Ever since the Japanese attack there has been debate as to how and why the United States had been caught off guard, and how much and when American officials knew of Japanese plans for an attack.
Several writers, including journalist Robert Stinnett and former United States Navy Rear Admiral Robert A. Theobald, have argued that various parties high in the U.S. 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 "back door." Evidence supporting this view is taken from quotations and source documents from the time and the release of newer materials.
Examination of information released since the War has revealed there was considerable intelligence information available to US and other nations' officials. Rather than attribute the lack of preparedness at the base to failure to process, it has been argued that the U.S. must have had some degree of advanced knowledge of the attack.
All ten reported incompetence, underestimation and misapprehension of Japanese capabilities & intentions, problems resulting from excessive secrecy about crypto, and lack of adequate manpower for intelligence (analysis, collection, processing, ...).
It is important to note that investigators prior to Clausen did not have the security clearance necessary to receive the most sensitive information, and some of those questioned were put in a difficult spot of having to lie (even under oath) to protect secrets they were charged with. Clausen reported that even though he had a letter from Secretary Stimson informing witnesses he had the necessary clearances to require their cooperation, he was repeatedly lied to until he produced copies of top secret decrypts, thus proving he indeed had the proper clearance.
The Stimson report to Congress (based on Clausen's work) was limited due to secrecy concerns (largely about crypto); a more complete account was not publicly available until the mid-1980s and not published until 1992, with Clausen's own account of the investigation. Reaction to the 1991 publication has varied; some regard it as a valuable addition to understanding, others note that (a) he didn't speak to General Walter Short, Army commander at Pearl Harbor during the attack, and (somewhat prematurely) (b) his investigation was "notoriously unreliable" in several aspects.
Nevertheless, U.S. cryptanalytic work continued after Stimson's action in two separate efforts: the Army's Signal Intelligence Service (SIS) and the Navy's Office of Naval Intelligence (ONI) crypto group, OP-20-G. Cryptanalytic work was kept secret to such an extent, however, commands such as the 14th Naval District at Pearl Harbor were prohibited from working on codebreaking by Admiral Kelly Turner as a consequence of the bureaucratic infighting in Washington.
By late 1941, those organizations had broken several Japanese ciphers, such as J19 and PA-K2, called Tsu and Oite respectively by the Japanese. The highest security diplomatic code, dubbed PURPLE by the U.S., had been broken, but American cryptanalysts had made little progress against the IJN's current D code (called JN-25 by the U.S.)
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. Translators were over-worked, cryptanalysts in short supply, staff generally stressed. Furthermore, there were difficulties retaining good intelligence officers and trained linguists; most did not remain on the job for the extended periods necessary to become truly professional. For career reasons, nearly all wanted to return to more standard assignments. However, concerning the manning levels, "... just prior to World War II, we had some 700 people engaged in the effort and we were, in fact, obviously having some successes. Of these, 85% were tasked to decryption and 50% to translation efforts against IJN codes. The nature and degree of these successes has led to great confusion among non-specialists. Furthermore, OP-20-GY "analysts relied as much on summary reports as on the actual intercepted messages.
The U.S. was also given decrypted messages by Dutch (NEI) intelligence, who like the others in the British-Dutch-U.S. agreement to share the cryptographic load, shared information with allies. The U.S. refused to do likewise. This was, at least in part, due to fears of compromise; sharing even between Navy and Army was restricted.
In any case, the eventual flow of intercepted and decrypted information was tightly and capriciously controlled to the point at times even President Roosevelt did not receive information from code-breaking activities. (This was in part due to fears of compromise as a result of poor security, after a memo dealing with MAGIC was tossed in the wastebasket of Brigadier General Edwin M. (Pa) Watson, the President's military aide.)
Most unfortunately, to date not all PURPLE messages have been released. This was noted as long ago as the Joint Congressional Hearings during the "Magic" testimony. This known fact is often missed, as well as other curious items, for example, the Hearing's questions regarding the missing 25 pages from the Roberts Commission report. Blanket or un-qualified statements on what decoded "Magic" messages revealed are, therefore, pre-mature.
