Timeline

1936

December 2 Yamamoto begins forging the naval air arm into a modern weapon.

1939

February 10 Japan occupies the Chinese island of Hainan.
August Adm. Isoroku Yamamoto appointed commander-in-chief of the Imperial Japanese Navy.
September 4 Yamamoto writes to V/Adm. Shimata to say that he is uneasy about "Japan's relations with Germany and Italy in the face of changes now taking place in Europe."

1940

Spring The US fleet transfers to Pearl Harbor as its permanent home base: to the Japanese, this is a thinly veiled threat. Yamamoto uses this to urge expansion of naval air power. Yamamoto begins thinking that it would be better to carry war to the US Navy rather than wait for them to choose the time and place for battle.
July Roosevelt has an embargo placed on all aviation fuel, steel and scrap iron to Japan.
August Lieutenant-Colonel. Friedman, a cryptographer, breaks the Japanese Purple Code (MAGIC).
September 3 Roosevelt gives Britain 50 old destroyers for the rights to establish US naval bases in British territories.
September 4 The US warns Japan not to attack French Indochina.
September 11 Ojiro Okuda is appointed acting consul general to Hawaii. He is in charge of reporting on movements of US ships in the harbor, much of which appears in American newspapers. Kohichi Seki studies Jane's Fighting Ships and travels around the island studying the base and airfields, but without trespassing on US government property though.
September 27 Japan joins the Tripartite Pact. Yamamoto tells Konoye: "I hope you will . . . avoid a Japanese-American war.
November 12 British torpedo bombers attack the Italian fleet at Taranto, disabling half of Italy's Mediterranean fleet.
December 10 Yamamoto writes to Shimada: "The probability is great . . . our operations against the Netherlands' Indies are almost certain to develop into a war with America, Britain and Holland before those operations are half-over. Consequently we should not launch . . . the southern operation unless we are prepared . . . and adequately equipped."
December 30 Rear Admiral Bloch sends a memo: "Any aircraft attacking Pearl Harbor will . . . be brought by carriers."

