Before the attack on Pearl Harbor, war between the Empire of Japan and the United States was a possibility 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, as it sought to join the great powers, all of which were Western nations. The Japanese government saw it necessary to become a colonial power in order to be modern and therefore Western. 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 U.S.
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 U.S. That move prompted the U.S. 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, was 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.
Background In the 1890s, both the American acquisition of Pacific colonies near Japan in the Spanish-American War and its brokering of a resolution to the Russo-Japanese War via the Treaty of Portsmouth that was unsatisfactory for Japan created a public perception in Japan that the United States was inappropriately foisting itself into Asian regional politics and intent on limiting Japan.
Tensions between Japan and the prominent Western countries (the United States, France, the 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 and promoted a Greater East Asia Co-Prosperity Sphere as part of Japan's alleged "divine right" to unify Asia under Emperor Hirohito's rule.
In the 1930s, Japan's increasingly expansionist policies brought it into renewed conflict with its neighbors, the Soviet Union and China. The latter was in 1922 disappointed by Germany's former Chinese colony of Shandong being transferred to Japan in the Treaty of Versailles. (Japan had fought the First Sino-Japanese War with China in 1894–1895 and the Russo-Japanese War with Russia in 1904–1905. Japan's imperialist ambitions had a hand in precipitating both conflicts after which Japan gained a large sphere of influence in Manchuria and saw an opportunity to expand its position in China.) In March 1933 Japan withdrew from the League of Nations in response to international condemnation of its conquest of Manchuria and subsequent establishment of the Manchukuo puppet government there. On January 15, 1936, the Japanese withdrew from the Second London Naval Disarmament Conference because the United States and the United Kingdom refused to grant the Japanese Navy parity with theirs.Lester H. Brune and Richard Dean Burns, Chronological History of U.S. Foreign Relations: 1932-1988, 2003, p. 504. A second war between Japan and China began with the Marco Polo Bridge Incident in July 1937.
Japan's 1937 attack on China was condemned by the U.S. and by several members of the League of Nations, including the United Kingdom, France, Australia, and the Netherlands. Japanese atrocities during the conflict, such as the notorious Nanjing Massacre that December, further complicated relations with the rest of the world. The U.S., the United Kingdom, France and the Netherlands all possessed colonies in East and Southeast Asia. Japan's new military power and willingness to use it threatened the Western economic and territorial interests in Asia.
In 1938, the U.S. began to adopt a succession of increasingly-restrictive trade restrictions with Japan, including terminating its 1911 commercial treaty with Japan in 1939, which was further tightened by the Export Control Act of 1940. Those efforts failed to deter Japan from continuing its war in China or from signing the Tripartite Pact in 1940 with Nazi Germany and Fascist Italy, which officially formed the Axis powers.
Japan would take advantage of Adolf Hitler's war in Europe to advance its own ambitions in the Far East. The Tripartite Pact guaranteed assistance if a signatory was attacked by any country not already involved in conflict with the signatory, which implicitly meant the U.S. and the Soviet Union. By joining the pact, Japan gained geopolitical power and sent the unmistakable message that any U.S. military intervention risked war on both shores: with Germany and Italy in the Atlantic and with Japan in the Pacific. The Franklin D. Roosevelt administration would not be dissuaded. Believing that the American way of life would be endangered if Europe and the Far East fell under fascist military dictatorship, it committed to help the British and the Chinese through loans of money and materiel and pledged sufficient continuing aid to ensure their survival. Thus, the United States slowly moved from being a neutral power to one preparing for war.Parkes, Henry Bamford. Recent America, A History Of The United States Since 1900 (Thomas Y. Crowell Company, 1946)Page. 635-645
In mid-1940, President Roosevelt moved the U.S. Pacific Fleet to Pearl Harbor, Hawaii, to deter Japan.Shift Of Our Fleet To Atlantic Studied, New York Times, June 23, 1940 On October 8, 1940, Admiral James O. Richardson, Commander in Chief, Pacific Fleet, provoked a confrontation with Roosevelt by repeating his earlier arguments to Chief of Naval Operations Admiral Harold R. Stark and Secretary of the Navy Frank Knox that Pearl Harbor was the wrong place for his ships. Roosevelt believed relocating the fleet to Hawaii would exert a "restraining influence" on Japan.