Superenciphered codes of this sort were widely used and were the state of the art in practical cryptography of the time. JN-25 was very similar in principle to the British "Naval Cypher No. 3", known to have been broken by Germany during WWII.
Once it was realized what sort of cryptosystem JN-25 was, the cryptanalytic approach was known. Stinnett, in fact, notes the existence of a USN handbook for attacks on such a system, produced by OP-20-G. Even so, breaking it was not easy in actual practice. It took much effort and time, not least in accumulating sufficient depth in intercepted messages prior to the outbreak of hostilities when IJN radio traffic increased abruptly and substantially; prior to December 7, 1941 IJN radio traffic was limited, since the IJN played only a minor role in the war against China and therefore was only rarely required to send radio messages in their highest level crypto system. (As well, interception of IJN traffic off China would have been at best spotty.) Rather oddly however, the official history of GYP-1 shows nearly 45,000 IJN messages intercepted during the period from 1 June 1941 until 4 December 1941. Thus, most Japanese encrypted broadcast military radio traffic was Army traffic associated with the land operations in China.
Breaking a superencrypted cipher like JN-25 was a three-step process: (a) determining the "indicator" method to establish the starting point within the additive cipher, (b) stripping away the superencryption to expose the bare code, and then (c) breaking the code itself. When JN-25 was first detected and recognized, such intercepted messages as were interceptable were collected (at assorted intercept stations around the Pacific by the Navy) in an attempt to accumulate sufficient depth to attempt to strip away the superencryption. Success at doing so was termed by the cryptographers a 'break' into the system. Such a break did not produce a cleartext version of the intercepted message. Only after breaking the underlying code (another difficult process) would the message be available, and even then its meaning — in an intelligence sense — might be less than fully clear.
When a new edition was released, the cryptographers were forced to start again. The original JN-25A system replaced the 'Blue' code (as Americans called it), and used five-digit numbers, each divisible by three (and so usable as a quick, and somewhat reliable, error check, as well as something of a 'crib' to cryptanalysts), giving a total of 33,334 legal code values. To make it harder to crack a code value, meaningless additives (from a large table or book of five-digit numbers) were added arithmetically to each five-digit cipher. JN-25B superseded the first release of JN-25 at the start of December 1940. JN-25B had 55,000 valid words, and while it initially used the same additive list, this was soon changed and the cryptanalysts found themselves entirely blacked out again.
Over the years, various claims have been made as to the progress made decrypting this system, and arguments made over when it was readable (in whole or part). Lt. "Honest John" Leitweiler, Commander of Station CAST, the Philippines, stated in November 1941 that his staff could “walk right across” the number columns of the coded messages. He is frequently quoted in support of claims JN-25 was then mostly readable. This comment, however, refers not to the message itself but to the superenciphering additives and referred to the ease of attacking the code using a new method for discovery of additive values.
The 16 November 1941 letter to L.W. Parks (OP-20-GY) sent by Leitweiler states, "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 document, 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 dispatch 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. Arthur McCollum knew this and gave it due consideration when he drafted the McCollum memo. Duane L. Whitlock, (traffic analyst at CAST), was not aware before the attack that IJN movement traffic code was being read. "Reading" in this context means being able to see the underlying code groups, not breaking out the messages into plaintext. The Hewitt Inquiry document also states, "The "5 numeral system" (JN-25B) yielded no information which would arouse even a suspicion of the Pearl Harbor raid, either before or afterward."
The claim that no pre-attack IJN message expressly mentioned Pearl Harbor is perhaps true. Similarly are the claims no PURPLE traffic likewise pointed to Pearl Harbor. To note, however, in both instances neither all traffic (IJN nor PURPLE) from these pre-attack intercepts have been declassified and released to the public domain. Hence, any such claims are now indeterminate, pending a fuller accounting.
