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Ever since the Japanese attack on Pearl Harbor on December 7, 1941, there has been much debate as to how and why the United States had been caught unaware, and how much and when American officials knew of Japanese plans and related topics. Some have argued that various parties high in the US and British governments knew of the attack in advance and may even have let it happen or encouraged it in order to force America into war via the so-called "back door." Evidence supporting this view is taken from quotations and source documents from the time and the release of newer materials. Each one must be looked at individually, and then in context, in order to be a truly legitimate point for or against.

Part of the controversy of the debate centers on the state of documents pertaining to the attack. While conventional wisdom would seem to dictate that all of the Pearl Harbor documents were declassified decades ago, there are now many identified records that have not been released even now in 2006, suggesting a pernicious and purposeful pattern which is deleterious to Pearl Harbor scholarship. Others may not exist as many documents were destroyed early during the war due to fears of an impending Japanese invasion of the Hawaiian Islands, still others are partial, crudely mutilated, or clearly tampered with. (See: Conclusions Section, "Signals Intelligence and Pearl Harbor: The State of the Question" from Intelligence and National Security, Prof. Villa and Dr. Wilford, Volume 21, Number 4, August 2006, pages 520-556).

Examination of information released since the War has revealed that there was considerable intelligence information available to US and other nations' officials. It was the failure to process and use this information effectively that has led some to invoke conspiracy theories rather than a less interesting mix of mistake and circumstance. The US government had nine official inquiries into the attack – the inquiry by Secretary of the Navy Knox (1941), the Roberts Commission (1941–42), the Hart Inquiry (1944), the Army Pearl Harbor Board (1944), the Naval Court of Inquiry (1944), the Hewitt investigation, the Clarke investigation, the Congressional Inquiry (1945–46) and the top-secret inquiry by Secretary Stimson authorized by Congress and carried out by Henry Clausen (the Clausen Inquiry) (1945).

It is important to note that investigators prior to Clausen did not have the security clearance necessary to receive the most sensitive information and that some of those questioned were put in a difficult spot of having to perjure to protect secrets they were charged with and their many denials about their whereabouts the night of December 6, 1941. Clausen reported that even though he had a letter informing witnesses that he had the necessary clearance, he was repeatedly lied to until he produced copies of top secret decrypts, proving that he indeed had the proper clearance.

The top-secret information that Clausen collected was not declassified until the mid-1980s and not published until 1992, when Clausen wrote his own account of the investigation. This has led to the propagation of misinformation as authors refer to older documents that have information that has been superseded and corrected.

Had the US broken Japanese military codes before the attack?

The JN-25 code is one of the most debated portions of Pearl Harbor lore. JN-25 is the US Navy's final designation for the cryptosystem the Japanese Navy sometimes referred to as Naval Code D, five-numeral, 5Num, five-digit, five-figure, AN, AN-1, ... It was an example of the then state of the art and is quite different than modern forms of message encryption in being a code (ie, battleship = 63982) and further being superencrypted with an additive cypher, also in a book. It was based upon the Latin alphabet, likely due to the greater complexity of other forms of Japanese writing. Superencrypted codes of this sort were widely used and can be considered the state of the art in practical military cryptography of the time. It should be noted that this code is very similar to the British Naval "Cypher No. 3" known to have been broken by Germany .

The original JN-25A system used five-digit numbers, divisible by three, giving a total of 33,333 valid code values. To make it harder to crack a code item, 'noise' was overlaid into the transmission in the form of additives (from a large table of five-digit numbers) that had no meaning; they were added to each codegroup to obscure the use of code values. JN-25B superseded the first release of the JN25 system at the start of December of 1940. JN-25B had 55,000 valid codes and while it initially used the same additives, this was soon changed and the code breakers found themselves back to square one.

Over the years, various claims have been made as to the progress of decrypting the code. Lt. John (nickname: "Honest John") Leitweiler, Commander of Station CAST, Corregidor is quoted often as stating in November of 1941 that his staff could “walk right across” the number columns of the coded messages. This, however, refers not to the message itself but to the superencrypting additives and was in relation to how easy a new method for attacking the code made the discovery of additives.

A couple of often sited claims of code breaking are: A 16 November 1941 letter (Navy Department, Philippines Operations Summaries, 3200/1-NSRS) to L.W. Parks (OP-20-GY) sent by Leitweiler, "We have stopped work on the period 1 February to 31 July as we have all we can do to keep up with the current period. We are reading enough current traffic to keep two translators very busy." Another exhibit No. 151 from the Hewitt Inquiry has a copy of the US Navy message OPNAV-242239 'Evaluation of Messages of 26 November 1941' which has in part: '1. Reference (a) advised that Com 16 intercepts were considered most reliable and requested Com 16 to evaluate reports on Japanese naval movements and send despatch to OPNAV, info CINCPAC. Com 16's estimates were more reliable than Com 14's, not only because of better radio interception, but because Com 16 was currently reading messages in the Japanese Fleet Cryptographic System ("5-number code" or "JN25") and was exchanging technical information and translations with the British C. I. Unit at Singapore. McCollum knew this and gave it due consideration when he drafted reference (a).' Whitlock, also at the time, was not even aware that the IJN code movement traffic was being read.

However reading is being defined as being able to see the underlying code groups, not breaking out the messages into plain text.

Additionally, there are no known decrypts of JN-25B traffic with any intelligence value prior to Pearl Harbor. Such breaks as recorded by Holmes and Blair were into the additive tables. The first 100 JN-25 decrypts from all sources in date/time order of translation have been released and are available in the National Archives. The first JN-25B Decrypt was in fact by Station HYPO (Hawaii) on 8 January 1942. (numbered #1 up JN-25B RG38 CNSG Library, Box 22, 3222/82 NA CP) The first 25 decrypts were very short messages or partial decrypts of marginal intelligence value. As Duane Whitlock stated "The reason that not one single JN-25 decrypt made prior to Pearl Harbor has ever been found or declassified is not due to any insidious cover-up . . . it is due quite simply to the fact that no such decrypt ever existed. It simply was not within the realm of our combined cryptologic capability to produce a usable decrypt at that particular juncture."