1941

January 1 In Japan, American ambassador Grew writes in his diary: "Japan . . . is on the warpath . . . If . . . Americans . . . could read . . . articles by leading Japanese . . . they . . . would realize the utter hopelessness of a policy of appeasement."
January 6 President Roosevelt declares the United States the "arsenal of democracy."
January 7 Yamamoto writes a letter to R/Adm. Takijiru Oikawa, saying: "A conflict with the United States . . . is inevitable." The Japanese navy should "destroy the US main fleet at the outset of the war." He continues that the Japanese Navy should strike so as to "decide the fate of the war on the very first day." His plan is to find the US Navy "at Pearl Harbor [and] attack it vigorously with our air force." He concludes that if the US Navy is not at Pearl Harbor, they should find them regardless of where they are. The Japanese First and Second Carrier Divisions should mount a "surprise attack with all their air strength, risking themselves on a moonlight night or at dawn." Oilers were needed for refueling at sea, destroyers would pick up survivors whose aircraft or ships went down, and submarines would attack vessels fleeing Pearl Harbor and attempt to sink Allied vessels at the entrance and block it. An attack on "the Philippines and Singapore should be made at almost the same time as . . . against Hawaii." At the end of the letter, Yamamoto requests: "I sincerely desire to . . . personally command that attack force."
January 24 Prince Fumimaro Konoye, the Japanese prime minister, asserts that "firm establishment of a Mutual Prosperity Sphere in Greater East Asia is . . . necessary to the continued existence of this country." Yamamoto hypothesizes that should war break out "between Japan and the United States, it would not be enough that we take Guam and the Philippines, nor even Hawaii and San Francisco. We would have to . . . dictate the terms of peace in the White House. I wonder if our politicians . . . are prepared to make the necessary sacrifices."
January 27 In secret talks with Britain, the US decides that if Japan enters the war on the German side, and if the US enters the war, Germany is to be defeated first, then Japan. Ambassador Grew, in Japan, is warned by his Peruvian counterpart that he has heard a Japanese worker in his embassy say that if war occurs the "Japanese military . . . [will] attempt a surprise mass attack on Pearl Harbor using all their military facilities." In Washington, military intelligence is surprised only that Grew puts credence in the source of the report and not in the supposition of the report. In Japan, Foreign Minister Yosuke Matsuoka says, "We must control the Western Pacific," and that the US should reconsider their prior actions: if the US does not, there is "no hope for Japanese-American relations." Aboard Nagato, Yamamoto discusses the logical and technical feasibility of an attack on Pearl Harbor. After this meeting, Onishi asks Maeda (his senior staff officer) the following question: if US capital ships were "moored around Ford Island, could a successful torpedo attack be launched against them?" Maeda says no, the water is too shallow for torpedoes to be effective. However, if the torpedoes were modified. . .
February 1 Kimmel replaces Richardson as CinCPAC; Short is promoted to commander of the Hawaiian Department.
February 5 Kimmel receives a letter from Secretary Knox stating: "If war eventuates with Japan . . . hostilities . . . would start . . . with a surprise attack on Pearl Harbor." The letter tells Kimmel to "increase the joint readiness of the army and navy to withstand a raid." He says that probable forms of attack are bombing, torpedo attacks, or both. Congressman Faddis of Pennsylvania states: "The Japanese are not going to risk a fight . . . where they must face the American Navy in open battle. Their navy is not strong enough."
February 12 Nomura presents his credentials, which appoint him Ambassador to Washington, to Cordell Hull.
February 15 Kimmel issues a Pacific Fleet Conference letter saying they are faced with a possible surprise attack on ships in Pearl Harbor.
Mid-February Onishi sends for Cmdr. Minoru Genda and presents Yamamoto's plan, mentioning that Yamamoto has given some thought to making it a one-way mission (katamechi kogami) to increase the striking distance to over 500 miles. Genda opposes treating aircraft as disposable: "Ditching . . . would be a waste of men and planes." He thinks Yamamoto should include dive-bombers and high altitude bombers as well as torpedo planes in the attack. "To obtain the best results, all carriers should approach as close to Pearl Harbor as possible." His last point is: "Our prime target should be US carriers." Onishi asks Genda to prepare a report about feasibility, component forces and manner of execution, and then report back in ten days.
Late February Genda gives Onishi a report containing ten main proposals. It must be a surprise attack; US carriers are its main objective; US aircraft on Oahu are an objective; and every available Japanese carrier should take part in the operation. Furthermore, all kinds of attack aircraft should be used, and Japanese fighters should play an active role in the attack; the attack should be in early morning; refueling vessels at sea is necessary for success; and all planning must be ultra-secret. The tenth proposal is for a full-scale invasion, which Onishi disagrees with because they could not maintain supply so far from their present bases. Yamamoto wants to cripple the US Navy whereas Genda feels they should annihilate it.
February 27 Okuda reports: "The fleet goes to sea for a week and stays in Pearl Harbor for one week. Every Wednesday those at sea and those in the harbor change places."
March 5 The Japanese foreign ministry wires Nomura to say that they feel fairly certain that the US "is reading your code messages."
March 10 Onishi gives Yamamoto a draft of his plan for attack, based on Genda's plan but with some modifications.
March 11-12 Congress passes the Lend Lease Act, which supplies materiel to governments fighting the Axis.
March 14 Kita is appointed consul general to Hawaii.
March 20 Nomura responds to the foreign ministry: "Though I do not know which ones, I have discovered that the United States is reading some of our codes." Nomura informs them he will tell them details in a "safe" way. Still they did not change the Purple Code. Matsuoka may have been suspicious of Nomura's warning, feeling it sprang from insecurity.
March 27 Takeo Yoshikawa, an intelligence officer, arrives in Pearl Harbor and realizes that battleships are berthed in pairs and that the in-shore ship is protected from torpedo attacks by the outboard one.
March 30 Roosevelt orders the Coast Guard to seize two German, 28 Italian and 35 Danish ships in US ports.