Additionally, no decrypts have come to light of JN-25B traffic, and importantly identified as such, with any intelligence value prior to Pearl Harbor. Such breaks as recorded by authors W.J. Holmes and Clay Blair, Jr., were into the additive tables, which was a required first step of two. 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 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 any JN-25B messages were fully decrypted before the start of the war. Tallied results for September, October, and November reveal roughly 3,800 code groups (out of 55,000, about 7%) had been recovered by the time of the attack on Pearl Harbor.
According to a 1942 Japanese after action report "In order to keep strict radio silence, through steps such as taking off fuses in the circuit, holding and sealing the keys were taken. During the operation, the strict radio silence was perfectly carried out.... The Kido Butai used the radio instruments for the first time on the day of the attack since they had been fixed at the base approximately twenty days before and proved they worked well. Paper flaps had been inserted between key points of some transmitters on board Akagi to keep the strictest radio silence..." Commander Genda, who helped plan the attack stated, "We kept absolute Radio Silence". For about two weeks before the attack, 31 ships "used flag and light signals" (semaphore and blinker), perfectly usable with task force ships remaining in line of sight. Kazuiyoshi Koichi, the Communications Officer for Hiei, dismantled vital transmitter parts and kept them in a box that he used as a pillow to prevent Hiei from making any radio transmissions until the attack on Pearl Harbor had commenced. Former Lieutenant Commander Chuichi Yoshoka, communications officer aboard the flagship, Akagi, said he did not recall any ship dispatching a radio message before the attack. Furthermore, Captain Kijiro, in charge of the three Kido Butai submarines, stated nothing of interest occurred during the transit to Hawaiian waters. Vice Admiral Ryunosuke Kusaka stated, "It is needless to say that the strictest radio silence was ordered to be maintained in every ship of the Task Force. To keep radio silence was easy to say, but not so easy to maintain." There is nothing in the Japanese logs or after action report that shows that radio silence was broken until after the attack. Which Kusaka worried about when briefly broken on the way home.
An often debated subject is also the appendix to the war-initiating operational order. The message of 25 November 41 from CinCCombined sent to All Flagships from Admiral Yamamoto stated, "Ships of the Combined Fleet will observe radio communications procedure as follows: 1. Except in extreme emergency the Main Force and its attached force will cease communicating. 2. Other forces are at the discretion of their respective commanders. 3. Supply ships, repair ships, hospital ships, etc., will report directly to parties concerned." Furthermore, "In accordance with this Imperial Operational Order, the CinC of the Combined Fleet issued his operational order, ... The Task Force then drew up its own operational order, which was given for the first time to the whole force at Hitokappu Bay. ..." However also stated is under the appendix to that document, paragraph four where the especially secret Strike Force was specifically directed to "maintain strict radio silence from the time of their departure from the Inland Sea. Their communications will be handled entirely on the general broadcast communications net."
Since to disobey the radio silence orders would mean the location of the entire attack force would have been exposed, the personnel reports fit well with the requirements of the objective. Unfortunately, the current whereabouts of either Lurline's log, or the reports delivered to the Navy or Coast Guard by Lurline's radio operator Leslie Grogan in Hawaii, are not known, so contemporaneously written evidence of what was recorded aboard Lurline is not now available - the original logs Lurline, surrendered to Lt. Cmdr. George W. Pease, 14th Naval District in Honolulu, have disappeared. Grogan commented on a signal source "moving" eastward in the North Pacific over several days as shown via "relative bearings" which then "bunched up" and stopped moving.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 (from the homeland) at the time. A recently discovered missing report by Grogan, dated 10 December 1941 and titled "Record for Posterity", also does not support claims of Kido Butai broadcasting.
