World War II lasted for.... Approximately 6 years over-all. Four years for the U.S. World War II OFFICIALLY began when Britain and France declared war on Germany on September 3rd, 1939. It began for the U.S. When the U.S. Declared war on Japan on December 7th, 1941. Shortly after Hitler needlessly declared war on the U.S.--starting the war in the European Theater for Americans. The War ended in the European Theater on May 8, 1945. In the Pacific Theater it ended on December ?15?, 1945. I'm not sure of the exact day of December--I don't study Pacific Theater myself. I study European theater very extensively, however.