Richardson asked Roosevelt if the United States was going to war and got this response: At least as early as October 8, 1940,... affairs had reached such a state that the United States would become involved in a war with Japan.... 'that if the Japanese attacked Thailand, or the Kra Peninsula, or the Dutch East Indies we would not enter the war, that if they even attacked the Philippines he doubted whether we would enter the war, but that they (the Japanese) could not always avoid making mistakes and that as the war continued and that area of operations expanded sooner or later they would make a mistake and we would enter the war.'Joint Congressional Hearings on the Pearl Harbor Attack, Part 40, ^p.506, "Conclusions Restated With Supporting Evidence".Richardson, "On the Treadmill", pp.425 and 434; Baker, "Human Smoke", p.239,
Japan's September 1940 move into Vichy France-controlled French Indochina further raised tensions. Along with Japan's war with China, withdrawal from the League of Nations, alliance with Germany and Italy, and increasing militarization, the move induced the United States to intensify its measures to restrain Japan economically. The United States placed an embargo on scrap-metal shipments to Japan and closed the Panama Canal to Japanese shipping.Hsu Long-hsuen and Chang Ming-kai, translated by Wen Ha-hsiung. History of The Sino-Japanese War (1937-1945), 2nd ed. (Taipei, Republic of China: Chung Wu Publishing, 1971), p.317, "Invasion of French Indochina". That hit Japan's economy particularly hard because 74.1% of Japan's scrap iron came from the United States in 1938, and 93% of Japan's copper in 1939 came from the United States. In early 1941 Japan moved into southern Indochina, thereby threatening British Malaya, North Borneo and Brunei.
Japan and the U.S. engaged in negotiations in 1941 in an effort to improve relations. During the negotiations, Japan considered withdrawal from most of China and Indochina after it had drawn up peace terms with the Chinese. Japan would also adopt an independent interpretation of the Tripartite Pact and would not discriminate in trade if all other countries reciprocated. However, War Minister General Hideki Tojo rejected compromises in China.Chapter V: The Decision for War Morton, Louis. Strategy and Command: The First Two Years Responding to Japanese occupation of key airfields in Indochina (July 24) after an agreement between Japan and Vichy France, the U.S. froze Japanese assets on July 26, 1941, and on August 1, it established an embargo on oil and gasoline exports to Japan.Worth, Roland H., Jr., No Choice But War: the United States Embargo Against Japan and the Eruption of War in the Pacific (Jefferson, North Carolina: McFarland, 1995). The oil embargo was an especially strong response because oil was Japan's most crucial import, and more than 80% of Japan's oil came from the United States.
Japanese war planners had long looked south, especially to Brunei for oil and Malaya for rubber and tin. In the autumn of 1940, Japan requested 3.15 million barrels of oil from the Dutch East Indies but received a counteroffer of only 1.35 million. The complete U.S. oil embargo reduced the Japanese options to two: seize Southeast Asia before its existing stocks of strategic materials were depleted or submission to American demands. Moreover, any southern operation would be vulnerable to attack from the Philippines, a U.S. colony, and so war against the U.S. seemed necessary in any case.
After the embargoes and the asset freezes, the Japanese ambassador to the United States, Kichisaburō Nomura, and U.S. Secretary of State Cordell Hull held multiple meetings to resolve Japanese-American relations. No solution could be agreed upon for three key reasons:
Japan honored its alliance to Germany and Italy in the Tripartite Pact. Japan wanted economic control and responsibility for Southeast Asia, as envisioned in the Greater East Asia Co-Prosperity Sphere. Japan refused to leave Mainland China unless it kept its puppet state of Manchukuo.La Feber, Walter. Polenberg, Richard The American Century, A History Of The United States Since the 1890s (John Wiley & Sons, Inc.), pp. 243-247.