Such an attack would certainly provoke American military involvement in the war. This would complicate matters for the Japanese and a preventive strike was planned which resulted in the attack. Planning for an 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. Over the next 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. The order to attack was issued at the beginning of December.
Since the 1930s, increasingly Japan's expansionist policies brought her into conflict with her neighbors, Russia and China, including with Russian influence expansion into Manchuria and Korea (which Japan saw as a threat and which helped spark the Russo-Japanese War which, after a surprise attack on the Russian fleet, resulted in Japanese victory). Japan's invasion and seizure of Manchuria in 1931, and persistent meddling in Chinese affairs, resulted in Japan leaving the League of Nations. A full-scale war began with the Marco Polo Bridge Incident faked by militarist expansionist Army officers in July 1937. Japanese atrocities during the succeeding invasion like the Rape of Nanking further complicated relations with the rest of the world, particularly the US.
In response to international condemnation particularly by the United States, Britain and the Netherlands of the 1931 conquest of Manchuria, the creation of the Manchukuo puppet government, in 1933 Japan withdrew from the League of Nations. On January 15, 1936, Japan withdrew from the Second London Naval Disarmament Conference which had refused parity of Japan's naval forces with other major navies (notably the U.S.). The 1937 Japanese attack against China was condemned by the U.S. and by several members of the League of Nations, particularly Britain, France, Australia, and the Netherlands. These states had economic and territorial interests, or formal colonies, in Southeast Asia, and had become increasingly alarmed at Japan's military power and willingness to use it, which they saw as threats to their control in Asia. In July 1939, the U.S. terminated the 1911 U.S.-Japan commercial treaty. These efforts failed to deter Japan from continuing the war in China nor from signing both the Anti-Comintern Pact with Nazi Germany and the Tripartite Pact with Germany and Italy forming the Axis Powers.
On October 8, 1940, Admiral Richardson, commander of the U.S. fleet, had a confrontation with President Roosevelt. Richardson repeated what he had said in his letter to Admiral Stark and his memo to Secretary Knox -- that Pearl Harbor was the wrong place for his ships. Roosevelt said he thought that having the fleet in Hawaii was a "restraining influence" on Japan. When Richardson asked the president whether the United States was going to war . "He replied," in Richardson's account, "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 the Japanese couldn't always avoid making mistakes, the president said. "Sooner or later they would make a mistake and we would enter the war."
The Tripartite Pact, war with China, increasing militarization and Japan's withdrawal from the League of Nations eventually led the U.S. to embargo scrap metal and gasoline shipments to Japan and to constrain its foreign policy actions and close the Panama Canal to Japanese shipping. In 1941, Japan moved into northern IndoChina. The U.S. responded by freezing Japan's assets in the U.S. and embargoing all oil exports to Japan. Oil was Japan's most crucial imported resource; more than 80 percent of Japan's oil imports at the time came from the United States To secure oil supplies, and other resources, Japanese planners had long been looking south, especially the Dutch East Indies. The Navy was certain any attempt to seize this region would bring the U.S. into the war and was reluctant to agree with other factions' plans for invasion. The complete US oil embargo changed to the Naval view to support of expansion toward support for an invasion of the Dutch East Indies and seizure of its oil fields. In August 1941, Japanese Prime Minister Fumimaro Konoe proposed a summit with President Roosevelt to discuss differences. Roosevelt replied Japan must leave China before a summit meeting could be held.
In July 1941, the Imperial Japanese Navy headquarters informed Hirohito that its reserve bunker oil would be exhausted within two years if a new source was not found. On September 6, 1941, at the second Imperial Conference concerning attacks on the Western colonies in Asia, Japanese leaders met to consider the attack plans prepared by Imperial General Headquarters, one day after the emperor had reprimanded General Sugiyama about the lack of success in China, and the speculated low chances of victory against the Western powers.
Prime Minister Konoe argued for more negotiations and possible concessions to avert war. Military leaders like Hideki Tojo, Sugiyama, and IJN Chief of Staff Osami Nagano argued time had run out and that additional negotiations would be pointless. They urged swift military actions against all American and European colonies in Southeast Asia and Hawaii. Tojo argued yielding to the American demand to withdraw troops would wipe out all the fruits of the Second Sino-Japanese war, depress Army morale, endanger Manchukuo and jeopardize control of Korea and so argued that doing nothing was the same as defeat and a loss of face.
On October 16, 1941, Konoe resigned and proposed Prince Naruhiko Higashikuni, who was also the choice of the Army and the Navy, as his successor. Hirohito choose Tojo instead, worried, as he told Konoe, about having the Imperial House being held responsible for a war against Western powers.
