Events leading to the attack on Pearl Harbor

A series of events led to the attack on Pearl Harbor. War between the Empire of Japan and the United States was a possibility for which each nation's military forces had planned for after World War I. The expansion of American territories in the Pacific had been a threat to Japan since the 1890s, but real tensions did not begin until the Japanese invasion of Manchuria in 1931.

Japan's fear of being colonized and the government's expansionist policies led to its own imperialism in Asia and the Pacific to join the great powers, all of which were Western nations. The Japanese government saw the need to be a colonial power to be modern and therefore Western.[1][2] In addition, resentment was fanned in Japan by the rejection of the Japanese Racial Equality Proposal in the 1919 Treaty of Versailles, as well as by a series of racist laws, which enforced segregation and barred Asian people (including Japanese) from citizenship, land ownership, and immigration to the US.[2]

In the 1930s, Japan expanded slowly into China, which led to the Second Sino-Japanese War in 1937. In 1940, Japan invaded French Indochina in an effort to embargo all imports into China, including war supplies that were purchased from the US. That move prompted the US to embargo all oil exports, which led the Imperial Japanese Navy (IJN) to estimate it had less than two years of bunker oil remaining and to support the existing plans to seize oil resources in the Dutch East Indies.

Planning had been underway for some time on an attack on the "Southern Resource Area" to add it to the Greater East Asia Co-Prosperity Sphere Japan envisioned in the Pacific.

The Philippines, then an American protectorate, were also a Japanese target. The Japanese military concluded that an invasion of the Philippines would provoke an American military response. Rather than seize and fortify the islands and wait for the inevitable US counterattack, Japan's military leaders instead decided on the preventive attack on Pearl Harbor, which they assumed would negate the American forces needed for the liberation and the reconquest of the islands. (Later that day [December 8, local time], the Japanese indeed launched their invasion of the Philippines).

Planning for the attack on Pearl Harbor had begun very early in 1941 by Admiral Isoroku Yamamoto. He finally won assent from the Naval High Command by, among other things, threatening to resign. The attack was approved in the summer at an Imperial Conference and again at a second Conference in the autumn. Simultaneously over the year, pilots were trained, and ships prepared for its execution. Authority for the attack was granted at the second Imperial Conference if a diplomatic result satisfactory to Japan was not reached. After the Hull note and the final approval by Emperor Hirohito, the order to attack was issued in early December.