Detailed month by month progress reports have shown no reason to believe that any messages were fully decrypted before the start of the war. Tallied results for September, October, and November reveal that roughly 3,800 code groups (out of 55,000, about 7%) had been recovered by the time of the attack on Pearl Harbor.

US signals intelligence in 1941 was both impressively advanced and uneven. The US MI8 cryptographic operation in New York City had been shut down by Henry Stimson (Hoover's newly appointed Secretary of State), which provoked its now ex-director, Herbert Yardley, to write a book (The American Black Chamber) about its successes in breaking other nations' crypto traffic. Most countries responded promptly by changing (and generally improving) their ciphers and codes, forcing other nations to start over in reading their signals. The Japanese were no exception. Nevertheless, US cryptanalytic work continued after Stimson's action in two separate efforts by the Army's Signal Intelligence Service (SIS) and the Office of Naval Intelligence's (ONI) crypto group, OP-20-G. Cryptanalytic work was kept secret to such extent, however, that major commands such as the 14th Naval District at Pearl Harbor were prohibited from working on code breaking of the Japanese Naval Operations Code by Admiral Turner and the flow of intercepted information was tightly and capriciously controlled to the point that at times even President Roosevelt did not receive information from code-breaking activities.

By late 1941, those organizations had broken several Japanese ciphers (such as "PA-K2", called Tsu by Japan, and the diplomatic code, dubbed "Purple" by the U.S.), but had made little progress against the Naval JN-25 code.

The break into the PURPLE cipher was a considerable cryptographic triumph and proved quite useful later in the War. It was the highest security Japanese Foreign Office cipher, but prior to Pearl Harbor carried little information about events planned by the Japanese; the military, who were essentially determining policy for Japan, didn't trust the Foreign Office and left it "out of the loop". Unfortunately for the US, the two US crypto groups generally competed rather than cooperated, and distribution of intelligence from the military to US civilian policy-level officials was poorly done (eg, capriciously selected for distribution) and furthermore in a way that prevented any of its recipients from developing a larger sense of the meaning of the decrypts. Along with obsessional security, there was little or no contextual analysis; decrypts were typically provided raw, completely without context.

In addition, there was a perennial shortage of manpower, thanks to penury on one hand and the perception of intelligence as a low-value career path on the other. Furthermore, there were difficulties retaining good intelligence officers and trained linguists; most did not remain in the job for the extended periods needed to become truly professional.

However, from publicly available documents there are other well understood aspects. For example, in SRH-255 "Oral History Interview with Mr. Robert D. Ogg" (still with censored portions), is Captain Irving Newman's (USN retired) expert comment on page 23 " ... just prior to World War II, we had some 700 people engaged in the effort and we were in fact, obviously having some successes. ..." So significant manpower and resources were assigned to the Japanese Naval codes, and that some current traffic was being read, that is, those "successes." Newman's revealing comment is well substantiated by SRH-149 "A Brief History of Communications Intelligence in the United States" (also, still heavily censored). There, on page 6 is a table entitled "COMPLEMENT OF THE NAVY COMMUNICATION INTELLIGENCE ORGANIZATION" showing for January 1941 (Total: 543 people), and for 7 December 1941 (Total: 730 people). Also in SRH-149, on page 19, is another table entitled "ALLOCATION OF NAVY COMINT EFFORT - EARLY DECEMBER 1941" showing in the "Decryption" category, 85% allocated to the "JAP NAVY" and in the "Translation" category, 50% allocated to the "JAP NAVY" - surfeit resources.

The US was also given decrypt codes by Dutch (NEI) intelligence, who were far more advanced in their decryption, and shared all information with allies. If only the US had done likewise... (C.H. Baker, "Nanyo" 1987)

Was the Japanese Kido Butai broadcasting?

As the Kido Butai (Special Attack Force) lead by Admiral Chuichi Nagumo neared Hawaii, there is claimed to have been a flurry of radio traffic that begat warnings to US intelligence and even directly to those with White House connections. For instance, the Matson liner SS Lurline, heading from San Francisco to Hawaii on its regular route, is said to have heard and plotted unusual radio traffic, noting here that Japanese Morse code is very different from International Morse code. That traffic, which is said to have persisted for several days, and noted as coming from a moving source and not shore stations, is further said to have been from the approaching Japanese fleet.

There are several problems with this analysis. Surviving officers from Nagumo's ships claim that there was no radio traffic to have been overheard by anyone; their radio operators had been left in Japan to fake traffic for the benefit of listeners (ie, military intelligence traffic analysts in other countries), and all radio transmitters aboard Nagumo's ships were claimed to have been physically disabled to prevent inadvertent broadcast and subsequent tracking of the attack force. But note also, this mythos is directly countered in Layton, for example: (page 317, " ... same ham-fisted operator ..." for CV AKAGI; and Chapter 21, "Self-Deception" Note No. 19 " ... Did the Japanese Paint Us a 'Picture.' ..." on page 547).

To disobey the radio silence orders would necessarily mean that the location of the entire attack force would have been jeopardized; the personnel reports fit well with this objective situation. Unfortunately, the current whereabouts of either Lurline's log, or reports delivered to the Navy by Lurline's radio operator Leslie Grogan in Hawaii, are not known, so contemporaneous written evidence of what was recorded aboard Lurline is not now available - the original logs "Lurline" surrendered to the USN in Honolulu have gone walk-about. Generally, however, the directions given by Grogan in a recreation of the logbook for the Matson Line are consistent with locations used for radio deception by the Japanese at that time, as some believe.

