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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 since 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 to estimate that 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 that Japan envisioned in the Pacific.
The Philippine islands, at that time an American territory, were 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 Pearl Harbor attack, which they assumed would negate the American forces needed for the liberation and reconquest of the islands.
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 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, Britain, and the Netherlands) increased significantly during the increasingly militaristic early rule 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.
Over the course of the 1930s, Japan's increasingly expansionist policies brought it into renewed conflict with its neighbors, Russia and China (Japan had fought the First Sino-Japanese War with China in 1894-95 and the Russo-Japanese War with Russia in 1904-05; Japan's imperialist ambitions had a hand in precipitating both conflicts). 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. On January 15, 1936, Japan withdrew from the Second London Naval Disarmament Conference because the United States and Great Britain refused to grant the Imperial Japanese Navy (IJN) parity with their navies. A second full-scale 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 several members of the League of Nations including Britain, France, Australia, and the Netherlands. Japanese atrocities during the conflict such as the notorious Nanking Massacre that December, served to further complicate relations with the rest of the world. The U.S., Britain, France and the Netherlands had including colonies in East and Southeast Asia. Japan's new military power and willingness to use it threatened Western economic and territorial interests in Asia.
Beginning in 1938 the U.S. adopted a succession of increasingly restrictive trade restrictions with Japan. This included terminating its 1911 commercial treaty with Japan in 1939, further tightened by the Export Control Act of 1940. These 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, officially forming the Axis Powers.
Japan would take advantage of Hitler's war in Europe to advance its ambitions in the Far East. The Tripartite Pact guaranteed assistance if a signatory was attacked by any country not already involved in their conflicts. (This implicitly meant the U.S.) By joining it, Japan simultaneously gained geo-political power, and it sent the unmistakable message any U.S. military intervention risked war on both of her shores—with Germany and Italy on the Atlantic, and with Japan on the Pacific. The Roosevelt administration would not be dissuaded. Believing the American way of life would be endangered if Europe and the Far East fell under military dictatorship, it committed to help the British and 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 to one preparing for war.
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 the President if the United States was going to war. Roosevelt's view was:
"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.' ... ".
Japan's 1940 move into Vichy-controlled Indochina further raised tensions. When combined with its war with China, withdrawal from the League of Nations, alliance with Germany and Italy, and increasing militarization, the move provoked an attempt to restrain Japan economically. The United States embargoed scrap metal shipments to Japan and closed the Panama Canal to Japanese shipping. This particularly hit Japan's economy hard because 74.1% of Japan's scrap iron came from the United States, documented in 1938. Japan's main source for copper was also heavily dependent on American trade; 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 during the course of 1941 in an effort to improve relations. During these negotiations Japan considered withdrawal from most of China and Indochina after Japan and Nationalist China drew up peace terms. Japan would also adopt an independent interpretation of the Tripartite Pact, and would not discriminate in trade provided all other countries reciprocated. However, these compromises in China were rejected by General Tojo, then War Minister. Responding to continuing Japanese aggression in China, the U.S. froze Japanese assets in the U.S. on 26 July 1941 and on 1 August established an embargo on oil and gasoline exports to Japan. The oil embargo was an especially strong response because oil was Japan's most crucial import, and more than 80 percent of Japan's oil at the time came from the United States.
Japanese war planners had long looked south, especially to Brunei for oil and to Malaya for rubber and tin. The Navy was (mistakenly) certain any attempt to seize this region would bring the U.S. into the war,[page needed] but the complete U.S. oil embargo removed any hesitancy. Moreover, any southern operation would be vulnerable to attack from the Philippines, then a U.S. commonwealth, so war with the U.S. seemed necessary in any case. In addition to this, Japan looked to the Dutch East Indies. In the autumn of 1940, Japan requested 3.15 million barrels of oil from the Dutch East Indies, but received a counter offer of only 1.35 million barrels. Therefore, the Japanese were interested in expansion to the East Indies, but Malaya and the Philippines had to have been cleared in order for the Japanese to secure the area. This meant inevitable U.S. intervention.
After the embargoes and the asset freezes, the Japanese Ambassador to Washington, Kichisaburō Nomura, and U.S. Secretary of State Cordell Hull held multiple meetings in order to resolve Japanese-American relations. But no solution could be agreed upon for three key reasons:
The U.S. embargoes gave Japan a sense of urgency. It would either have to agree to Washington's demands or use force to gain access to the resources it needed.
In their final proposal, on 20 November, Japan offered to withdraw their forces from southern Indochina and not to launch any attacks in southeast Asia provided that the U.S., Britain, and the Netherlands ceased aiding China and lifted their sanctions against Japan. The American counterproposal of 26 November (the Hull note) required Japan to evacuate all of China without conditions and conclude non-aggression pacts with Pacific powers.
