Pearl Harboris alagoonharboron the island ofOahu,Hawaii, west ofHonolulu. Much of the harbor and surrounding lands is aUnited States Navydeep-water naval base. It is also the headquarters of theUnited States Pacific Fleet. The U.S. government first obtained exclusive use of the inlet and the right to maintain a repair and coaling station for ships here in 1887.Theattack on Pearl Harborby theEmpire of Japanon December 7, 1941 was the immediate cause of theUnited States' entry intoWorld War II.