One suggested example of a Kido Butai transmission is the November 30, 1941 COMSUM14 report in which Captain Joseph Rochefort of Hawaii's code-breaking Station Hypo mentioned that a "tactical" circuit was heard calling "marus" (a term for Japanese commercial vessels. However, the usage SHIRIYA MARU, an IJN tanker and not a member of Japan's merchant fleet, appears in Parts XIII, pages 407, 420, 462-464, and Part XXX, page 2743, 2787, and 2792 in the Hearing before the Joint Committee; further message SRN-116476 refers to two "marus" - so the usage was not definitive). However this is more likely to have been part of the extensive IJN radio deception program to mislead enemy intelligence into believing that the units and commands of the Kido Butai were continuing to exercise off Kyūshū and the Western Inland Sea.

As shown in Prange's "At Dawn We Slept" on page 416, as given by IJN Admiral Toimioka, are identified the oil tankers supporting the Kido Butai: Kyokuto Maru, Kenyo Maru, Kokuyo Maru, Shinkoku Maru, Toho Maru, Nihon Maru, Toei Maru, and Shiriya. Prange also on page 435, regarding HIJMS Shiriya, has "... naval unit designed as an auxiliary to the Pearl Harbor task force ... The skipper of Shiriya was Captain Minour Togo, son of the great Admiral Heihachiro Togo. ..."

Circumstantial evidence

High-Ranking Officials

One quote from Gordon Prange's Pearl Harbor: The Verdict of History is often used to add legitimacy to the notion that the British Government knew in advance that the attack was coming. Oliver Lyttelton, the British Minister of War Production, is quoted on page 35 as saying, " ... Japan was provoked into attacking the Americans at Pearl Harbor. It is a travesty of history ever to say that America was forced into the war. Everyone knows where American sympathies were. It is incorrect to say that America was truly neutral even before America came into the war on an all-out basis."

When considering information like this as a point for or against, the reader must keep in mind questions such as, was this official privy to information about the US government, did he have communications with high-level administration figures such as President Roosevelt or Ambassador Grew, or is this just a strongly held personal opinion?

For instance, one position discussed in author Robert Stinnet's book Day of Deceit suggests that a memorandum prepared by Office of Naval Intelligence Lieutenant Commander Arthur McCollum was central to US Government policy in the immediate pre-war period. The memo suggests that only a direct attack on US interests would sway the American public to favor direct involvement in the European war, specifically in support of the British.

Although the memo was passed to Captains Walter Anderson and Dudley Knox, two of Roosevelt's military advisors on October 7, 1940, there is no evidence available to suggest that Roosevelt ever saw the memo. Nor can it be proved the Roosevelt did not see it. Moreover, although Anderson and Knox offered eight specific plans to aggrieve the Japanese Empire and added "If by these means Japan could be led to commit an overt act of war, so much the better," of the eight "plans" (actions to be taken) offered in the memo only one was ever implemented in any fashion, and there is considerable doubt that the memo was the source of the inspiration for that implementation. Nonetheless, as shown in the Notes section of Stinnett's "Day of Deceit" (paperback edition) for Chapter Two (FDR's Back Door to War), pages 321-322, are notes 7, 8, and 11 which show in fact that all action items are indeed enacted.

ONI, in Washington, is further said to have been aware of Japanese carrier movements. First publicly reported in John Toland’s Infamy, was a Dutch Rear Admiral's claim to have been present at the US Office of Naval Intelligence not long before the 7th. According to Toland, and perhaps entries in J.E. Meijer Ranneft’s diary, Naval Intelligence informed Ranneft of two Japanese carriers. Toland claims the diary states these ships were northwest of Honolulu. Later, Toland claims the diary states the ships were north. However, among other problems with the story, the diary (photo in hardback version, removed in later paperback printings) states “beW. Honolulu.” meaning “Westerly of”. Also none of the witnesses present on those dates support’s Ranneft’s claims, clouding their meaning considerably. Some speculate the diary could have been reference to carriers near the Marshall Islands. Toland has also made other conflicting and incorrect claims about the diary as well.

However, a rather obvious question is generally glossed over in the above. Namely, given the three acknowledged cases of Ogg, Grogan, and Ranneft (and their respective circumstances) - each independent of the others and each occuring at approximately the same time - why was there was no urgent alert sent to the Pacific Fleet to investigate further?

Japanese Intelligence

Japanese intelligence (that is, espionage) efforts against Pearl Harbor included at least two German Abwehr agents. One of them, Otto Kuhn, was a sleeper agent living in Hawaii with his family; he and they were essentially incompetent. The other, Dusko Popov, a Yugoslavian businessman, was thought quite effective by the Abwehr, but was actually a double agent whose loyalty was to the British. He worked for the XX Committee of MI5. In August 1941 he was tasked by the Abwehr with specific questions about Pearl (John Cecil Masterman's book on the Double Cross operation and Prange's Verdict both reproduce it), but the FBI seems to have evaluated the effort as of negligible importance. There has been no report that its existence, or even Popov's availability as a double agent, was passed on to US military intelligence or to civilian policy officials. J. Edgar Hoover dismissed Popov's importance noting that his British codename, Tricycle, was connected with his sexual tastes. In any case, he was not allowed to continue on to Hawaii and to develop more intelligence for the UK and US. Regardless, Prange demonstrates Popov's claim to have provided warning is overblown, and a case that his notorious questionnaire was a product of Abwehr thoroughness.

