For how long did World War II last?

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.