Japanese decision for war in 1941
While the Empire of Japan was largely committed to forcible expansion by the late 1930s, specific plans, decisions and preliminary operations (e.g. French Indochina), by the Empire of Japan, were made to begin large-scale operations of World War Two in the Pacific in December 1941, primarily in 1941 but some in 1940. These wre more detailed than broad strategic directions such as the Strike-North and Strike-South Factions, or a decision to consolidate in China and Manchuria.
The decision to go to war, however, is rarely completely rational. In a 2009 paper, Jeffrey Record, of the U.S. Air War College, observes a number of points that led to Japan's decision, and are lessons not to be forgotten by future policymakers:
- Fear and honor, “rational” or not, can motivate as much as interest.
- There is no substitute for knowledge of a potential adversary’s history and culture.
- Deterrence lies in the mind of the deterree, not the deterrer.
- Strategy must always inform and guide operations.
- Economic sanctioning can be tantamount to an act of war.
- The presumption of moral or spiritual superiority can fatally discount the consequences of an enemy’s material superiority.
- “Inevitable” war easily becomes a self-fulfilling prophesy.
It is the central conclusion of this monograph
that the Japanese decision for war against the United States in 1941 was dictated by Japanese pride and the threatened economic destruction of Japan by the United States. The United States sought to deter Japanese imperial expansion into Southeast Asia by employing its enormous leverage over the Japanese economy; it demanded that Japan withdraw its forces from both Indochina and China—in effect that Japan renounce its empire in exchange for a restoration of trade with the United States and acceptance of American principles of international behavior. Observed B.H. Liddell HartSir Basil Henry Liddell Hart in retrospect: “No Government, least of all the Japanese, could be expected to swallow suchhumiliating conditions, and utter loss of face.”
An irony of the situation was that Japanese expansion into Indochina did not directly threaten the United States. The Japanese had two reasons for doing so: to cut off the southern supply route to China, and to obtain bases for the strikes into Southeast Asia. It was American policy, however, to side strongly with China.Karel von Wolferen, who has long explained Japanese political behavior, even after the war, in the lack of central authority led to the decision.  argues that the decision came directly from rivalry and lack of information exchange between the Imperial Japanese Army and Navy. The Army, having been fighting in East Asia for a number of years, was the more militant of the two services, while Isoroku Yamamoto was transferred from vice-Navy Minister to Commander-in-Chief, Combined Fleet, to better protect him against radical assassins angry at his advice against war.
The Army, on setting Japan on the path to war took it for granted that the Navy would take care of defense against U.S. might, while the Navy, wishing to hold onto its power and its credibility as a patriotic institution, refrained from directly expressing the belief, widespread among its ranks, that victory was impossible. 
Japan believed her growth was constrained by resources, but, in 1941, she saw resources as adequate for what was hoped to be a short war. The principal goal was obtaining four critical resources in which Japan was not self-sufficient, so economic geography largely dictated the targets of the war:
- Rice, which could be obtained, given sufficient shipping, from Thailand and French Indochina
- Essential war materials.
- "Iron.-- ... iron ore for could be supplied from Japan, Manchuria, China (including Hainan Island) and French Indochina.
- "Nickel.-- The shortage is great and there are no measures for obtaining supplies within our present sphere of influence. It must be obtained from occupied territories in the south.
- "Crude rubber.--On the basis of an economic agreement, the amount expected from Thailand and French Indochina is 45,000 tons. Domestic requirements are 65,000 tons and the amount on hand is less than 500 tons. Unless the amount procured from French Indochina and Thailand is increased, or a supply of over 20,000 tons is secured from the Netherlands East Indies, the shortage will have a great effect upon domestic industry and especially upon the progress of military preparations.
- "Tin.--Unless a supply of about 10,000 tons annually is procured from Thailand and French Indochina, not only would it be impossible to meet national requirements, but even peacetime military preparations would come to a standstill. After 2 years of war, present stockpiles would be completely depleted.
- "Copper.--Supplies on hand will soon be cut in half if the present trend continues. Thus, it will be necessary to develop sources in the Philippines for supplies of copper.
- "Lead.--If the present situation continues, the supply will be halved. If more lead can be obtained from Burma, the supply will be sufficient.
- "Cobalt.--Cobalt must be procured from the Netherlands East Indies.
- Transport capacity." The estimate did not make it clear from where this would come. U.S. submarine operations against Japan were devastating because they concentrated on transport, not warships."
Who were the enemies?
Originally, the Strike-South was to have been of moderate scope, confronting only Britain. This soon expanded, though, to an inevitable confrontation with the Netherlands over Indonesia (then the Dutch East Indies). Confrontation with Australia and the United States were more complex. Japan never had serious intent to invade Australia, but recognized that Australia would be threatened by nearby activities and indeed might compete for some of the same resources. If Japan attack the Philippines, that would be a direct attack on a U.S. territory, but, even if Japan did not, there was the danger that the U.S. would support its Western allies.
To what extent Japan understood it is unclear, but Franklin D. Roosevelt saw anything that interfered with Britain's ability to fight Nazi Germany as of vital strategic interest to the U.S. Malaya and Singapore provided Britain with essential resources.