More to the point, the Japanese did not need Abwehr assistance, having an active consulate in Hawaii which included on its staff an undercover IJN intelligence officer, Takeo Yoshikawa. The consulate had been making reports to IJN Intelligence for years, and Yoshikawa increased the rate of reports after his arrival in Japan. (Sometimes called a "master spy", he was in fact quite young, and his reports not infrequently contained errors.) Pearl Harbor base security was so lax that Yoshikawa, and indeed anyone else, had no difficulty obtaining access to it, even taking the Navy's own harbor tourboat. (Even had he not, hills overlooking the Harbor were perfect for observation or photography, and were not off-limits.) Gossip with taxi drivers is supposed to have been one of his sources. Some of his information, and presumably other material from the Consulate, was hand-delivered to intelligence officers aboard Japanese commercial vessels calling at Hawaii prior to the War; at least one is known to have been deliberately routed to Hawaii for this purpose during the summer. Most, however, seem to have been transmitted to Tokyo, almost certainly via cable. Many of those messages were intercepted and decrypted by the US; most were evaluated as the sort of intelligence gathering all nations routinely do about potential opponents and not as evidence of an attack plan. None of those currently known, including those decrypted after the Attack when there was time to return to those remaining undecrypted, explicitly stated anything about an attack on Pearl; the only exception was a message sent from the Hawaiian Consulate on 6 December, which was not decrypted until after the 7th and was thus moot with regard to any US foreknowledge question. No cable traffic (the usual communication method to/from Tokyo) was intercepted in Hawaii until after David Sarnoff of RCA agreed to assist during a visit to Hawaii immediately before the 7th; such interception was illegal under US law, though it had been going on in New York for some time. Farago's POSTSCRIPT offers the viewpoint of RCA personnel. In the final analysis, illegal co-operation of American cable companies likely changed little or nothing, since intercept stations were picking up some of the traffic, and American intelligence didn't make optimum use of their information, in any case.

Grid Planning

The Japanese had, during the days prior to the attack, asked Yoshikawa to divide the waters of Pearl Harbor into sections, a grid, to save time (and telegraph bills). He could say "six DDs section AA" instead of "six destroyers in the area northwest of Ford Island". Some people have referred to this as the "bomb plot" message. Note that, importantly, while Washington knew beginning the fall of 1941 of the "Kita Messages" or bomb plots - neither Admiral Kimmel or General Short were told. However, in the actual attack, the Japanese Navy did not use the "bomb plot" as the exact locations of the ships in Pearl had never been forwarded to them. However, in Stinnett's book on page 106, is a figure "A. Sketch Used Reporting Enemy Anchoring Formation" obtained from a downed Japanese aircraft on December 7, 1941, so it seems SUZUKI's trip to Hitokappu Wan (as given in SRN 116643 in the National Archives, SRNs - Special Reseach Navy - Japanese Naval messages said to have been translated in 1945/1946), prior to the attack, yielded the precise berthing locations. (A harbor chart reproduced in I-boat Captain, presumably based on Yoshikawa, erroneously lists some of them.)

While the Japanese, in addition to Hawaii, had the Panama Canal Zone and the Philippines under surveillance, Pearl Harbor was the only installation now known to have been divided into a grid pattern. However, examination of an aerial photo of Pearl Harbor at the time of the attack makes it plain a pilot could easily determine the locations of individual ships and their classes without reference to a guide. This Naval Historical Center photo of Pearl Harbor from the air is a clear example. It also handily debunks the claim that Japanese residents of Hawaii chopped hundred-foot-long arrows in nearby sugar cane fields to point the way to Pearl. Even in the absence of any evidence of this claim (none has come to light), it is immediately apparent a hundred-foot arrow would appear negligibly small compared to the much larger capital ships clearly visible in the harbor.

Allied Intelligence

Locally, Naval Intelligence in Hawaii had been tapping telephones at the Japanese Consulate before the 7th, and overheard a most peculiar discussion of flowers in a call to Tokyo (the significance of which is still publicly opaque and which was discounted in Hawaii at the time), but the Navy's tap was discovered and was disconnected in the first week of December. The local FBI field office was informed of neither the tap nor its removal; the local FBI agent in charge later claimed he would have had installed one of his own, had he known the Navy's had been disconnected.

Throughout 1941, the US, Britain, and the Netherlands collected a considerable range of evidence suggesting that Japan was planning some additional military adventure. The Japanese attack on the US in December was essentially a side operation to the main Japanese thrust to the South against Malaya and the Philippines—many more resources, especially Imperial Army resources, were devoted to these attacks as compared to Pearl Harbor. Many in the Japanese military (both Army and Navy) had disagreed with Adm. Isoroku Yamamoto's idea of attacking the US Fleet at Pearl Harbor when it was first proposed in early 1941, and remained reluctant through the Imperial Conferences in September and November which first approved it as policy (allocation of resources, preparation for execution), and then authorized the attack. The Japanese focus on South-East Asia was quite accurately reflected in US intelligence assessments; there were warnings of attacks against Thailand (the Kra Peninsula), Malaya, French Indochina, the Dutch East Indies (Davao-Weigo Line), even Russia.

There are reports of concern at the War Department and in the White House about Japanese plans for the SE Asian region. There had even been a specific claim of a plan for an attack on Pearl Harbor from the Peruvian Ambassador to Japan in early 1941. (The source for this bit of intelligence has been traced to the Ambassador's Chinese cook. It was treated with the skepticism such gossip deserves.) Since not even Yamamoto had yet then decided to even argue for an attack on Pearl Harbor, discounting US Ambassador Grew's report to Washington in early 1941, was quite sensible. Later reports from a Korean labor organization also seem to have been regarded as unlikely, though they may have had better grounding in actual IJN events. There has been uncovered no record of a serious belief or conviction by anyone in US or UK military intelligence, or among US civilian policy officials, prior to the attack, that Pearl Harbor or the US West Coast would be attacked. The so-called "Winds Code" announcing the direction of new hostilities via a broadcast weather 'forecast', remains a curious and confused episode, demonstrating the uncertainty of meaning inherent in most intelligence information, and its handling/mis-handling - and in this case, even uncertainty about the existence of some intelligence information, or of its active removal from official records, especially some years after the event. At most, however, that the Winds system was to be implemented only if the communications between Japan and Washington were cut, and they were never out of touch with their embassy prior to the attack, there was not a need for it. To be noted, however, in Safford's testimony on this topic he states very clearly that London (not Washington) was the addressee of this message. Given this, any talk of intercepted "Winds" messages would appear to be specious. However, there are two very specific issues. Firstly there is Admiral King's endorsement of the Navy Court of Inquiry, page 344, reading in part as: " ... (3) Admiral Kimmel was not informed of the implementation of the "Winds Message". Admiral Stark says he never got this information himself, but it is clear that it did reach Admiral Stark's office. This, together with the handling of other matters of information, indicates lack of efficiency in Admiral Stark's organization. ..." And, secondly, there exists more documentation for the "Winds Execute" - Briggs' Special Research History - SRH-051 (still a very heavily censored document in the US National Archives) - which relates an account of interception of the "Winds Execute", passing the message to ONI, and later, his direct order from Captain Harper (see page 16 of SRH-051) not to testify in support of Safford at the Pearl Harbor hearings.