Another technique confirmed to be used by the Japanese was the practice of radio deception. Susumu Ishiguru, intelligence and communications officer for Carrier Division Two, stated, "Every day false communications emanated from Kyushu at the same time and same wavelength as during the training period." This gave Rochefort the idea First Air Fleet remained in Home Waters for routine training. The ships left their own regular wireless operators to carry on "routine" radio traffic. Captain Sadatoshi Tomioka stated, "The main force in the Inland Sea and the land-based air units carried out deceptive communications to indicate the carriers were training in the Kyushu area." The main Japanese naval bases (Yokosuka, Kure, and Sasebo) all engaged in considerable radio deception. By analyzing DF bearings (when such bearings were obtained) from Navy directional finder stations, claimed breaks of radio silence can be accounted for, and by plotting the bearings directions, they match up with the various locations previously mentioned, instead of the actual locations of those ships on the relevant dates.
Rochefort claims there were no dummy messages used anytime throughout 1941 and that there was no effort by the Japanese to use serious deception. He also stated, when asked after the attack just how he knew where Akagi was, he recognized her "same ham-fisted" radio operators. However, the critical DF tracked radio transmissions have clearly shown bearings that could have not come from the strike force. Emissions monitored from CAST or CAST's report Akagi was off Okinawa on 8 December 1941, are examples, though some transmissions continue to be debated.
One suggested example of a Kido Butai transmission is the November 30, 1941 COMSUM14 report in which Commander Joseph Rochefort, commanding HYPO, mentioned a "tactical" circuit heard calling "marus". (a term often used for commercial vessels or non-combat units). Further, the perspective of U.S. naval intelligence at the time was, "... The significance of the term, 'tactical circuit' is that the vessel itself, that is AKAGI, was using its own radio to call up and work directly the other vessels rather than work them through shore stations via the broadcast method which was the common practice in Japanese communications. The working of the AKAGI with the Marus, indicated that she was making arrangements for fuel or some administrative function, since a carrier would rarely address a maru.
Another "eye witness viewpoint" akin to Beatty's is provided by Roosevelt's administrative assistant at the time of Pearl Harbor, Jonathan Daniels; it is the telling comment about FDR's reaction to the attack - "The blow was heavier than he had hoped it would necessarily be. ... But the risks paid off; even the loss was worth the price. ..."
"Ten days before the Attack on Pearl Harbor", Henry L. Stimson, United States Secretary of War at the time "entered in his diary the famous and much-argued statement - that he had met with President Roosevelt to discuss the evidence of impending hostilities with Japan, and the question was 'how we should maneuver them [the Japanese] into the position of firing the first shot without allowing too much danger to ourselves.'" "
Mark Parillo, in his essay The United States in the Pacific, wrote that "[t]hese theories tend to founder on the logic of the situation. Had Roosevelt and other members of his administration known of the attack in advance, they would have been foolish to sacrifice one of the major instruments needed to win the war just to get the United States into it.
One quote is often used to add legitimacy to the notion the British Government knew in advance the attack was coming. Oliver Lyttelton, the British Minister of War Production, said, "... 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. If Britain, did, indeed know and chose to conceal, "withholding this vital intelligence only ran the risk of losing American trust", and with it any further American aid, which would be reduced after the attack in any event.
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 U.S. government? Did he have communications with high-level administration figures such as President Roosevelt or Ambassador Joseph Grew? Is this just a strongly held personal opinion? Or were there measures justifying this view?
Lend-Lease, enacted in March 1941, informally declared the end of American neutrality in favor of the Allies by agreeing to supply Allied nations with war materials. The signing of the bill into existence, because the materials would be used to combat the Axis powers, made the United States a de facto hostile and opened her to future attack.
Furthermore, one position discussed in author Robert Stinnett's Day of Deceit suggests a memorandum prepared by Office of Naval Intelligence Lieutenant Commander Arthur McCollum was central to U.S. policy in the immediate pre-war period. The memo suggests only a direct attack on U.S. interests would sway the American public (or Congress) to favor direct involvement in the European war, specifically in support of the British. An attack by Japan would not, could not, do that, as history would prove.