On November 3, 1941, Nagano presented a complete plan for the attack on Pearl Harbor to Hirohito. At the Imperial Conference on 5 November, Hirohito approved the plan for a war against the United States, Great Britain and Holland, scheduled to start at the beginning of December if an acceptable diplomatic settlement were not achieved before then.
On 30 November 1941, Prince Nobuhito Takamatsu warned his brother, Hirohito, that the Navy felt the Empire could not fight more than two years against the United States and wished to avoid war. After consulting with Koichi Kido (who advised him to take his time until he was convinced) and Tojo, the Emperor called Shigetaro Shimada and Nagano who reassured him war would be successful. On December 1, Hirohito finally approved a "war against United States, Great Britain and Holland", during another Imperial Conference, to commence with a surprise attack on the US Pacific Fleet at its main forward base at Pearl Harbor in Hawaii.
On February 3, 1940, Yamamoto briefed Captain Kanji Ogawa of Naval Intelligence on the potential attack plan, asking him to start intelligence gathering on Pearl Harbor. Ogawa already had spies in Hawaii, including Japanese Consular officials with an intelligence remit, and he arranged for help from a German already living in Hawaii who was an Abwehr agent. None had been providing much militarily useful information. He planned to add 29-year-old Ensign Takeo Yoshikawa. By the spring of 1941, Yamamoto officially requested additional Hawaiian intelligence, and Yoshikawa boarded the liner Nitta-maru at Yokohama. He had grown his hair longer than military length, and assumed the cover name Tadashi Morimura.
Yoshikawa began gathering intelligence in earnest by taking auto trips around the main islands, and toured Oahu in a small plane, posing as a tourist. He visited Pearl Harbor frequently, sketching the harbor and location of ships from the crest of a hill. Once, he gained access to Hickam Field in a taxi, memorizing the number of visible planes, pilots, hangars, barracks and soldiers. He was also able to discover that Sunday was the day of the week on which the largest number of ships were likely to be in harbor, that PBY patrol planes went out every morning and evening, and that there was an antisubmarine net in the mouth of the harbor. Information was returned to Japan in coded form in Consular communications, and by direct delivery to intelligence officers aboard Japanese ships calling at Hawaii by consulate staff.
Expecting war, and seeing an opportunity in the forward basing of the US Pacific Fleet at Hawaii, the Japanese began planning in early 1941 for an attack on Pearl Harbor. For the next several months, planning, and organizing a simultaneous attack on Pearl Harbor and invasion of British and Dutch colonies to the South occupied much of the Japanese Navy's time and attention. The Pearl Harbor attack planning arose out of the Japanese expectation the U.S. would be inevitably drawn into the war after a Japanese attack against Malaya and Singapore.
The intent of a preemptive strike on Pearl Harbor was to neutralize American naval power in the Pacific, thus removing it from influencing operations against American, British, and Dutch colonies to the south. Successful attacks on colonies were judged to depend on successfully dealing with the American Pacific Fleet. Planning had long anticipated that a battle between the two Fleets would happen in Japanese home waters after the US Fleet traveled across the Pacific, under attack by submarines and other forces all the way. The US Fleet would be defeated in a climactic battle, just as had the Russian Fleet in 1905. A surprise attack posed a twofold difficulty compared to long standing expectations. First, the US Pacific Fleet was a formidable force, and would not be easy to defeat or to surprise. Second, for aerial attack, Pearl Harbor's shallow waters made using conventional air-dropped torpedoes ineffective. On the other hand, Hawaii's isolation meant a successful surprise attack could not be blocked or quickly countered by forces from the continental U.S.
Several Japanese naval officers had been impressed by the British Operation Judgement, in which 21 obsolete Fairey Swordfish disabled half the Regia Marina. Admiral Yamamoto even dispatched a delegation to Italy, which concluded a larger and better-supported version of Cunningham's strike could force the U.S. Pacific Fleet to retreat to bases in California, thus giving Japan the time necessary to establish a "barrier" defense to protect Japanese control of the Dutch East Indies. The delegation returned to Japan with information about the shallow-running torpedoes Cunningham's engineers had devised.
Japanese strategists were undoubtedly influenced by Admiral Togo's surprise attack on the Russian Pacific Fleet at Port Arthur in 1905, and may have been influenced by U.S. Admiral Harry Yarnell's performance in the 1932 joint Army-Navy exercises, which simulated an invasion of Hawaii. Yarnell, as commander of the attacking force, placed his carriers northwest of Oahu and simulated an air attack. The exercise's umpires noted Yarnell's aircraft were able to inflict serious "damage" on the defenders, who for 24 hours after the attack were unable to locate his force.