Advance Warning and Code Breaking

Despite all these fumbles and confusions, in late November 1941, both the US Navy and Army sent war warnings to all Pacific commands. Although these clearly stated the high probability of war with Japan, and instructed recipients to be on alert for war, they did not mention the likelihood of an attack on Pearl Harbor itself, instead focusing on the probability of an attack on the Far East. Washington forwarded none of the raw intelligence it had, and little of its intelligence estimates (after analysis), to Hawaii, and did not solict the views of the local Hawaii commanders, Admiral Husband E. Kimmel and General Walter C. Short about likelihood of war or Hawaiian special concerns. Washington's war warning messages have also been criticised by some (e.g., the US Army Pearl Harbor Board - "Do/Don't Messages") as containing "conflicting and imprecise" language. Since the Army was officially responsible for the security of the Hawaiian facilities, and so of the Navy's ships in port, Army actions are of particular interest. Short reported to Washington that he had increased his alert level (but his earlier change in meaning for those levels was not understood in Washington and led to misunderstanding there about what he was really doing). In addition, Short's main concern seems to have been sabotage from fifth columnists, which probably accounts for his orders that Army Air Corps planes be parked close together close to the center of the airfields. There seems to have been no increased urgency in the Army about getting its radar equipment emplaced and properly integrated in the year it had been available and operational in Hawaii before the Attack; leisurely training continued and the early warning center was left minimally (preliminarily) staffed. Anti-aircraft guns remained in a state of low readiness. Long range patrol aircraft were not used effectively, remaining on a peacetime maintenance and use schedule.

The mistakes made by both Hawaii and Washington meant little was done to prepare for an air attack. Inter-service rivalries between Kimmel and Short did not improve the situation. Particularly, most intelligence information was sent to Kimmel, assuming he would relay it to Short, and vice versa. Hawaii did not have a PURPLE machine, so it was dependent on Washington from intelligence from that (somewhat limited) source. However, since Short had no liaison with Kimmel's intelligence staff, he was usually left out of the loop. Henry Clausen reported that the war warnings could not be more precise because Washington could not risk Japan guessing the U.S. was reading important parts of their traffic (ie, most importantly PURPLE despite the fact that there was no tactical or strategic info in that traffic). Additionally, Clausen says that military men of Kimmel and Short's seniority and background should have understood the significance of the warnings and that they should have been more vigilant than they were, as for instance in scouting plane flights from Hawaii, which were partial at best in the period before the attack. All other Pacific commands took appropriate measures for their situations. Like most commentators, Clausen ignores what the "war warnings" (and their context) explitely warn, though indistinctly against: DC, with more complete intelligence than any field command, expected an attack anywhere on this list (Pearl Harbor not included). He, like most, also ignores what Kimmel, Short, and Admiral Claude C. Bloch (Commander, Fourteenth Naval District, responsible for Hawaii) did do. They took precautions against sabotage, widely expected as precursor to war, and reported their preparations, although in Short's case, the most important of the three, Washington thought he had ordered something other than he was actually doing. They did not anticipate an air attack; despite good reason to have done so, no one did so explicitly.

One major point often omitted from the debate (though Costello covers it thoroughly) is that MacArthur, unlike Kimmel, had complete access to decrypted PURPLE traffic (indeed, Stinnet quotes Whitlock to that effect), and was caught unprepared and with planes on the ground nevertheless, nine hours after the Pearl Harbor attack. Caidin and Blair also raise the issue.

One of the main considerations making an attack against Pearl Harbor unimaginable was the depth of Pearl Harbor — generally less than 40 feet. Depths of less than 150 feet had been (in the USN and others) widely believed insufficient for torpedo attack; at the time, torpedoes dropped from planes dove deeply before attaining running depth and in water that was not deep enough (like Pearl Harbor) would contact the bottom, detonating or embedding themselves in harbor mud. The British had proved that torpedoes, modified for shallow water, could be effective in their attack on the Regia Marina at Taranto on November 11, 1940 (Battle of Taranto). The US Navy discussed this new development as can be seen in a July 1941 memo, but as Taranto was about 75 feet deep and Pearl less than 40, it was not considered that the British attack method was relevant to an attack at Pearl Harbor. The RN (Royal Navy) had used Swordfish torpedo planes (fabric-covered biplanes), and their slow speed was part of the reason the Taranto attack succeeded. The IJN (Imperial Japanese Navy) no longer had any planes like them, thus other methods were required. The Japanese independently developed shallow water torpedo modifications ("Thunder Fish") during the planning and training for the attack in 1941. Wooden fins had been added to the tail and roll "flippers" kept the torpedo upright once in the water. The fins kept the torpedo's nose level in the air and broke off when they hit the water. The level flight of the torpedoes helped keep them from diving too deeply and embedding themselves in the mud (but, despite these modifications, several torpedoes did indeed dive to the bottom and several remain unaccounted for). This very simple modification of the Japanese torpedoes was not anticipated by the USN, and Admiral Bloch (commander of the 14th Naval District at Pearl) never pushed to install torpedo nets at Pearl. Due to the shallow anchorage (which even today must be constantly dredged to allow sufficient depth for the passage of capital ships), the installation of torpedo nets would have severely restricted the mobility of vessels entering, leaving and maneuvering in the harbor.

