The attack on Pearl Harbor (or Hawaii Operation, Operation Z, as it was called by the Japanese Imperial General Headquarters) was an unannounced military strike conducted by the Japanese navy against the United States naval base at Pearl Harbor, Hawaii on the morning of December 7, 1941. It resulted in the United States entry into World War II. The attack was intended as a preventive action to keep the U.S. Pacific Fleet from influencing the war the Empire of Japan was planning in Southeast Asia against Britain and the Netherlands, as well as the U.S. in the Philippines. The attack consisted of two aerial attack waves totaling 353 aircraft, launched from six Japanese aircraft carriers.
The attack sank four U.S. Navy battleships (two of which were raised and returned to service later in the war) and damaged four more. The Japanese also sank or damaged three cruisers, three destroyers, and one minelayer, destroyed 188 aircraft, and caused personnel losses of 2,402 killed and 1,282 wounded. The power station, 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 hit. Japanese losses were minimal, with 29 aircraft and five midget submarines lost, and 65 servicemen killed or wounded. One Japanese sailor was captured.
The attack was a major engagement of World War II. It took place before a formal declaration of war by Japan and before the last part of a 14-part message had been delivered to the State Department in Washington, D.C. The Japanese Embassy in Washington had been instructed to deliver it immediately prior to the scheduled time of the attack in Hawaii. The attack, and especially its surprise nature, were both factors which swayed U.S. public opinion from isolationism to support for direct participation in the war. Germany’s prompt declaration of war, unforced by any treaty commitment to Japan, quickly brought the United States into the European Theater as well. Despite numerous historical precedents of unannounced military action, the lack of any formal declaration prior to the attack led President Franklin D. Roosevelt to proclaim “December 7th, 1941 — a date which will live in infamy“. [via]