Part of the Japanese plan for the attack included breaking off negotiations with the United States 30 minutes before the attack began. Diplomats from the Japanese Embassy in Washington, including the Japanese ambassador, Admiral Kichisaburō Nomura, and special representative Saburō Kurusu, had been conducting extended talks with the State Department regarding the U.S. reactions to the Japanese move into French Indochina in the summer.
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, 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.
The Japanese Ambassador asked for an appointment to see the Secretary at 1:00 p.m., but later telephoned and asked that the appointment be postponed to 1:45 as the Ambassador was not quite ready. The Ambassador and Mr. Kurusu arrived at the Department at 2:05 p.m. and were received by the Secretary at 2:20. The Japanese Ambassador stated that he had been instructed to deliver at 1:00 p.m. the document which he handed the Secretary, but that he was sorry that he had been delayed owing to the need of more time to decode the message. The Secretary asked why he had specified one o'clock. The Ambassador replied that he did not know but that that was his instruction. After the Secretary had read two or three pages he asked the Ambassador whether this document was presented under instructions of the Japanese Government. The Ambassador replied that it was. The Secretary as soon as he had finished reading the document turned to the Japanese Ambassador and said:
The Ambassador and Mr. Kurusu then took their leave without making any comment. 
There were Japanese records, admitted into evidence during Congressional hearings on the attack after the War, that 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).
In July 1941, the IJN headquarters informed Hirohito that its reserve bunker oil would be exhausted within two years if a new source was not found. 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. On September 6, 1941, at the second Imperial Conference concerning attacks on the Western colonies in Asia and Hawaii, Japanese leaders met to consider the attack plans prepared by Imperial General Headquarters, one day after the emperor had reprimanded General Hajime Sugiyama, the Chief of the Imperial Japanese Army (IJA) General Staff, about the lack of success in China, and the speculated low chances of victory against the United States, British Empire, and their allies.
Prime Minister Konoe argued for more negotiations and possible concessions to avert war. However, military leaders like Sugiyama, Minister of War General Hideki Tōjō, and Chief of the IJN General Staff Admiral Osami Nagano asserted that 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. Tōjō opined 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; hence, 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 chose Hideki Tōjō 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 the Netherlands, scheduled to start at the beginning of December if an acceptable diplomatic settlement were not achieved before then. The following weeks, the military regime of new Prime Minister Tōjō offered a final deal to the United States. They offered to leave only Indochina, but in return for large American economic aid. On 26 November, the so-called Hull Memorandum rejected the offer and demanded that in addition to them leaving Indochina they must leave China (without Manchoukuo) and agree to an Open Door Policy in the Far East.
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 Kōichi Kido (who advised him to take his time until he was convinced) and Tojo, the Emperor called Shigetarō 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.
In June 1941, German and Italian consulates were closed, and there were suggestions Japan's should be closed, as well. They were not, because they continued to provide valuable information (via MAGIC) and neither President Roosevelt nor Secretary Hull wanted trouble in the Pacific. Had they been closed, however, it is possible Naval General Staff, which had opposed the attack from the outset, would have called it off, since up-to-date information on the location of the Pacific Fleet, on which Yamamoto's plan depended, would no longer have been available.
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 plans for the Pearl Harbor attack 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 preventive 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 U.S. 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 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 aerial 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. Yamamoto's emphasis on destroying the American battleships was in keeping with the Mahanian doctrine shared by all major navies during this period, including the U.S. Navy and Royal Navy.
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, Yamamoto requested Admiral Takijiro Onishi, chief of staff of the Eleventh Air Fleet, study the technical feasibility of an attack against the American base. Onishi gathered together as many facts as possible about Pearl Harbor.
After consulting first with Kosei Maeda, an expert on aerial torpedo warfare, and being told the harbor's shallow waters rendered such an attack almost impossible, Onishi summoned Commander Minoru Genda. After studying the original proposal put forth by Yamamoto, Genda agreed: "the plan is difficult but not impossible". Yamamoto gave the bulk of the planning to Rear Admiral Ryunosuke Kusaka, who was very worried about the area's air defenses. Yamamoto encouraged Kusaka by telling him that "Pearl Harbor is my idea and I need your support." Genda emphasized that it should be carried out early in the morning and in total secrecy, employing an aircraft carrier force and several different types of bombing.
Although attacking the US Pacific Fleet while it was at anchor in Pearl Harbor would achieve surprise, it also carried two distinct disadvantages: The targeted ships would be sunk or damaged in very shallow water, meaning that it would be quite likely that they could be salvaged and possibly returned to duty (as six of the eight battleships eventually were); and most of the crews would survive the attack, since many would be on shore leave or would be rescued from the harbor afterward. Despite these concerns, Yamamoto and Genda pressed ahead.
