Events leading to the attack on Pearl Harbor  A series of events led to the attack on Pearl Harbor. War between Japan and the United States had been a possibility that each nation's military forces planned for in the 1920s, though real tension did not begin until the 1931 invasion of Manchuria by Japan. Over the next decade, Japan expanded slowly into China, leading 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 purchased from the U.S. This move prompted the United States to embargo all oil exports, leading 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, at that time an American protectorate, were also a Japanese target. The Japanese military concluded an invasion of the Philippines would provoke an American military response. Rather than seize and fortify the islands, and wait for the inevitable U.S. counterattack, Japan's military leaders instead decided on the preventive Pearl Harbor attack, which they assumed would negate the American forces needed for the liberation and reconquest of the islands. (Later that same day [December 8th local time], the Japanese indeed launched their invasion of the Philippines). Planning for the attack on Pearl Harbor had begun in very early 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 fall. 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 final approval by Emperor Hirohito the order to attack was issued at the beginning of December. Tensions between Japan and the prominent Western countries (the United States, France, United Kingdom and the Netherlands) increased significantly during the increasingly militaristic early reign of Emperor Hirohito. Japanese nationalists and military leaders increasingly influenced government policy, promoting a Greater East Asia Co-Prosperity Sphere as part of Japan's alleged "divine right" to unify Asia under Hirohito's rule.