In a letter dated January 7, 1941 Yamamoto finally delivered a rough outline of his plan to Koshiro Oikawa, then Navy Minister, from whom he also requested to be made Commander in Chief of the air fleet to attack Pearl Harbor.
A few weeks later, in yet another letter, this time directed at Takijiro Onishi—chief of staff of the Eleventh Air Fleet—Yamamoto requested Onishi study the technical feasibility of an attack against the American base. After consulting first with Kosei Maeda, an expert on aerial torpedo warfare, and being told the harbour's shallow waters rendered such an attack almost impossible, Onsihi summoned Commander Minoru Genda. After studying the original proposal put forth by Yamamoto, Genda agreed: "the plan is difficult but not impossible".
During the following weeks, Genda expanded Yamamoto's original plan, highlighting the importance of it being carried out early in the morning and in total secrecy, employing an aircraft carrier force, several different types of bombing, among other aspects which included an actual landing in Hawaii, aimed at forcing American forces to retreat towards the West Coast.
By April 1941, the Pearl Harbor plan became known as Operation Z, after the famous Z signal given by Admiral Tōgō at Tsushima.
Over the summer, pilots trained in earnest near Kagoshima City on the Japanese island of Kyūshū. Genda had chosen it because its geography and infrastructure presented most of the same problems bombers would face at Pearl Harbor. In training, each crew flew over the 5000-foot (1500 m) mountain behind Kagoshima, dove down into the city, dodging buildings and smokestacks before dropping to an altitude of 25 feet (7 m) at the piers. Bombardiers released torpedoes at a breakwater some 300 yards (270 m) away.
Yet even skimming the water did not solve the problem of torpedoes bottoming in the shallow waters of Pearl Harbor. Japanese weapons engineers created and tested modifications allowing successful shallow water drops. The effort resulted in a heavily modified version of the Type 91 torpedo which inflicted most of the ship damage during the attack. Japanese weapons technicians also produced special armor-piercing bombs by fitting fins and release shackles to 14 and 16 inch (356 and 406 mm) naval shells. These were able to penetrate the lightly armored decks of the old battleships.
On November 26 1941, the day the Hull note was received from United States Secretary of State Cordell Hull, the Japanese carrier battle group under the command of Admiral Chuichi Nagumo, already assembled in Hitokappu Wan in the Kurile Islands, sortied for Hawaii, under strict radio silence.
The Kido Butai, the Combined Fleet's main carrier force of six aircraft carriers carriers (the most powerful carrier force with the greatest concentration of air power in the history of naval warfare to date), embarked 359 airplanes, organized as the First Air Fleet. The carriers Akagi (flag), Kaga, Sōryū, Hiryū, and the newest, Shōkaku and Zuikaku, had 135 Mitsubishi A6M Type 0 fighters (Allied codename "Zeke", commonly called "Zero"), 171 Nakajima B5N Type 97 torpedo bombers (Allied codename "Kate"), and 108 Aichi D3A Type 99 dive bombers (Allied codename "Val") aboard. Two fast battleships, two heavy cruisers, one light cruiser, nine destroyers, and three fleet submarines provided escort and screening. In addition, the Advanced Expeditionary Force included 20 fleet and five two-man Ko-hyoteki-class midget submarines, which were to gather intelligence and sink U.S. vessels attempting to flee Pearl Harbor during or soon after the attack. It also had eight oilers for underway fueling.
On December 1, 1941, after the striking force was en route, Chief of Staff Nagano gave a verbal directive to commander of the Combined Fleet Admiral Isoroku Yamamoto, informing him: Upon completion, the force was to return to Japan, re-equip, and re-deploy for "Second Phase Operations".
Finally, Order number 9, issued on 1 December 1941 by Nagano, instructed Yamamoto to crush hostile naval and air forces in Asia, the Pacific and Hawaii, promptly seize the main U.S., British, and Dutch bases in East Asia and "capture and secure the key areas of the southern regions".
On the home leg, the force was ordered to be alert for tracking and counterattacked by the Americans, and to return to the friendly base in the Marshall Islands, rather than the Home Islands.
By 1941, U.S. signals intelligence, through the Army's Signal Intelligence Service and the Office of Naval Intelligence's OP-20-G, had intercepted and decrypted considerable Japanese diplomatic and naval cipher traffic, though nothing actually carrying significant information about Japanese military plans in 1940-41. Decryption and distribution of this intelligence, including such decrypts as were available, was capricious and sporadic, some of which can be accounted for by lack of resources and manpower. At best, the information available to decision makers in Washington was fragmentary, contradictory, or poorly distributed, and was almost entirely raw, without supporting analysis. It was thus, incompletely understood. Nothing in it pointed directly to an attack at Pearl Harbor, and a lack of awareness of Imperial Navy capabilities led to a widespread underlying belief Pearl Harbor was not a possible attack target. Only one message from the Hawaiian Japanese consulate (sent on 6 December), in a low level consular cipher, included mention of an attack at Pearl; it was not decrypted until 8 December.