The situation with France also was complex, as Japan saw Vichy as an ally. Operations in French Indochina, taken in 1940, were transitional, in that they directly bore on the Second Sino-Japanese War, but also would establish bases for Strike-South. Strike-North had largely been rejected due to the rough handling of Japanese troops, by Soviet forces, on the border, including such things as the Nomohan Incident.As seen by the U.S. Strategic Bombing Study, Japan, led by the radical officers, headed by Hideki Tojo, who had grown from its successes in Manchuria and an "uneasy coalition with the conservatives" in the Konoe governments,
. ...took Japan to war without concrete minimum or maximum objectives nor any clear conception as to how the war could be brought to a close. The decision to attack was roughly calculated as a two-way gamble. If the European Axis defeated Soviet Russia, Japan would require chips to play on the winning side at the peace settlement with the United States and Britain that might well follow. On the other side, and independent of European events, a quick drive to the Southern Resources Area accompanied by a series of stunning and crippling defeats of the United States forces would redress Japan's relative strength and create a situation in which the United States might be willing to negotiate a peace by trading out the issues on terms favorable to Japan. Great confidence was put in the eventual superiority of Japan's fighting spirit over the potential material superiority of the United States.
Prince Saionji would normally recommended the new Prime Minister, after the fall of Hirota government, but was reluctant to come to the Palace. Told he would not to have to travel to Tokyo. Saionji immediately recommended General Kazushige Ugaki, "who, as Governor General of Korea, had made himself the leader of the army officers who took a moderate position between the Strike North and Strike South Factions. He had the support of the political parties and the cartels. Most important, he was outspoken in his opposition to the planned war of aggression in China." Ugaki did not want the job, and resigned from the reserve. On January 29, Hirohiro convinced him to keep his commission, and he became foreign minister in the cabinet after next. 
Hirohito debated if it was more important to get the support of Hitler, or to alienate the Soviet Union. He told Marquis Kido that it was the most fateful of his reign. If he made the wrong decision, he might find him "left alone and stripped of my closest retainers and elder statesmen."
The Imperial Japanese Army urged him to sign the Tripartite Pact, and Navy Minister Mitsumasu Yonai added to the pro-Nazi argument by "stating flatly that the Navy, in its present state of development, could not hope to win in the Pacific even against the British fleet alone.
The Inner Cabinet approved the pact on April 25, with two provisos:
- The clauses of the alliance directed against the Allies should be kept secret
- Japan's entry into WWII should not necessarily immediately follow Germany's
Hitler rejected these provisos, and the Pact would not be signed for another four months.
After preparatory meetings, Prince Konoe was authorized, on 17 July 1940 by Hirohito, to form a cabinet. In a meeting six days before, he had gained the support of President of the Privy Council Yoshimichi Hara, as well as four other prior prime ministers, Senjuro Hayashi, Koki Hirota, Keisuke Okada and Reijiro Watasuki. His cabinet succeeded that of Mitsumasu Yonai.
Japan hoped, especially after the fall of France, to work diplomatically with Vichy France, even before the Tripartite Pact was signed, both to cut off supplies to Chiang Kai-shek and to establish airbases in French Indochina, needed to strike further south and east.
Georges Catroux, French governor of Indochina, did close the border with China. A Japanese verification group, headed by Major General Issaku Nishimura entered Indochina on June 25. On the same day that Nishimura arrived, Vichy dismissed Catroux, for independent foreign contact. He was replaced by Vice Admiral Jean Decoux, who commanded French naval forces in the Far East, and was based in Saigon. Ducoux and Catroux were in general agreement about policy, and considered managing Nakamura the first priority.  Ducoux had additional worries. The senior British admiral in the area, on the way from Hong Kong to Singapore, visited Ducoux and told him that he might be ordered to sink Ducoux's flagship, with the implicit suggestion that Ducoux could save his ships by taking them to Singapore, which appalled Ducoux. While the British had not yet attacked French ships that would not go to the side of the Allies, that would happen at Mers-el-Kabir in North Africa within two weeks; it is not known if that was suggested to, or suspected by, Ducoux. Deliberately delaying, Ducoux did not arrive in Hanoi until July 20, while Catroux stalled Nishimura on basing negotiations, also asking for U.S. help. 
Admiral Isoroku Yamamoto, as Navy Vice-Minister, had opposed the war. As Commander-in-Chief, Combined Fleet, he considered the Pearl Harbor attack essential if he had to fight.
If I am told to fight regardless of the consequences, I shall run wild considerably for the first six months or a year, but I have utterly no confidence for the second and third years. . . . Now that the situation has come to this pass [the Tripartite Pact] I hope you will endeavor for avoidance of an American-Japanese war. — Yamamoto to Prime Minister Prince Konoye, October 1940
The Pearl Harbor striking force set sail on 27 November, but still could be recalled.