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 Roosevelt ever saw it, nor any he did not. 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 the memo was the inspiration. Nonetheless, as shown in Day of Deceit Stinnett claims all action items were implemented.
ONI 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 ONI not long before the 7th. According to Toland, entries in J.E. Meijer Ranneft’s diary made on December 2nd and on December 6th, show that ONI informed Ranneft about two Japanese carriers. Toland claims the diary places these ships northwest of Honolulu. Later, Toland claims the diary states the ships were north. However the diary clearly states beW meaning “Westerly" of Honolulu, Hawaii. So the documentary evidence cuts Toland's own ground from under him. Also none of the witnesses present on those dates support Ranneft’s claims, clouding their actual significance considerably. David Kahn's review of Toland's book (The NY Review of Books May 27, 1982) and Zimmerman (IaNS Vol. 17, No 2) suggested 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 during lectures at IHR.
The diary states at 02:00 (6-12-41) Turner fears a sudden Japanese attack on Manila. At 14:00 the diary states "Everyone present on O.N.I. I speak to Director Admiral Wilkinson, Captain MacCollum, Lt. Cdr. Kramer...They show me – on my request – the place of the 2 carriers (see 2–12–41) West of Honolulu. I ask what the idea is of these carriers on that place. The answer was: ‘perhaps in connection with Japanese rapports [sic] on eventual American actions’. There is not one of ours who speaks about a possible air attack on Honolulu. I myself did not think of it because I believed everyone on Honolulu to be 100% on the alert, as everyone here on O.N.I. There prevails a tense state of mind at O.N.I." These diary entries are provided (in Dutch) in the photo section in George Victor's The Pearl Harbor Myth: Rethinking the Unthinkable.
Main article : McCollum memo
On October 7, 1940, Lieutenant Commander Arthur McCollum of the Office of Naval Intelligence submitted a memo to Navy Captains Walter Anderson and Dudley Knox, which details eight actions which might have the effect of provoking Japan into attacking the United States. The memo remained classified until 1994. Sections 9 and 10 of the memo are cited as the "Smoking Gun", and a primary thesis of an influential book (Stinnett's, see further reading) suggests that it was central to the high level conspiracy to lure the Japanese into an attack. Evidence that the memo or derivative works actually reached President Roosevelt, senior administration officials, or the highest levels of US Navy command, is largely conjectural.
From section 10 of the memo:
If by these means Japan could be led to commit an overt act of war, so much the better. At all events we must be fully prepared to accept the threat of war. -- A. H. McCollum
Japanese intelligence (espionage) efforts against Pearl Harbor included at least two 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 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 Abwehr with specific questions about Pearl (Masterman's The Double-Cross System 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 its existence, or even Popov's availability as a double agent, was passed on to U.S. military intelligence or to civilian policy officials. J. Edgar Hoover dismissed Popov's importance, noting (without basis) his British codename, Tricycle, was connected with his sexual tastes. In any case, he was not allowed to continue to Hawaii and to develop more intelligence for the UK and U.S. Regardless, Prange demonstrates Popov's claim to have provided warning is overblown, and makes a case the 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 eventually 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 from 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, indeed anyone, 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 IJN 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 (the usual communication method to/from Tokyo). Many of those messages were intercepted and decrypted by the U.S.; most were evaluated as the sort of intelligence gathering all nations routinely do about potential opponents, and not as evidence of an active attack plan. None of those currently known, including those decrypted after the attack when there was finally 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 questions of U.S. foreknowledge. No cable traffic 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 U.S. law, though it had been going on sub rosa in New York for some time. Farago's postscript offers a viewpoint from RCA personnel. In the final analysis, illegal co-operation of American cable companies likely changed little or nothing, since radio intercept stations were picking up some of the consular traffic anyway, and American intelligence failed to make optimum use of their information in any case.
