Events leading to the attack on Pearl Harbor

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 in the 1920s. The expansion of American territories in the Pacific had been a threat to Japan since the 1890s, though the real tension did not begin until the rejection of Japan’s Racial Equality Proposal and the 1931 invasion of Manchuria by Japan. 
Japan’s fear of being colonized and the government’s expansionist policies led to its own Imperialism in Asia and Pacific in order to join the Great Powers, which only constituted of white nations. The Japanese government saw the need to be a colonial power to be modern, therefore, Western. In addition, series of racist laws fanned further resentment in Japan. These laws enforced segregation and barring Japanese (and often Chinese) from citizenship, land ownership and immigration. 
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 (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, at that time an American protectorate, were also a Japanese target. The Japanese military concluded an invasion of the Philippines would provoke an American military response. Rather than seize and fortify the islands, and wait for the inevitable U.S. 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. (Later that same day [December 8, local time], the Japanese indeed launched their invasion of the Philippines).
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 the Hull note and final approval by Emperor Hirohito the order to attack was issued at the beginning of December.
Background to conflict.
Tensions between Japan and the prominent Western countries (the United States, France, 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, promoting a Greater East Asia Co-Prosperity Sphere as part of Japan's alleged "divine right" to unify Asia under Hirohito's rule.
During 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 there. On January 15, 1936, Japan 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. 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 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 each possessed colonies in East and Southeast Asia. Japan's new military power and willingness to use it threatened these 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 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; this implicitly meant the U.S. By joining the pact, Japan gained geopolitical power and sent the unmistakable message that any U.S. military intervention risked war on both of her shores—with Germany and Italy in the Atlantic, and with Japan in the Pacific. The Roosevelt administration would not be dissuaded. 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 power to one preparing for war.
In mid-1940 Roosevelt moved the U.S. Pacific Fleet to Pearl Harbor, Hawaii, to deter Japan. 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. 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 embargoed scrap-metal shipments to Japan and closed the Panama Canal to Japanese shipping. This hit Japan's economy particularly hard because 74.1% of Japan's scrap iron came from the United States in 1938. Also, 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 drawing up peace terms with the Chinese. Japan would also adopt an independent interpretation of the Tripartite Pact, and would not discriminate in trade, provided all other countries reciprocated. However General Tojo, then Japanese War Minister, rejected compromises in China. Responding to Japanese occupation of key airfields in Indochina (July 24) following an agreement between Japan and Vichy France, the U.S. froze Japanese assets on July 26, 1941, and on August 1 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% 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 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 Navy was certain any attempt to seize this region would bring the U.S. into the war, but the complete U.S. oil embargo reduced 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, then a U.S. commonwealth, so war with the U.S. seemed necessary in any case.
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. No solution could be agreed upon for three key reasons:
In their final proposal on November 20, Japan offered to withdraw its forces from southern Indochina and not to launch any attacks in southeast Asia provided the U.S., Britain, and the Netherlands ceased aiding China and lifted their sanctions against Japan. The American counterproposal of November 26 (the Hull note) required Japan to evacuate all of China, unconditionally, and to conclude non-aggression pacts with Pacific powers.
Breaking off negotiations.
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 cypher machine, in a cipher named by U.S. cryptanalysts), with instructions to deliver it to Secretary of State Cordell Hull at . The last part arrived late Saturday night (Washington time), but because of decryption and typing delays, as well as Tokyo's failure to stress the crucial necessity of the timing, embassy personnel did not deliver the message to Secretary Hull until several hours after the attack.
The United States had decrypted the 14th part well before the Japanese managed to, and long before embassy staff composed a clean typed copy. The final part, with its instruction for the time of delivery, had been decoded Saturday night but was not acted upon until the next morning (according to Henry Clausen).
Ambassador Nomura asked for an appointment to see Hull at , but later asked it be postponed to 1:45 as the ambassador was not quite ready. Nomura and Kurusu arrived at and were received by Hull at 2:20. Nomura apologized for the delay in presenting the message. After Hull had read several pages, he asked Nomura whether the document was presented under instructions of the Japanese government; the Ambassador replied it was. After reading the full document, Hull turned to the ambassador and said:
I must say that in all my conversations with you...during the last nine months I have never uttered one word of untruth. This is borne out absolutely by the record. In all my fifty years of public service I have never seen a document that was more crowded with infamous falsehoods and distortions--infamous falsehoods and distortions on a scale so huge that I never imagined until today that any Government on this planet was capable of uttering them.
Japanese records, admitted into evidence during congressional hearings on the attack after the war, established that Japan had not even written a declaration of war until hearing news of the successful attack. The two-line declaration was finally delivered to U.S. ambassador Joseph Grew in Tokyo about ten hours after the completion of the attack. Grew was allowed to transmit it to the United States, where it was received late Monday afternoon (Washington time).
War.
In July 1941, IJN headquarters informed Emperor Hirohito 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 , 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. The summit occurred one day after the emperor had reprimanded General Hajime Sugiyama, chief of the IJA General Staff, about the lack of success in China and the speculated low chances of victory against the United States, the British Empire and their allies.