U.S. economic warfare
By July, the Imperial Japanese Navy, although generally less pro-war than the Army, faced the situation that its oil reserves were dropping. If they did not find alternate sources fairly soon, they would not have enough to conduct a war. Navy Chief of Staff Osami Nagumo said there was a chance of victory with the U.S. that
will diminish as time goes on. By the latter half of next year, it will be difficult to cope with the United States. After that the situation will become progressively worse. The United States will probably drag things out until her defenses are built up and then will try to settle. If we could settle matters without war, there would be nothing better. But if we conclude conflict cannot ultimately be avoided, then I would like you to understand that time is not on our side. Furthermore, if we occupy the Philippines, it will be easier, from the standpoint of the Navy, to have prosecuted the war fully from the start.
Franklin D. Roosevelt, as he simultaneously tried to aid Britain, China, and the Soviet Union, while mobilizing the United States, wanted to contain Japan but not trigger war. He had intended to revoke permits to export strategic goods but allow them to reapply for controlled amounts, but this did not work in practice. Oil export licenses were cancelled on August 1, and Roosevelt went to travel the next day. When he returned, he did not formally approve the embargo until September 5, without notice to Japan. Japanese tankers waited in U.S. ports until November. 
September conferenceAfter war games in September, the Army and Navy Chiefs of Staff, Sugiyama and Nagano, briefed the Emperor. The chiefs were optimistic. When Sugiyama responded to a query of what to do if the weather was bad with "That is an obstacle we must overcome", the Emperor responded
But you do think that you can do what you plan? Remember that when you were war minister [in 1937] you said that Chiang Kai-Shek would surrender immediately. But you still haven't made him do it.
If you call the interior of China broad, isn't the Pacific Ocean even broader? How can you confidently say [this time it will take only] five months?
Konoe proposed a mid-October meeting, but the State Department opposed it because they did not believe the Imperial Japanese Army would agree to the American requirements. The policy of the Atlantic Charter resembled the anti-fascist doctrine of Baron Shidehara after the Washington Naval Conference. 
Final conferenceA December 1 conference brought together Emperor Hirohito, the Cabinet, and others for a total of nineteen leaders. The first question was posted by Privy Council President Yoshimichi Hara, who referred to a 26 November document from Hull to Japanese Ambassador Kichisaburo Nomura, which became one of the key factors in the Japanese choice of war, quoting it (emphasis added) as "the United States has demanded we withdraw troops from all of China." The original English text read only "China". Hara asked for clarification, and Foreign Minister Togo said it was unclear if the United States had separated Manchuria from China. Throughout the Nomura-Hull talks, China and Manchuria had always been separated, due to a confusing explanation from Togo, no one at the conference assumed so. Hara concluded that war was preferable to accepting the American proposal because
If we were to give in [to the United States, then we would not only give up the fruits of the Sino-Japanese War, and the Russo-Japanese War, but also abandon the results of the Manchurian Incident. There is no way we could endure this....It is clear that the existence of our empire is threatened, that the great achievements of the Emperor Meiji would all come to naught, and that there is nothing else we can do.
- Jeffrey Record (February 2009), Japan's Decision for War in 1941: Some Enduring Lessons, Strategic Studies Institute, U.S. Army War College, p. ix
- Record, p. 6
- B.H. Liddell Hart, Strategy,Frederick A. Praeger, 1967, p. 269, quoted in Record, p. 6
- Karel van Wolferen (1989), The Enigma of Japanese Power: People and Power in a Stateless Nation, Alfred E. Knopf, pp. 39-40
- Sadao Asada, "The Japanese Navy and the United States" in Dorothy Berg & Okamoto Shumpei (eds.), Pearl Harbor as History, Columbia University Press, 1973, pp. 258-259, quoted in von Wolferen, pp. 39-40
- Chairman's Office (1 July 1946), APPENDIX A-1. Estimate of Japanese National Strength at the Outbreak of the Greater East Asian War as of December 1941, Japan's Struggle to End the War, United States Strategic Bombing Survey, pp. 14-16
- Chairman's Office (1 July 1946), Japan's Struggle to End the War, United States Strategic Bombing Survey
- David Bergamini (1971), Japan's Imperial Conspiracy, Morrow, p. 679-680
- Bergamini, pp. 701-702
- Merion and Susie Harris (1991), Soldiers of the Sun: the Rise and Fall of the Imperial Japanese Army, Random House, pp. 275-276
- Arthur J. Dommen (2001), The Indochinese Experience of the French and the Americans, Indiana University Press pp. 47
- Martin Gilbert (1989), The Second World War, Stoddart, p. 107
- Dommen, p. 48
- Frederick D. Parker (2001), A Priceless Advantage: U.S. Navy Communications Intelligence and the Battles of Coral Sea, Midway, and the Aleutians, National Security Agency, p.1
- Bergamini, pp. 774-776
- Harris & Harris, pp. 291-292
- Bergamini, p. 782
- Harris & Harris, pp. 135-136
- Document Handed by the Secretary of State (Hull) to the Japanese Ambassador (Nomura) (26 November 1941), Outline of Proposed Basis for Agreement between the United States and Japan
- Herbert P. Bix (2001), Hirohito and the making of modern Japan, Harper Perennial, ISBN 978-0060931308, pp. 431-422