing another such conflict. The self-determination spawned by the war accelerated decolonization movements in Asia and Africa, while Europe itself began moving toward integration.