AWOL Carriers

Another piece of supposed circumstantial evidence is that US aircraft carriers were out of port at the time of the attack. This has been alleged by some as evidence of advance knowledge of the attack: the carriers were supposedly away so as to save them (the most valuable ships) from the attack. In fact, the two carriers then operating from Pearl Harbor, Enterprise and Lexington, were on missions to deliver fighters to Wake and Midway Islands. These assignments sent the carriers west, toward Japan and the Japanese Navy, lightly escorted. At the time of the attack, the Enterprise was about 200 miles west of Pearl Harbor, heading back to Pearl Harbor. In fact, the Enterprise was scheduled to be back on December 6th, but was delayed by weather. A rescheduling had the Enterprise's estimated time of arrival as 7:00, almost an hour before the attack, but the ship was also unable to make this schedule. Furthermore, at the time, aircraft carriers were classified as scouting elements of the fleet, not capital ships; the most important vessels in naval planning even as late as Pearl Harbor were battleships. As the only large warships available in the fleet, carriers became the Navy's capital ships during the gap between Pearl Harbor and the return of the battleships to duty in early 1943.

American Immediate Reaction

Closer to the moment of the attack, the attacking planes were detected and tracked as they approached by an Army radar installation being operated that morning as a training exercise. The Opana Point radar station, operated by two enlisted men (Pvts. Lockard and Elliot) plotted the approaching force, and their relief team plotted them returning to the carriers. These radar returns were thought, by the ill-trained junior officer (Lt. Kermit A. Tyler) in charge at the barely-operational information center at Pearl Harbor, to be a flight of American bombers expected from the mainland. In fact those bombers did arrive, in the middle of the attack. Additionally, submarines were sighted and attacked outside the Pearl Harbor entrance a few hours before the planes arrived and at least one was sunk—all before the planes came within bombing range. It has been argued that this failure to follow up saved the USS Enterprise. If she had been directed to investigate the source of the planes, she might have run into the six-carrier Japanese strike force. Detection of one of the subs by the USS Ward might have provided enough notice to disperse aircraft and fly off reconnaisance, except, yet again, reactions of the duty officers were tardy.

War with Germany

Conspiracy theorists repeatedly point to the fact that Roosevelt 'desperately' wanted the United States to intervene in the war against Germany. This is undoubtedly true, if perhaps over-dramaticly phrased, but a basic understanding of the political situation of 1941 precludes this as good evidence that Roosevelt 'allowed' the Pearl Harbor attack. No attack by Japan was a guarantee the US would declare war on Germany. American public anger would be directed at Japan, not Germany, in the case of a Japanese attack. The Tripartite Pact alliance between Germany and Japan called for one to aid the other in case of attack, and so Japan could not reasonably claim that America had attacked Japan in the event of a Japanese attack on the US. In addition, Germany had been at war with the U.K. since 1939, and with the U.S.S.R. since June of 1941 without Japanese assistance. There was a serious if low-level naval war going on in the Atlantic between Germany and the US Navy in the summer of 1941, in any case. Nevertheless, it was only Hitler's, unforced by treaty, declaration of war several days after the Pearl Harbor attack that brought the US into the European war.

Henry Clausen and Bruce Lee's "Pearl Harbor: Final Judgement" contains some interesting information on what intelligence Winston Churchill possessed prior to the attack. On page 367 in the Appendix is a PURPLE message, dated 29 November 1941, from the Japanese Ambassador in Berlin to the Japanese Foreign Minister in Tokyo. A closing paragraph reads: " ... He (sic RIBBENTROP) also said that if JAPAN were to go to war with AMERICA, GERMANY would, of course, join in immediately, and HITLER's intention was that there should be absolutely no question of GERMANY making a separate peace with ENGLAND. ..." Churchill (having full access to PURPLE, while Hawaii commanders - Short and Kimmel - did not) was well aware of this message, noting it in red ink (See David Irving's "Churchill's War" - Volume Two, page 220.)

Controversy

Each of these points remains controversial.

Conflicting stories regarding FOIA (Freedom of Information Act) requests for the source materials used, e.g., Sheet Number 94644, or materials available at the National Archives are also common among the debate. However much information was said to be automatically destroyed under a destruction of classified information policy. Note that various authors have continued to bring classified Pearl Harbor materials into the sunlight via the FOIA mechanism.

Recognize that Sheet No. 94644 derives uniquely from its reference in the FOIA-released Japanese Navy Movement Reports of Station H in November 1941. Entries for 28 November 1941 have several more items of interest, each being a "code movement" with specific message details given by associated Sheet Numbers. Examples are: Sheet No. 94069 has information on "KASUGA MARU" - this being hand-written (KASUGA MARU became CVE TAIYO); Sheet No. 94630 is associated for the IJN oiler SHIRIYA; and finally for Sheet No. 94644 there is another hand-written remark "FAF using AKAGI xtmr" (i.e., First Air Fleet using AKAGI's transmitter"). From Pelletier (see Pelletier, Cryptolog, Summer 1992, page 5), it is well known that the "code movement" reports were largely readable at the time. The three documents mentioned above (i.e., Sheet Numbers 94069, 94630, and 94644) constitute a prima facia example of identified materials which yet, even after decades and numerous FOIA requests, have not been declassified fully and made available to the public.

Another issue in the debate is the fact that neither Admiral Kimmel or General Short ever faced court martial. It is alleged that this was to avoid disclosing information conspirators would not want to see made public. When asked, Kimmel replied as (See Brownlow, pages 178-179): "Will historians know more later? Kimmel's reply to this was: ' ... I'll tell you what I believe. I think that most of the incriminating records have been destroyed. ... I doubt if the truth will ever emerge.' ..."

