Sunday morning, December 7, 1941. The attack led to the United States' formal entry into World War II 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.
Japan intended the attack as a preventive action to keep the United States Pacific Fleet from interfering with its planned military actions in Southeast Asia against overseas territories of the United Kingdom, the Netherlands, and the United States. Over the course of seven hours there were coordinated Japanese attacks on the U.S.-held Philippines, Guam and Wake Island and on the British Empire in Malaya, Singapore, and Hong Kong.
The attack commenced at 7:48 a.m. Hawaiian Time (18:18 GMT).[nb 4] The base was attacked by 353 Imperial Japanese aircraft (including fighters, level and dive bombers, and torpedo bombers) in two waves, launched from six aircraft carriers. All eight U.S. Navy battleships were damaged, with four sunk. All but USS Arizona were later raised, and six were returned to service and went on to fight in the war. The Japanese also sank or damaged three cruisers, three destroyers, an anti-aircraft training ship,[nb 5] and one minelayer. 188 U.S. aircraft were destroyed; 2,403 Americans were killed and 1,178 others were wounded. Important base installations such as the power station, dry dock, shipyard, maintenance, and fuel and torpedo storage facilities, as well as the submarine piers and headquarters building (also home of the intelligence section) were not attacked. Japanese losses were light: 29 aircraft and five midget submarines lost, and 64 servicemen killed. One Japanese sailor, Kazuo Sakamaki, was captured.
Japan announced a declaration of war on the United States later that day (December 8 in Tokyo), but the declaration was not delivered until the following day. The following day, December 8, Congress declared war on Japan. On December 11, Germany and Italy each declared war on the U.S., which responded with a declaration of war against Germany and Italy.
There were numerous historical precedents for the unannounced military action by Japan, but the lack of any formal warning, particularly while peace negotiations were still apparently ongoing, led President Franklin D. Roosevelt to proclaim December 7, 1941, "a date which will live in infamy". Because the attack happened without a declaration of war and without explicit warning, the attack on Pearl Harbor was later judged in the Tokyo Trials to be a war crime.