World War II or the Second World War (often abbreviated as WWII or WW2), was a global military conflict lasting from 1939 to 1945, which involved most of the world's nations, including all of the great powers: eventually forming two opposing military alliances, the Allies and the Axis. It was the most widespread war in history, with more than 100 million military personnel mobilised. In a state of "total war," the major participants placed their entire economic, industrial, and scientific capabilities at the service of the war effort, erasing the distinction between civilian and military resources. Marked by significant events involving the mass death of civilians, including the Holocaust and the only use of nuclear weapons in warfare, it was the deadliest conflict in human history, resulting in 50 million to over 70 million fatalities.
The war ended with the total victory of the Allies over Germany and Japan in 1945. World War II altered the political alignment and social structure of the world. The United Nations (UN) was established to foster international cooperation and prevent future conflicts (such as World War III). The Soviet Union and the United States emerged as rival superpowers, setting the stage for the Cold War, which would last for the next 46 years. Meanwhile, the influence of European great powers started to decline, while the decolonization of Asia and Africa began. Most countries whose industries had been damaged moved towards economic recovery. Political integration emerged as an effort to stabilise postwar relations.