World War II lasted six years in Europe and eight years in the Pacific. American involvement in the war lasted four years from December 1941, when Japan attacked Pearl Harbor, to 1945, when both Japan and Germany were defeated.
Germany's invasion of Poland in September 1939 sparked the war in Europe. The war began in the Pacific in July 1937 when Japan invaded Manchuria. After being surrounded by Soviet forces, Germany surrendered in May 1945, bringing an end to the war in the European theater. In Asia, the war came to an end in September 1945 with Japan's surrender after the U.S. detonation of two atomic bombs over the cities of Hiroshima and Nagasaki.
The war pitted the Axis Powers – Germany, Italy, Japan, Bulgaria, Hungary and Romania – against the Allied Powers – Great Britain, France, the Soviet Union, China, the United States and other nations. The most destructive war in history, civilian and military casualties totalled an estimated 50 million. The Allied victory sparked the creation of the United Nations, the emergence of the United States and Soviet Union as world powers and the eventual Cold War between the two.