Prime Minister Konoe argued for more negotiations and possible concessions to avert war. However, military leaders such as Sugiyama, Minister of War General Hideki Tōjō, and chief of the IJN General Staff Fleet Admiral Osami Nagano asserted 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 argued that yielding to the American demand to withdraw troops would wipe out all the gains 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 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 November 5, 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. Over the following weeks, Tōjō's military regime offered a final deal to the United States. They offered to leave only Indochina, but in return for large American economic aid. On November 26, the so-called Hull Memorandum (or Hull Note) rejected the offer and demanded that, in addition to leaving Indochina, the Japanese must leave China (without Manchoukuo) and agree to an Open Door Policy in the Far East.
On November 30, 1941, Prince Takamatsu warned his brother, Hirohito, 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 Tōjō, the Emperor called Shigetarō Shimada and Nagano, who reassured him that 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 U.S. Pacific Fleet at its main forward base at Pearl Harbor, Hawaii.
Intelligence gathering.
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" ) and neither President Franklin D. Roosevelt nor Secretary of State Cordell 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.
Planning.
Expecting war, and seeing an opportunity in the forward basing of the U.S. Pacific Fleet in 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 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. Successful attacks on colonies were judged to depend on successfully dealing with the Pacific Fleet. Planning had long anticipated a battle in Japanese home waters after the U.S. fleet traveled across the Pacific, under attack by submarines and other forces all the way. The U.S. fleet would be defeated in a "decisive battle", as Russia's Baltic Fleet had been in 1905. A surprise attack posed a twofold difficulty compared to longstanding expectations. First, the Pacific Fleet was a formidable force, and would not be easy to defeat or to surprise. Second, Pearl Harbor's shallow waters made using conventional aerial torpedoes ineffective. On the other hand, Hawaii's distance 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 action in the Battle of Taranto, in which 21 obsolete Fairey Swordfish disabled half the "Regia Marina" (Italian Navy). 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 1904. 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 as many facts as possible about Pearl Harbor.
After first consulting 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: "[T]he 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, "Pearl Harbor is my idea and I need your support." Genda emphasized the attack should be carried out early in the morning and in total secrecy, employing an aircraft carrier force and several types of bombing.
Although attacking the U.S. Pacific Fleet anchor would achieve surprise, it also carried two distinct disadvantages. The targeted ships would be sunk or damaged in very shallow water, meaning it would be quite likely that they could be salvaged and possibly returned to duty (as six of the eight battleships eventually were). Also, 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 Kyūshū. Genda chose 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 mountain behind Kagoshima and dove into the city, dodging buildings and smokestacks before dropping to at the piers. Bombardiers released torpedoes at a breakwater some away.
However, even this low-altitude approach would not overcome 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 eventual 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.
Concept of a Japanese invasion of Hawaii.
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 Japan must follow any attack on Pearl Harbor with an invasion of Hawaii or risk losing the war. He viewed Hawaii as a base to threaten the west coast of North America, and perhaps as a negotiating tool for ending the war. He believed, following a successful air attack, 10,000-15,000 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 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 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 a massive carrier-based three wave airstrike against Pearl Harbor to destroy the 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, 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 the invasion plan unrealistic, after the war called his rejection of it the "biggest mistake" of his life.
Strike force.
On November 26, 1941, the day the Hull note (which the Japanese leaders saw as an unproductive and old proposal) was received, the carrier force under the command of Vice Admiral Chuichi Nagumo (already assembled in Hitokappu Wan) sortied for Hawaii under strict radio silence.
In 1941, Japan was one of the few countries 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 (flag), , , , and the newest, and , 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.
Execute order.
On December 1, 1941, after the striking force was "en route", Chief of Staff Nagano gave a verbal directive to the 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 counterattacks by the Americans, and to return to the friendly base in the Marshall Islands, rather than the Home Islands.
Lack of preparation.
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 Harbor 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. In a 1932 fleet problem, a surprise airstrike led by Admiral Harry E. Yarnell had been judged a success and to have caused considerable damage, a finding corroborated in a 1938 exercise by Admiral Ernest King. In October 1941, Lord Louis Mountbatten visited Pearl Harbor. While lecturing American naval officers on Royal Navy tactics against the Germans, an officer asked when and how the United States would enter the war. Mountbatten pointed to Pearl Harbor on a map of the Pacific and said "right here", citing Japan's surprise attack on Port Arthur, and the British attack on Taranto. In Washington he warned Stark about how unprepared the base was against a bomber attack; Stark replied, "I'm afraid that putting some of your recommendations into effect is going to make your visit out there very expensive for the U.S. Navy".
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. While the Japanese "Diplomatic" codes (Purple code) could be read, the current version (JN-25C) of the Japanese "Naval" code (JN-25) which had replaced JN-25B on 4 December 1941 could not be read until May 1942.
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 Japan make the first hostile act. It was felt war would most probably start with attacks in the Far East: the Philippines, Indochina, Thailand, or the Russian Far East; Pearl Harbor was never mentioned as a potential target. 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.
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. 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 three American aircraft carriers (, , and ) 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.