The Empire of Japan launched a surprise military attack on the US Pacific Fleet at its naval base at Pearl Harbor in Oahu, Hawaii, on December 7, 1941. At the time, the US was a neutral country in World War II. The air raid on Pearl Harbor, which was launched from aircraft carriers, prompted the US to declare war on Japan the next day. The Japanese military leadership referred to the attack as the Hawaii Operation and Operation AI, and as Operation Z during its planning.