On December 7, 1941, the Empire of Japan launched a surprise military strike on the United States Pacific Fleet at its naval base at Pearl Harbor on Oahu, Hawaii. 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, prompted the U.S. 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.