In 1924, General William L. Mitchell produced a 324-page report warning that future wars (including with Japan) would include a new role for aircraft against existing ships and facilities. He even discussed the possibility of an air attack on Pearl Harbor, but his warnings were ignored. Navy Secretary Knox had also appreciated the possibility of an attack at Pearl in a written analysis shortly after taking office. American commanders had been warned that tests had demonstrated shallow-water aerial torpedo attacks were possible, but no one in charge in Hawaii fully appreciated this. And a war game surprise attack against Pearl Harbor in 1932 had been judged a success and to have caused considerable damage.
Nevertheless, because it was believed Pearl Harbor had natural defenses against torpedo attack (e.g., the shallow water), the Navy did not deploy torpedo nets or baffles, which were judged to inconvenience ordinary operations. And as a result of limited numbers of long-range aircraft (including Army Air Corps bombers), reconnaissance patrols were not being made as often or as far out as required for adequate coverage against possible surprise attack; they improved considerably, with far fewer remaining planes, after the attack. The Navy had 33 PBYs in the islands, but only three on patrol at the time of the attack. Hawaii was low on the priority list for the B-17s finally becoming available for the Pacific, largely because General MacArthur in the Philippines was successfully demanding as many as could be made available to the Pacific (where they were intended as a deterrent). The British, who had contracted for them, even agreed to accept fewer to facilitate this buildup. At the time of the attack, Army and Navy were both on training status rather than operational alert.
There was also confusion about the Army's readiness status as General Short had changed local alert level designations without clearly informing Washington. Most of the Army's mobile anti-aircraft guns were secured, with ammunition locked down in armories. To avoid upsetting property owners, and in keeping with Washington's admonition not to alarm civil populations (e.g., in the late November war warning messages from the Navy and War Departments), guns were not dispersed around Pearl Harbor (i.e., on private property). Additionally, aircraft were parked on airfields to lessen the risk of sabotage, not in anticipation of air attack, in keeping with Short's interpretation of the war warnings.
Chester Nimitz said later, "It was God's mercy that our fleet was in Pearl Harbor on December 7, 1941.". Nimitz believed if Kimmel had discovered the Japanese approach, he would have sortied to meet them. With the American carriers absent and Kimmel's battleships at a severe disadvantage to the Japanese carriers, the likely result would have been the sinking of the American battleships at sea in deep water, where they would have been lost forever with tremendous casualties (as many as twenty thousand dead), instead of in Pearl Harbor, where the crews could easily be rescued, and six battleships ultimately raised.
In the days before the attack, a long 14-part message was sent to the Embassy from the Foreign Office in Tokyo (encrypted with the Type 97 cryptographic machine, in a cipher named PURPLE by U.S. cryptanalysts), with instructions to deliver it to Secretary of State Cordell Hull at 1 p.m. Washington time. The last part arrived late Saturday night (Washington time) but due to decryption and typing delays, and to Tokyo's failure to stress the crucial necessity of the timing, her Embassy personnel did not deliver the message breaking off negotiations to Secretary Hull until several hours after the attack.
The United States had decrypted the 14th part well before the Japanese Embassy managed to, and long before the Embassy managed a fair typed copy. The final part, with its instruction for the time of delivery, had been decoded that night, but was not actioned until the next morning; according to Clausen, who also denied the claim by Bratton that General Marshall couldn't be found (as he was out for a morning horseback ride).
It prompted General George Marshall, Army Chief of Staff, to send that morning's warning message, with assurances that it would be received by all recipients by 1 pm Washington time. There were delays in the message sent to Hawaii because of trouble with the Army's long distance communication system, a decision not to use the Navy's parallel facilities despite an offer to permit it, and various troubles during its travels over commercial cable facilities (somehow its "urgent" marking was misplaced, adding additional hours to its travel time). It was actually delivered to General Walter Short, by a young Japanese-American cycle messenger, several hours after the attack had ended.
There were Japanese records, admitted into evidence during Congressional hearings on the attack after the War, established that the Japanese government had not even written a declaration of war until hearing news of the successful attack. The two-line declaration of war was finally delivered to U.S. Ambassador Grew in Tokyo about 10 hours after the attack was over. He was allowed to transmit it to the United States where it was received late Monday afternoon (Washington time).