Locally, Naval Intelligence in Hawaii had been tapping telephones at the Japanese Consulate before the 7th, and, among much routine traffic, 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 U.S., Britain, and the Netherlands collected a considerable range of evidence suggesting Japan was planning some additional military adventure. The Japanese attack on the U.S. 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 Admiral Isoroku Yamamoto's idea of attacking the U.S. Fleet at Pearl Harbor when it was first proposed in early 1941, and remained reluctant after the Navy approved planning and training for an attack beginning in spring 1941, and through the highest level 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 U.S. intelligence assessments; there were warnings of attacks against Thailand (the Kra Peninsula), Malaya, French Indochina, the Dutch East Indies (Davao-Weigo Line), the Philippines, even Russia. Pearl Harbor was not mentioned.
The U.S. Navy was aware of the traditional planning of the Imperial Japanese Navy for a war with the U.S., as maintained throughout the 1930s and into the 1940s. The Japanese made no secret of it, and in the 1930s American radio intelligence gave U.S. war planners considerable insight in Japanese naval exercises. These plans presumed there would be a large decisive battle between Japanese and U.S. battleships, but this would be fought near Japan, after the numerical superiority of the U.S. Pacific Fleet (assured by the Washington Naval Treaty, and still taken as given) was whittled down by primarily night attacks by light forces, such as destroyers and submarines. This strategy expected the Japanese fleet to take a defensive posture, awaiting US attack, and it was confirmed by the Japanese Navy staff only three weeks before Pearl Harbour . In the 1920s, the decisive battle was supposed to happen near the Ryukyu islands; in 1940 it was expected to occur in the central Pacific, near the Marshall islands. War Plan Orange reflected this in its own planning for an advance across the Pacific. Yamamoto's decision to shift the focus of the confrontation with the U.S. as far east as Pearl Harbour, and to use his aircraft carriers to cripple the American battleships, was a radical enough departure from previous doctrine to leave analysts in the dark.
There had 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 skepticism, and properly so, given the now known nascent state of planning for the attack at the time.) Since Yamamoto had not 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 (in Hawaii or DC or elsewhere), or among US civilian policy officials, prior to the attack, that Pearl Harbor or the US West Coast was on a target list.
There are two problems with this evaluation. First, there is Admiral King's endorsement of the Navy Court of Inquiry, page 344, reading in part, " ... (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 some lack of efficiency in Admiral Stark's organization. ..." Second, there exists more documentation for the "Winds Execute which relates an account of interception of the "Winds Execute", passing the message to ONI, and later, his direct order from Captain Harper not to testify in support of Safford at one of the Pearl Harbor hearings. Also see RG457 SRH-255 Archives II College Park MD where Ogg's statements during an interview by Captain Irwin G. Newman were considerably different from Toland's attributions and amplified by authors Stinnett and Wilford, and where neither Ogg or Newman ever mention the Winds code topic.
Since the Army was officially responsible for the security of the Pearl Harbor facilities and Hawaiian defense generally, and so of the Navy's ships while in port, Army actions are of particular interest. Short reported to Washington 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 was sabotage from fifth columnists, which accounts for his orders that Army Air Corps planes be parked close together near the center of the airfields. There seems to have been no increased Army urgency about getting its existing radar equipment properly integrated with the local command and control in the year it had been available and operational in Hawaii before the Attack. Leisurely radar training continued and the recently organized early warning center was left minimally staffed. Anti-aircraft guns remained in a state of low readiness, with ammunition in secured lockers. Neither Army long range bombers nor Navy PBYs were used effectively, remaining on a peacetime maintenance and use schedule. In Short's defense, it should be noted he had training responsibilities to meet, and the best patrol aircraft, B-17s and B-24s, were in demand in the Philippines and Britain, both of which had higher priority.
The mistakes made by both Hawaii and Washington meant little was done to prepare for 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; this assumption was honored mostly in the breach. Hawaii did not have a PURPLE cipher machine (although, by agreement at the highest levels between US and UK cryptographic establishments, four had been delivered to the British by October 1941), so Hawaii remained dependent on Washington for intelligence from that (militarily limited) source. However, since Short had no liaison with Kimmel's intelligence staff, he was usually left out of the loop. Henry Clausen reported 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 (i.e., most importantly PURPLE, despite the fact there was no known tactical or strategic info in that traffic).
