when did the u.s. join world war 2?
The United States entered World War II after the Japanese bombed the country's navy in Pearl Harbor on December 7, 1941. On December 7, 1941, the United States became active in World War II. The war had originally begun between countries in 1939 (Germany struck Poland), but after the Japanese had attacked the fleet in Pearl Harbor, Hawaii, the U.S. officially declared war on Japan a day later (December 8, 1941).