The United States halted shipments of airplanes, parts, machine tools, and aviation gasoline to Japan, which the latter perceived as an unfriendly act. In 1940, Japan invaded French Indochina, attempting to stymie the flow of supplies reaching China. In 1938, following an appeal by President Roosevelt, US companies stopped providing Japan with implements of war. The US unsuccessfully proposed a joint action with the British to blockade Japan. Starting in December 1937, events such as the Japanese attack on USS Panay, the Allison incident, and the Nanking Massacre swung Western public opinion sharply against Japan. Pearl Harbor on October 30, 1941, looking southwest. The ' Southern Operation' was designed to assist these efforts. Japan spent considerable effort trying to isolate China and endeavored to secure enough independent resources to attain victory on the mainland. Over the next decade, Japan expanded into China, leading to the Second Sino-Japanese War in 1937. Tensions did not seriously grow until Japan's invasion of Manchuria in 1931. Īlthough Japan had begun to take a hostile policy against the United States after the rejection of the Racial Equality Proposal, the relationship between the two countries was cordial enough that they remained trading partners. Japan had been wary of American territorial and military expansion in the Pacific and Asia since the late 1890s, followed by the annexation of islands, such as Hawaii and the Philippines, which they felt were close to or within their sphere of influence. War between Japan and the United States had been a possibility that each nation had been aware of, and planned for, since the 1920s.

Main article: Events leading to the attack on Pearl Harbor Diplomatic background 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. Roosevelt to proclaim December 7, 1941, ' a date which will live in infamy'. There were numerous historical precedents for the unannounced military action by Japan, but the lack of any formal warning (required by part III of the Hague Convention of 1907), particularly while peace negotiations were still apparently ongoing, led President Franklin D. On December 11, though they had no formal obligation to do so under the Tripartite Pact with Japan, Germany and Italy each declared war on the US, which responded with a declaration of war against Germany and Italy. The British government declared war on Japan immediately after learning that their territory had also been attacked, while the following day (December 8) the United States Congress declared war on Japan. Japan announced declarations of war on the United States and the British Empire later that day (December 8 in Tokyo), but the declarations were not delivered until the following day. Kazuo Sakamaki, the commanding officer of one of the submarines, was captured. Japanese losses were light: 29 aircraft and five midget submarines lost, and 64 servicemen killed. 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. 2,403 Americans were killed and 1,178 others were wounded. More than 180 US aircraft were destroyed.

The Japanese also sank or damaged three cruisers, three destroyers, an anti-aircraft training ship, and one minelayer. All but USS Arizona were later raised, and six were returned to service and went on to fight in the war. Of the eight US Navy battleships present, all were damaged, with four sunk. 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. Over the course of seven hours there were coordinated Japanese attacks on the US-held Philippines, Guam, and Wake Island and on the British Empire in Malaya, Singapore, and Hong Kong. Its aim was to prevent 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 those of the United States. Japan intended the attack as a preventive action. The Japanese military leadership referred to the attack as the Hawaii Operation and Operation AI, and as Operation Z during its planning. The United States was a neutral country at the time the attack led to its formal entry into World War II the next day. The attack on Pearl Harbor was a surprise military strike by the Imperial Japanese Navy Air Service upon the United States against the naval base at Pearl Harbor in Honolulu, Territory of Hawaii, just before 8:00 a.m., on Sunday, December 7, 1941.

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