Pearl Harbor advance-knowledge conspiracy theory

One perspective is given by Rear Admiral Frank Edmund Beatty Jr., who at the time of the Pearl Harbor attack was an aide to the Secretary of the Navy Frank Knox and was very close to President Franklin D. Roosevelt's inner circle, remarked that:

Prior to December 7, it was evident even to me... that we were pushing Japan into a corner. I believed that it was the desire of President Roosevelt, and Prime Minister Churchill that we get into the war, as they felt the Allies could not win without us and all our efforts to cause the Germans to declare war on us failed; the conditions we imposed upon Japan—to get out of China, for example—were so severe that we knew that nation could not accept them. We were forcing her so severely that we could have known that she would react toward the United States. All her preparations in a military way — and we knew their over-all import — pointed that way.[26]

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 a 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. ..."[27]

"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.'"[28] However Stimson, in reviewing his diary after the war, recalled that the commanders at Pearl Harbor had been warned of the possibility of attack, and that the poor state of readiness that the attack had revealed was a surprise to him:

[Yet] General Short had been told the two essential facts: 1) a war with Japan is threatening, 2) hostile action by Japan is possible at any moment. Given these two facts, both of which were stated without equivocation in the message of Nov. 27, the outpost commander should be on the alert to make his fight ... To cluster his airplanes in such groups and positions that in an emergency they could not take the air for several hours, and to keep his antiaircraft ammunition so stored that it could not be promptly and immediately available, and to use his best reconnaissance system, radar, only for a very small fraction of the day and night, in my opinion betrayed a misconception of his real duty which was almost beyond belief. ...[29]

Robert Stinnett's Day of Deceit suggests a memorandum prepared by Commander McCollum was central to U.S. policy in the immediate pre-war period. Stinnett claims 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, aid Britain. 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 to suggest Roosevelt ever saw it, while Stinnett's claims of evidence he did is nonexistent.[30] 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, many if not all were implemented, but there is considerable doubt the McCollom memo was the inspiration. Nonetheless, in Day of Deceit Stinnett claims all action items were implemented.[31] Yet there were numerous instances of members of the Roosevelt Administration insisting on not provoking Japan. Mark Parillo, in his essay The United States in the Pacific, wrote, "[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."[32] Furthermore, on 5 November 1941, in a joint memo, Stark, CNO, and Marshall, Army Chief of Staff, warned, "if Japan be defeated and Germany remain undefeated, decision will still not have been reached.... War between the United States and Japan should be avoided...."[33] Additionally, in a 21 November 1941 memo, Brigadier Leonard T. Gerow, head of Army War Plans, stated, "one of our present major objectives [is] the avoidance of war with Japan...[and to] insure continuance of material assistance to the British."[34] He concluded, "[I]t is of grave importance to our war effort in Europe..."[34] Furthermore, Churchill himself, in a 15 May 1940 telegram, said he hoped a U.S. commitment to aid Britain would "quiet" Japan, following with a 4 October message requesting a USN courtesy visit to Singapore aimed at "preventing the spreading of the war"[35] And Stark's own Plan Dog expressly stated, "Any strength that we might send to the Far East would...reduce the force of our blows against Germany..."[36] Roosevelt could scarcely have been ignorant of Stark's views, and war with Japan was clearly contrary to Roosevelt's express wish to aid Britain.

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."[37] How this demonstrates anything with regard to Japan is unclear. Rather, it refers to other aid to Britain. 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. In addition, Roosevelt authorized a so-called Neutrality Patrol, which would protect the merchantmen of one nation, namely Britain, from attack by another, Germany. This made shipping legitimate target of attack by submarine.[38] Furthermore, Roosevelt ordered U.S. destroyers to report U-boats, then later authorized them to "shoot on sight". This made the U.S. a de facto belligerent. None was the act of a disinterested neutral, while all are unquestionably of assistance to Britain.

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? If Britain, did, indeed know and chose to conceal, "withholding this vital intelligence only ran the risk of losing American trust",[39] and with it any further American aid, which would be reduced after the attack in any event.

There is also a claim, first asserted in Toland's Infamy, that ONI knew about Japanese carrier movements. Toland cited entries from the diary of Rear Admiral J. E. Meijer Ranneft of the Dutch Navy for 2 December and 6 December. Ranneft attended briefings at ONI on these dates. According to Toland, Ranneft wrote that he was told by ONI that two Japanese carriers were northwest of Honolulu. However, the diary uses the Dutch abbreviation beW, meaning "westerly", contradicting Toland's claim. Nor did any other persons present at the briefings report hearing Toland's version. In their reviews of Infamy, David Kahn[40] and John C. Zimmerman[41] suggested Ranneft's reference was to carriers near the Marshall Islands. Toland has made other conflicting and incorrect claims about the diary during lectures at the Holocaust denial organization the Institute for Historical Review.