April 1 Naval Intelligence in Washington alerts district commanders to the fact that "the Axis Powers often . . . [attack on] Saturday and Sunday or on national holidays" and that commanders should put "proper watches and precautions . . . in effect."
April 10 The IJN reorganizes into the 1st Air Fleet, consisting of the First Carrier Division (Kaga and Akagi and four destroyers), the Second Carrier Division (Hiryu and Soryu and four destroyers) and the Fourth Carrier Division (Ryuho and two destroyers).
April 13 Japan and Russia sign a neutrality pact giving Japan the green light for southward expansion.
April 15 The US begins shipping lend-lease goods to China.
April 21 US, English, and Dutch officers coordinate the proposed roles of each in the military defense against Japan in case of a Japanese attack on Singapore.
April 23 Marshall disagrees with Roosevelt's decision to keep the US fleet in Hawaii because "our heavy bombers and . . . pursuit planes . . . could put up such a defense that the Japs wouldn't dare attack Hawaii."
April 28 When queried about the US choice to strengthen the Atlantic Fleet by removing vessels from the Pacific, the British reply that the "reduction . . . would not unduly encourage Japan." New Mexico, Mississippi, Idaho, Yorktown, four light cruisers, 17 destroyers, three oilers, three transports, and ten auxiliaries are transferred by the end of summer.
May 20 Nomura confirms to Tokyo: "the US is reading some of our codes."
May 26 Yoshikawa reports that three battleships and three light cruisers have disappeared from Pearl Harbor. Kimmel fires off an 11-page memo noting that 72 percent of the new officers for the Atlantic came from the Pacific Fleet and that the Pacific Fleet's needs are subordinated to those of Britain and the Atlantic Fleet.
May 27 Roosevelt declares the US to be in an unlimited state of national emergency.
June 14 The US freezes German and Italian assets.
June 16 German consulates in the US are shut down.
June 17 Germany moves against US property in Germany.
June 20 The US stops oil shipments from Gulf and East Coast ports to all destinations except Latin America and Britain.
June 22 Italian consulates in the US are closed.
June 26 Vichy France permits Japan to occupy French Indochina. The US impounds Japanese credits in the US. Roosevelt nationalizes the Philippine Army.
July 17 A new Japanese government is formed.
July 28 The US puts an embargo on oil sales, freezes assets, and closes ports to Japanese vessels.
August 18 An amendment to the 1940 Selective Service Law extends the length of service for US inductees from one year to two-and-a-half years.
September 24 A message from Tokyo to the Consulate General instructs the spy to report on US vessels in Pearl Harbor.
October 16 Konoye resigns and Gen. Tojo sets up a new government with himself as prime minister. Stark warns Kimmel of the possibility of Japanese activities.
November 5 Yamamoto issues Top Secret Order No.1 to the Combined Fleet, detailing the plan for the attack on Pearl Harbor.
November 7 Congress repeals sections of the Neutrality Act concerning arming US cargo ships and transporting war goods to warring nations.
November 10 Britain states that should Japan go to war with the US, they will declare war on Japan "within the hour."
November 22 The US intercepts a message telling Nomura that the deadline of November 22 has been extended to November 25, 1941.
November 25 No US-Japanese agreement is reached: consequently, Nagumo's task force sails from the Kuriles.
November 26 A large Japanese fleet under Vice-Admiral Chuichi Nagumo (Nagumo, Chuichi) leaves the Kurile Islands for Hawaii, maintaining radio silence and taking a northerly route to avoid detection. It has orders to attack the United States Pacific Fleet, in Pearl Harbor.
November 27 Argentina decides not to sell tungsten to Japan. Kimmel and Short are advised that US-Japanese negotiations have failed and that they should be prepared for any eventuality. Kimmel is ordered to deliver 25 aircraft to Wake and Midway.
November 28 An intercepted dispatch from Tokyo to the Japanese embassy in Washington confirms that Japan was now preparing to go to war.
November 29 This day marks the Japanese government's secret deadline for US compliance with their ultimatum of 20 November.
November 30 General Tojo (Tojo, Hideki)addresses a public rally in Japan, attacking the Chinese nationalist leader Chiang Kai-shek and his allies. "Chiang Kai-shek is dancing to the tune of Britain, America, and communism at the expense of able-bodied and promising young men."
December 1 President Franklin Delano Roosevelt returns hurriedly to Washington from vacation in Georgia, after suspicion heightens that the Japanese will attack. A meeting of the Imperial conference in Tokyo decides that Japan should go to war against the United States, the United Kingdom, and the Netherlands.
December 2 Nagumo gets the go-ahead. The US intercepts a message to the Japanese Embassy to destroy all codes.
December 3 In the afternoon, Hawaiian time, the Japanese strike force stops at 43 degrees North, 170 degrees East to refuel one last time before the attack. Their tanks are filled to capacity.
December 4 Naval commanders at Pearl Harbor receive a dispatch:
"Highly reliable information has been received that categoric and urgent instructions were sent yesterday to Japanese diplomatic and consular posts at Hong Kong, Singapore, Batavia, Manila, Washington, and London to destroy most of their codes at once and to burn all other important, confidential, and secret documents."
December 5 At a cabinet meeting on this day, Secretary Hull tells the president that 'With every hour that passes, I become more convinced that they [the Japanese] are not playing in the open...I'm convinced that they don't intend to make any honorable agreement with us about anything, or to come to any understanding. I think it is useless and futile.'
December 6 Roosevelt is given the partly deciphered 14-part message. Instructions state it is not to be given to Hull until 1300 hrs Washington time on December 7.
December 7 The Japanese Navy attacks Pearl Harbor.
December 8 Roosevelt calls the attack on Pearl Harbor a day that will "live in infamy," and Congress declares war on Japan. Gen. Yamashita's 25th Army lands near the borders of Thailand and Malaya and begins the battle for Singapore.
December 9 The Japanese strike force moves homeward ater completing their mission.
December 10 News of victorious attacks on Hong Kong, Singapore, Pearl Harbor, and the British warships the Prince of Wales and the Repluse reach a jubilant Tokyo.
December 11 Italy and Germany declare war on the US.
December 12 Japanese forces occupy Guam.
December 23 Japanese forces capture Wake Island.
December 25 Hong Kong falls to the Japanese.

1942

February 15 Singapore surrenders.