Some Remaining Classified Materials Relating to Pearl Harbor

A. All trans-oceanic telephone conversations (transcripts and recordings) between President Roosevelt and Churchill during November (of interest is especially that for November 26th) and December 1941 (the first week in particular).

B. Full and "true copy" RDF (Radio Direction Finder) records from all of the US Navy's Pacific facilties for November 1941 and December 1941.

C. Complete and "true copy" of all raw intercepts of IJN Operations traffic for 1941, including all communications information (e.g., Frequencies, Call Signs, TOI, Originator, Action/Information, ..., etc.). SRN-115376 and SRN-116643 are of particular interest for some.

Further reading

It has been suggested that this section be split out into another article titled People engaged in the Pearl Harbor advance-knowledge debate. (Discuss)

• Roberta Wohlstetter, Pearl Harbor: Warning and Decision. Stanford Univ, Press, 1962. Bancroft Prize Winner which The American Historical Review judged admirable both as history and analysis. She argues that divided and confused command responsibilities, and poor liaison, among the Army, Fleet, and Naval Base commands in and around Pearl were a major factor in the unreadiness for an air attack.
• John Toland, Infamy: Pearl Harbor and Its Aftermath (Berkeley, reissue edition 1991 - the so-called tenth investigation) is an account of the various investigations of the US failure to be prepared at Pearl Harbor. He claims that Roosevelt had advance knowledge of the attack. Some of Toland's sources have since said that his interpretation of their experiences is incorrect.
• Donald Grey Brownlow, The Accused: The Ordeal of Rear Admiral Husband Edward Kimmel, USN Vantage Press, New York, 1968. Dedicated to Admiral Kimmel's wife, Dorothy Kinkaid Kimmel, this modest book is considered one of the earliest "Pearl Harbor" text and contains materials based on extensive interviews and personal letters with many of the Pearl Harbor principles.
• James Rusbridger and Eric Nave, Betrayal at Pearl Harbor: How Churchill Lured Roosevelt into WWII (Summit, 1991) which posits that while the Americans couldn't read the Japanese naval code (JN-25), the British could, and Churchill deliberately withheld warning because the UK needed US help. Sir Nave was an Australian cryptographer whose diaries were used in writing this book; he later distanced himself from its content. A check against them has made clear that some of the charges Rusbridger makes here are unsupported by Nave's diaries of the time. The Appendix is of interest to note, especially a true copy of SRN-116741.
• Henry C. Clausen and Bruce Lee, Pearl Harbor: Final Judgement (Crown, 1992) is Clausen's own account of his investigation. It contains previously-unpublished information that was still classified in the mid 1980s. He reports that previously-published books are based on deliberate lies and omissions (due to the top-secret nature of the information). Clausen concludes that the notion that Roosevelt knew of the attack in advance and allowed it to happen is unfounded.
• Ladislas Farago, The Broken Seal (Bantan Books Edition - paperback, first printing: 1968). Contains an account of Lurline's "interception" and the "disappearing logbook".
• Edwin T. Layton (with Pineau and Costello), And I Was There - Pearl Harbor and Midway - Breaking the Secrets (William Morrow and Company, 1985) Layton was Kimmel's Intelligence Chief.
• Robert Stinnett, Day Of Deceit: The Truth About FDR and Pearl Harbor (Free Press, 2001) is a recent examination which begins claims that Roosevelt deliberately steered Japan into war with America. Stinnett's understanding of materials (cryptography/cryptanalysis) has been heavly criticized. Stinnett's effort has given the public many new materials beyond the McCollum action memorandum. Afterword section of the paperback edition has newer materials.
• L. S. Howeth, USN (Retired), History of Communications - Electronics in the United States Navy, GPO (Government Printing Office), Washington, DC, 1963. A very good source of material, especially on equipment and capabilities. Note: Chapter XV comments on identifying transmitters by their unique "tone" and a Navy radio operator's court-martial, conviction resulting.
• Frederick D. Parker, Pearl Harbor Revisited - United States Navy Communications Intelligence 1924-1941 from the Center for Cryptologic History, National Security Agency, 1944 - now available online . Of note are the SRNs given, and there to especially highlight are: (a) the clear distinction the IJN made between shortware versus longwave radio transmissions (see SRN-115397 on page 59)
• Mark Emerson Willey, Pearl Harbor - Mother of All Conspiracies (self-published in 1999, now available as a paperback). Has a detailed timeline of events leading to Pearl Harbor, discusses codebreaking and radio silence, with Appendix A highlighting the many contextural differences as evidenced in SRH-406 - Pre-Pearl Harbor Japanese Naval Dispatches. Known for having some of the more "outlandish" claims.
• A. J. Barker, Pearl Harbor - Battle Book No. 10 (Ballantine's Illustrated History of World War II from 1969). An interesting approach to the sequence of events, rare photographs, having as military consultant/historian the well-known Captain Sir Basil Liddell Hart. Claims others are mistaken as the belief of Lurline's radioman, based on an inadequate grasp of naval communications. Note that the highly experienced radioman with decades of martime service, Leslie Grogan, was a reserve officer in the US Navy having an extensive naval communications background from World War I.
• Stephen Budiansky, Battle of Wits - The Complete Story of Codebreaking in World War II, (Free Press, 2000). A look at some of the early machines designed/used in codebreaking during World War II. Uncovered a vast amount of detailed information regarding JN-25.
• Michael V. Gannon, Pearl Harbor Betrayed - The True Story of a Man and a Nation under Attack (Henry Holt and Company, 2001). Includes letter written by Admiral Kimmel but never sent - "You betrayed the officers and men of the Fleet by not giving them a fighting chance for their lives and you betrayed the Navy in not taking responsibility for your actions; you ..." This letter was intended for Admiral Stark.
• Gordon W. Prange, with Donald W. Goldstein and Katherine V. Dillon, At Dawn We Slept (1981), Verdict of History, Pearl Harbor Papers, Miracle at Midway Very outdated and well known as having many factual errors. See Layton, pages 495-526, for a critique of Prange (and associates) approach.
• John Prados, Combined Fleet Decoded - The Secret History of American Intelligence and the Japanese Navy in World War II (Random House, 1995). Pages 167-172 has more on the "Winds" Message, and on pages 698-699 is a recounting the recovery of the CA NICHI papers by US Navy divers from the USS CHANTICLEER in Manila Bay (last two photographs prior to page 423).
• David Kahn, The Codebreakers - The Story of Secret Writing (The MacMillian Company, 1967). A seminal text for the Pearl Harbor topic and codes in general.
• Fred B. Wrixon, Codes, Ciphers & Other Cryptic & Clandestine Communication: Making & Breaking of Secret Messages from Hieroglyphs to the Internet (Black Dog and Leventhal Publishers, 1998). A text with many examples - and on page 104 and page 114, are descriptions of the 1943 BRUSA Agreement and 1947 UKUSA Agreement respectively. If you wondered why no pre-Pearl Harbor Japanese Naval raw intercepts has been declassified, these Agreements might pertain.
• Timothy Wilford, Pearl Harbor Redefined: USN Radio Intelligence in 1941, (University Press of America, 2001); from his award winning Masters thesis in History from the University of Ottawa - the thesis is available online with additional materials not included in the book, e.g., the Appendix materials. Presents newer material recently declassified on radio silence, codebreaking, e.g., RFP (Radio Finger-Printing) "Fundamental Ripple" displays.
• Philip H. Jacobsen Pearl Harbor: Radio Officer Leslie Grogan of the SS Lurline and his Misidentified Signals (Cryptologia April 2005) Details errors, and conflicting stories within the works of Villa, Wilford, Stinnett, Toland, and Farago. Also puts forward the missing report of Leslie Grogan dated Dec. 10 1941 titled "Record for Posterity" and compares this with the 26 year old "remembrances" within Farago's "The Broken Seal". Jacobsen comes the conclusion what Grogan heard were from Japanese commercial ships sending routine plain language radio messages in their specialized Kata Kana telegraphic code.
• Philip H. Jacobsen Radio Silence and Radio Deception: Secrecy Insurance for the Pearl Harbor Strike Force (Intelligence and National Security, Vol. 19, No.4 Winter 2004) Author reviews the works of Robert Stinnett and most notably the works of Timothy Wilford regarding radio silence; noted for gross suppositions and flawed logic.
• John C. Zimmerman Pearl Harbor Revisionism: Robert Stinnett's Day of Deceit (Intelligence and National Security, Vol 17, No.2 Summer 2002) Of note, after publication of article, Stinnett would not provide author information supporting claim of 129 radio silence violations.
• History of GYP-1 General History of OP-20-3-GYP; Activities and Accomplishments of GY-1 During 1941, 1942 and 1943, RG38 CNSG Library, Box 115, 570/197 NA CP "JN-25 has no part to play in the story of Pearl Harbor".
• Duane L. Whitlock, The Silent War Against the Japanese Navy available online from the Corregidor Historical Society. Between June 1939 and December 1941 Washington did decrypt a few JN-25 messages, but they provided little insight into the current intelligence picture.
• Caidin, Martin. Ragged, Rugged Warriors. Bantam, 1979. Raises the issue of why MacArthur was unprepared.
• Costello, John. Days of Infamy. Pocket Books hardback, 1994. Covers the issue of why MacArthur was unprepared in detail, including mention of access to intelligence.
• Orita, Zenji, IJN, with Harrington, Joseph. I-boat Captain. Canoga Park, CA: Major Books, 1976.
• G. Edward Griffin