Additionally, Clausen claims military men of Kimmel and Short's seniority and background should have understood the significance of the warnings, and 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 just 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) explicitly warn, though indistinctly, against. Washington, with more complete intelligence than any field command, expected an attack anywhere on a list of possible locations (Pearl Harbor not among them), and since the Japanese were already committed to Thailand, it seems to have been expected another major operation by them was impossible. Clausen, like most, also ignores what actions Kimmel, Short, and Admiral Claude C. Bloch (Commander, Fourteenth Naval District, responsible for Naval facilities in Hawaii) actually took. They took precautions against sabotage, widely expected as a precursor to war, and reported their preparations. The Hawaiian commanders 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 the Philippines where MacArthur, unlike Kimmel or Short, had complete access to decrypted PURPLE traffic (indeed, Stinnet quotes Whitlock to that effect), and was nonetheless caught unprepared and with all planes on the ground nevertheless, nine hours after the Pearl Harbor attack. Caidin and Blair also raise the issue.
Although it has been argued that there was sufficient intelligence at the time to give commanders at Pearl Harbor a greater level of alert, some factors may take on unambiguous meaning not clear at the time, lost in what Roberta Wohlstetter in her masterful examination of the situation called "noise", "scattered amid the dross of many thousands of other intelligence bits, some of which just as convincingly pointed to a Japanese attack on the Panama Canal.
Not all in the US Navy shared this opinion. A mock air attack on Pearl Harbor during war games in the 1930s was judged to have been a success. Shortly after taking office, Navy Secretary Knox wrote an overview memo which specifically noted the possibility of an attack at Pearl Harbor. Neither observation led to formal policy recommendations to forestall such an attack, however.
The British proved torpedoes, modified for shallow water, could be effective in their attack on the Regia Marina at Taranto on November 11, 1940. The US Navy discussed this new development as can be seen in a June 1941 memo, as Taranto was about deep and Pearl less than 40. It was not considered the British attack method was relevant to a torpedo attack at Pearl.
The Royal Navy had used Swordfish torpedo planes, and their low speed was part of the reason the Taranto attack succeeded. The Imperial Japanese Navy no longer had any similar planes, so they had to develop other methods, both hardware and delivery technique. They independently developed shallow water torpedo modifications (called "Thunder Fish") during the planning and training for the attack in 1941. Wooden fins were added to the tail and anti-roll "flippers" kept the torpedo upright once in the water. The fins kept the torpedo's nose level in the air and broke off on entering the water. The flatter "flight" trajectory helped keep them from diving so deeply as to encounter bottom mud (but, despite these modifications, some Japanese torpedoes did indeed reach the bottom and several remain unaccounted for). These simple modifications were not anticipated by the USN, and Admiral Bloch (commander of the Pearl Harbor Naval District) didn't push to install torpedo nets or baffles at Pearl. Nor, it seems, did anyone else. Practical considerations also were an influence. Due to the shallow anchorage (which continues to require regular dredging), installation of torpedo nets would have severely restricted the mobility of vessels entering, leaving, and maneuvering in the harbor.
Kimmel and his staff testified regarding torpedo nets, booms, ...: "(m) Fact XV ... The decision not to install baffles appears to have been made by the Navy Department.. That is, in Washington, DC, rather than in Hawaii.
In fact, the two carriers then operating with the Pacific Fleet, Enterprise and Lexington, were on missions to deliver fighters to Wake and Midway Islands, which were intended in part to protect the route used by planes (including B-17s) bound for the Philippines. (The third, Saratoga, was in routine refit in Puget Sound, at the Bremerton shipyard.) At the time of the attack, Enterprise was about 200 miles west of Pearl Harbor, heading back. In fact, Enterprise had been scheduled to be back on December 6th, but was delayed by weather. A new arrival estimate, put her arrival at Pearl around 7:00AM, almost an hour before the attack, but she was also unable to make this schedule.