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.[42]

CBS correspondent Edward R. Murrow had a dinner appointment at the White House on 7 December. Because of the attack he and his wife only ate with Mrs. Roosevelt, but the president asked Murrow to stay afterwards. As he waited outside the Oval Office, Murrow observed government and military officials entering and leaving. He wrote after the war:[43]

There was ample opportunity to observe at close range the bearing and expression of Mr. Stimson, Colonel Knox, and Secretary Hull. If they were not surprised by the news from Pearl Harbor, then that group of elderly men were putting on a performance which would have excited the admiration of any experienced actor. … It may be that the degree of the disaster had appalled them and that they had known for some time…. But I could not believe it then and I cannot do so now. There was amazement and anger written large on most of the faces.[43]

One historian has written, however, that when Murrow met Roosevelt with William J. Donovan of the OSS that night, while the magnitude of the destruction at Pearl Harbor horrified the president, Roosevelt seemed slightly less surprised by the attack than the other men. According to Murrow, the president told him, "Maybe you think [the attack] didn't surprise us!" He said later, "I believed him", and thought that he might have been asked to stay as a witness. When allegations of Roosevelt's foreknowledge appeared after the war, John Gunther asked Murrow about the meeting. Murrow reportedly responded the full story would pay for his son's college education and "if you think I'm going to give it to you, you're out of your mind". Murrow did not write the story, however, before his death.[43]

The McCollum memo

On October 7, 1940, Lieutenant Commander Arthur H. McCollum of the Office of Naval Intelligence submitted a memo to Navy Captains Walter S. 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 and contains the notable line, "If by these means Japan could be led to commit an overt act of war, so much the better."

Sections 9 and 10 of the memo are said by Gore Vidal to be the "smoking gun" revealed in Stinnett's book, suggesting it was central to the high level plan to lure the Japanese into an attack. Evidence the memo or derivative works actually reached President Roosevelt, senior administration officials, or the highest levels of U.S. Navy command, is circumstantial, at best.

U.S. propaganda poster calling for revenge for the Pearl Harbor attack.

Theorists challenging the traditional view that Pearl Harbor was a surprise repeatedly note that Roosevelt wanted the U.S. to intervene in the war against Germany, though he did not say so officially. A basic understanding of the political situation of 1941 precludes any possibility the public wanted war. Thomas Fleming argued President Roosevelt wished for Germany or Japan to strike the first blow, but did not expect the United States to be hit as severely as it was in the attack on Pearl Harbor.[44]

An attack by Japan on the U.S. could not guarantee the U.S. would declare war on Germany.[45] After such an attack, 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 defense; Japan could not reasonably claim America had attacked Japan if she struck first.[46] For instance, Germany had been at war with the UK since 1939, and with the USSR since June 1941, without Japanese assistance. There had been a serious, if low-level, naval war going on in the Atlantic between Germany and the U.S. since summer of 1941, as well.[47] Nevertheless, it was only Hitler's declaration of war on 11 December, unforced by treaty, that brought the U.S. into the European war.

Clausen and Lee's Pearl Harbor: Final Judgement reproduces 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. ..."[48]

While theorists challenging the conventional view that the attack 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.

The Japanese code dubbed "Purple", which was used by the Foreign Office and only for diplomatic (but not for military) messages, was broken by Army cryptographers in 1940. A 14-part message using this code, sent from Japan to its embassy in Washington, was decoded in Washington on 6 and 7 December. The message, which made plain the Japanese intention to break off diplomatic relations with the United States, was to be delivered by the Japanese ambassador at 1 p.m. Washington time (dawn in the Pacific). Colonel Rufus S. Bratton, then serving as an aide to Marshall, took this to mean that the Japanese intended to attack at dawn somewhere in the Pacific. Marshall ordered a warning message sent to American bases in the area, including Hawaii. Due to atmospheric transmission conditions the message was sent out via Western Union over its undersea cable rather than over the military radio channels; the message was not received until the attack was already underway.[58]

The claim no pre-attack IJN message expressly mentioned Pearl Harbor is perhaps true. The claims that no Purple traffic pointed to Pearl Harbor may also be true, as the Foreign Office was not well thought of by the military and, during this period was routinely excluded from sensitive or secret material including war planning. It is also possible any such intercepts were not translated until after the attack, or indeed, after the war ended; some messages were not.[59] In both instances, all traffic from these pre-attack intercepts has not yet 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 with any intelligence value prior to Pearl Harbor, and certainly no such has been identified. Such breaks as recorded by authors W. J. Holmes and Clay Blair Jr., were into the additive tables, which was a required second step of three (see above). 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 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."[60]

