The attack on Pearl Harbor was a surprise military strike by the Empire of Japan on the United States Pacific Fleet at its naval base at Pearl Harbor on Oahu, Hawaii, on December 7, 1941. At the time, the U.S. was a neutral country in World War II. The air raid on Pearl Harbor, which was launched from aircraft carriers, resulted in the U.S. entering the war on the side of the Allies on the day following the attack. The Japanese military leadership referred to the attack as the Hawaii Operation and Operation AI, and as Operation Z during its planning.