Furthermore, at the time, aircraft carriers were classified as fleet scouting elements, and hence relatively expendable. They were not capital ships. The most important vessels in naval planning even as late as Pearl Harbor were battleships (per the Mahan doctrine followed by both the U.S. and Japanese navies at the time). However, as the only major strike power remaining available in the Pacific fleet, carriers became the Navy's most important ships only following the attack.
One question to consider here is that, at the time, naval establishments all over the world regarded battleships, not carriers, as the most powerful and significant elements naval power. So, had the US wanted to preserve its key assets from an invited attack, it would almost certainly have focused on protecting battleships, not carriers.
After the attack, the search for the attack force was concentrated south of Pearl Harbor, continuing the confusion and ineffectiveness of the American response.
Theorists challenging the traditional view that Pearl Harbor was a surprise repeatedly note Roosevelt wanted (though did not say so officially) the United States to intervene in the war against Germany. A basic understanding of the political situation of 1941 precludes such an understanding as reasonable evidence Roosevelt knew of, allowed, or even invited, the Pearl Harbor attack.
First, no attack by Japan on the U.S. was a guarantee the U.S. would declare war on Germany. In such a case, American public anger would be directed at Japan, not Germany, just as happened. The Tripartite Pact (Germany, Italy, Japan) called for each to aid another in case of attack; Japan could not reasonably claim America had attacked Japan if it attacked first. For instance, Germany had been at war with the UK since 1939, and with the U.S.S.R. since June 1941 without Japanese assistance. There was a serious, if low-level, naval war going on in the Atlantic between Germany and the U.S. Navy in the summer of 1941, in any case. Nevertheless, it was only Hitler's declaration of war, unforced by the Axis treaty, several days after the Pearl Harbor attack, that brought the U.S. into the European war.
Clausen and Lee's Pearl Harbor: Final Judgement contains some interesting information on the intelligence available to Roosevelt and Churchill 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 Tokyo. A closing paragraph reads, " ... He (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. ..." According to David Irving, Churchill (having full access to PURPLE traffic) was well aware of this message, noting it in red ink. While theorists challenging the conventional view that the Attack on Pearl Harbor was a surprise treat this as a guarantee to join after Japan's attack, it can as easily be taken as a guarantee to come to Japan's aid, as Germany had done for Italy in Libya.
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 has been said to have been automatically destroyed under a destruction of classified information policy during the war itself. Note various authors have nevertheless continued to bring classified Pearl Harbor materials to light via FOIA.
For instance, Sheet No. 94644 derives 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 "movement code" message (indicating ship movements or movement orders), with specific details given by associated Sheet Numbers. Examples are: Sheet No. 94069 has information on "KASUGA MARU" - this being hand-written (Kasuga Maru was later converted to CVE Taiyo); Sheet No. 94630 is associated with IJN oiler Shiriya; and finally for Sheet No. 94644 there is another hand-written remark "FAF using Akagi xtmr" (First Air Fleet using Akagi's transmitter). It is known the movement reports were largely readable at the time.
These three documents (Sheet Numbers 94069, 94630, and 94644) are examples of materials which yet, even after decades and numerous specific FOIA requests, have not been declassified fully and made available to the public. Sheet Number 94644, for example, noted as coming from Akagi's transmitter and as being a "movement code" report, would have likely contained a reported position.
Another issue in the debate is the fact neither Admiral Kimmel nor General Short ever faced court martial. It is alleged this was to avoid disclosing information showing the U.S. had advanced knowledge of the attack. When asked, "Will historians know more later?", Kimmel replied, "' ... 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.' ... It is equally, probably more, likely this was done to avoid disclosing the fact Japanese codes were being read, given there was a war on.