(Those who change this information are encouraged to discuss edits in discussion)

External links

• Did Roosevelt know in advance about the attack on Pearl Harbor yet say nothing? — The Straight Dope, Straight Dope Science Advisory Board, February 28, 2001
• The Independent Institute: Pearl Harbor Archive — Mostly a Stinnett site, but also has Pearl Harbor articles, debates, interviews, transcripts, book reviews, books, and Pearl Harbor documents
• The National Defense Authorization Act (where it is noted that available intelligence regarding an impending attack was not conveyed to the American commanders at Pearl Harbor;page 121, section 546).
• Too Late for Pearl Harbor — An article detailing the amount of Japanese code that was readable to allies before the attack, refuting key points in Stinnett's "Day of Deceit." Shown as a red herring non-starter after work of NSA historians were highlighted, in particular NSA's "some" versus Budiansky's "none" or "zero."
• Communism at Pearl Harbor — An article proposing that the Russians maneuvered the US into war
• An Interview with Robert Stinnett and WWII Vet O'Kelly McCluskey.
• Foreknowledge of Pearl Harbor? No!: The story of the U. S. Navy's efforts on JN-25B — Excellent in depth article illustrating the problems with Stinnett and Wilford's claims regarding JN-25.
• The Myths of Pearl Harbor — Very good site debunking claims of advance knowledge of the attack.
• Pearl Harbor - Mother of All Conspiracies Pro conspiracy site.
• The Great Pacific War — 16 years before Pearl Harbor attack, an English naval expert Hector Bywater uncannily prophesied in detail the war in the Pacific.
• Pearl Harbor In the Wake of the Prophet — It prophesied the Pacific War in detail, not only its operational modes, but its inevitable result.
• Pearl Harbor: The Controversy Continues Top-notch article on foreknowledge as well as steps to provoke Japan

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• Military history of the United States 1900-1999
• Asia and the Pacific 1941-42
• Conspiracy theories
• Espionage
• Attack on Pearl Harbor