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, organised into 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 action against civilians, including the Holocaust and the only use of nuclear weapons in warfare, it was the deadliest conflict in human history. Estimates range from fifty million to over seventy million fatalities. The war ended with the total victory of the Allies over Germany and Japan in 1945.

The war was fought between the Allies -- the United States, the United Kingdom, British Commonwealth forces and the Soviet Union -- against the Axis powers: Germany, Italy and Japan. The Allies were supported militarily by communist-led resistance movements throughout Europe and the Far East. British wartime leader Winston Churchill, in his voluminous history of World War II, depicts a generally cordial relationship between the Western allies and their Soviet allay. Documents declassified after the war provide a different perspective. In secret wartime correspondence between Soviet leader Josef Stalin and Churchill, Stalin complained repeatedly that by land, sea and air, the Western allies were failing to use their military forces to good effect while, as a result, the Soviet Union suffered appalling losses on the eastern or Russian-German front. Russian historians contend that the Eastern Front was the principal and decisive front of the war. Revisionist historians propose that, because of the wartime tensions that existed between Stalin and the Western leaders, the roots of the Cold War can be traced to events in World War II.

The end of World War II brought profound changes in international relations and political alliances. The League of Nations was replaced by the United Nationswith the intention of fostering international cooperation and prevent future conflicts. The Bretton Woods Agreement was signed to reform international financial institutions, and tariffs and trade, which were identified as having been among the main economic factors that had led to World War II. The Soviet Union and the United States emerged as rival superpowers in a nuclear arms race, while Western Europe began moving toward economic recovery and political integration.