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.
At several stages during 1941, Japan's military leaders discussed the possibility of launching an invasion to seize the Hawaiian Islands; this would provide Japan with a strategic base to shield its new empire, deny the United States any bases beyond the West coast and further isolate Australia and New Zealand.
Genda, who saw Hawaii as vital for American operations against Japan after war began, believed that Japan must follow any attack on Pearl Harbor with an invasion of Oahu or would lose the war. His country could use Hawaii as a base to threaten the contiguous United States, and perhaps as a negotiating tool for ending the war. He believed that after a successful air attack 10-15,000 men could capture Hawaii, and saw the operation as a precursor or alternative to a Japanese invasion of the Philippines. In September 1941 Commander Yasuji Watanabe of the Combined Fleet staff estimated that two divisions (30,000 men) and 80 ships, in addition to the carrier strike force, could capture the islands. He identified two possible landing sites, near Haleiwa and Kaneohe Bay, and proposed that both be used in an operation that would require up to four weeks with Japanese air superiority.
Although this idea gained some support, it was soon dismissed for several reasons:
With an invasion ruled out, it was agreed that a massive carrier-based airstrike against Pearl Harbor to cripple the American Pacific Fleet would be sufficient. Japanese planners knew that Hawaii, with its strategic location in the Central Pacific, would serve as a critical base from which the United States could extend its military power against Japan; however, as before, the confidence of Japan's leaders that the conflict would be over quickly and that the United States would choose to negotiate a compromise, rather than fight a long, bloody war, overrode this concern. Watanabe's superior, Captain Kameto Kuroshima, who believed that the invasion plan was unrealistic, after the war called his rejection of it the "biggest mistake" of his life.
On November 26, 1941, the day the Hull note was received from United States Secretary of State Cordell Hull, which the Japanese leaders saw as an unproductive and same old proposal, the carrier battle group under the command of then Vice Admiral Chuichi Nagumo, already assembled in Hitokappu Wan in the Kurile Islands, sortied for Hawaii under strict radio silence.
In 1941, Japan was one of the few countries that was capable of carrier aviation. The Kido Butai, the Combined Fleet's main carrier force of six aircraft carriers (at the time, the most powerful carrier force with the greatest concentration of air power in the history of naval warfare), 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:
Japan has decided to open hostilities against the United States, United Kingdom, and the Netherlands early in December...Should it appear certain that Japanese-American negotiations will reach an amicable settlement prior to the commencement of hostilities, it is understood that all elements of the Combined Fleet are to be assembled and returned to their bases in accordance with separate orders. [The Kido Butai will] proceed to the Hawaiian Area with utmost secrecy and, at the outbreak of the war, will launch a resolute surprise attack on and deal a fatal blow to the enemy fleet in the Hawaiian Area. The initial air attack is scheduled at 0330 hours, X Day.
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".
U.S. civil and military intelligence had, amongst them, good information suggesting additional Japanese aggression throughout the summer and fall before the attack. At the time, no reports specifically indicated an attack against Pearl Harbor. Public press reports during summer and fall, including Hawaiian newspapers, contained extensive reports on the growing tension in the Pacific. Late in November, all Pacific commands, including both the Navy and Army in Hawaii, were separately and explicitly warned war with Japan was expected in the very near future, and it was preferred that Japan make the first hostile act as they were apparently preparing to do. It was felt that war would most probably start with attacks in the Far East: the Philippines, Indochina, Thailand, or the Russian Far East. The warnings were not specific to any area, noting only that war with Japan was expected in the near future and all commands should act accordingly. Had any of these warnings produced an active alert status in Hawaii, the attack might have been resisted more effectively, and perhaps resulted in less death and damage. On the other hand, recall of men on shore leave to the ships in harbor might have led to still more being casualties from bombs and torpedoes, or trapped in capsized ships by shut watertight doors (as the attack alert status would have required), or killed (in their obsolete aircraft) by more experienced Japanese aviators. When the attack actually arrived, Pearl Harbor was effectively unprepared: anti-aircraft weapons not manned, most ammunition locked down, anti-submarine measures not implemented (e.g., no torpedo nets in the harbor), combat air patrol not flying, available scouting aircraft not in the air at first light, Air Corps aircraft parked wingtip to wingtip to reduce sabotage risks (not ready to fly at a moment's warning), and so on.
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 air raid against Pearl Harbor in 1932 led by Admiral Harry E. Yarnell had been judged a success and to have caused considerable damage, a finding corroborated in a 1938 war game by Admiral Ernest King.
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.