The JN-25 superencrypted code, and its cryptanalysis by the US, is one of the most debated portions of Pearl Harbor lore. JN-25 is the U.S. Navy's last of several names for the cryptosystem of the Imperial Japanese Navy, sometimes referred to as Naval Code D.[61] Other names used for it include five-numeral, 5Num, five-digit, five-figure, AN (JN-25 Able), and AN-1 (JN-25 Baker), and so on.[62]

Superenciphered codes of this sort were widely used and were the state of the art in practical cryptography at the time. JN-25 was very similar in principle to the British "Naval Cypher No. 3", known to have been broken by Germany during World War II.[63]

Once it was realized what sort of cryptosystem JN-25 was, how to attempt breaking into it was known. Stinnett notes the existence of a USN handbook for attacks on such a system, produced by OP-20-G. Even so, breaking any such code was not easy in actual practice. It took much effort and time, not least in accumulating sufficient 'cryptanalytic depth' in intercepted messages prior to the outbreak of hostilities when IJN radio traffic increased abruptly and substantially; prior to 7 December 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 whatever the highest level crypto system might have been. (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, none of which used IJN cryptography. [64]

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 always produce a cleartext version of the intercepted message; only a break in the third phase could do so. 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 element. 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 locked 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" Leitwiler,[65] 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[66] to L.W. Parks (OP-20-GY) sent by Leitwiler 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 (Memoranda from Captain L. F. Safford) from the Hewitt Inquiry[67] has a copy of the U.S. 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 Japanese-to-English translations[68] with the British unit (the Far East Combined Bureau) then at Singapore. Lt. Cdr. Arthur H. McCollum was aware of this, and it may have been part of his thinking when he drafted the McCollum memo. Duane L. Whitlock, traffic analyst at CAST,[69] was not aware before the attack 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 usable plaintext.[70] 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."

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. In all, the U.S. intercepted 26,581 messages in naval or related systems, not counting PURPLE, between September and December 1941 alone.[71]

So convinced were U.S. Navy planners Japan could only stage a single operation at a time,[72] after intercepts indicated a Japanese buildup for operations in the Dutch East Indies, for more than two weeks (between 1 November and 17 November), no JN-25 message not relating to that expected operation was even examined for intelligence value.[73]

There are claims that, as the Kido Butai (the Striking Force) steamed toward Hawaii, radio signals were detected that alerted U.S. intelligence to the imminent attack. For instance, the Matson liner SS Lurline, heading from San Francisco to Hawaii on its regular route, is said to have heard and plotted, via "relative bearings", unusual radio traffic in a telegraphic code very different from International Morse[79] which persisted for several days, and came from signal source(s) moving in an easterly direction, not from shore stations—possibly the approaching Japanese fleet. There are numerous Morse Code standards including those for Japanese, Korean, Arabic, Hebrew, Russian, and Greek. To the experienced radio operator, each has a unique and identifiable pattern. For example, kana, International Morse, and "Continental" Morse all have a specific rhythmic sound to the "dit" and "dah" combinations. This is how Lurline's radiomen, Leslie Grogan, a U.S. Navy reserve officer in naval communications, and with decades of maritime service in the Pacific[80] identified the mooted signal source as Japanese and not, say, Russian.

There are several problems with this analysis. Surviving officers from the Japanese ships state there was no radio traffic to have been overheard by anyone: their radio operators had been left in Japan to send fake traffic, and all radio transmitters aboard the ships (even those in the airplanes) were physically disabled to prevent any inadvertent or unauthorized broadcast.[81]

The Kido Butai was constantly receiving intelligence and diplomatic updates.[82] Regardless of whether the Kido Butai broke radio silence and transmitted, there was a great deal of radio traffic picked up by its antennas. In that time period, it was known for a radio signal to reflect from the ionosphere (an atmospheric layer); ionospheric skip could result in its reception hundreds or even thousands of miles away. Receiving antennas were sometimes detected passively 'rebroadcasting' signals that reached them (at much lower amplitudes, sufficiently low that the phenomenon was not of practical importance, nor even of much significance. Some have argued that, since the Kido Butai contained a large number of possible receiving antennas, it is conceivable the task force did not break radio silence but was detected anyway.

Such detection would not have helped the Americans track the Japanese fleet. A radio direction finder (DF or RDF) from that time period reported compass direction without reference to distance. (Moreover, it was common for the receiving stations to report erroneous reciprocal bearings.)[83] To locate the source, a plotter needed two such detections taken from two separate stations to triangulate and find the target. If the target was moving, the detections must be close to one another in time. To plot the task force's course with certainty, at least four such detections must have been made in proper time-pairs, and the information analyzed in light of further information received by other means. This complex set of requirements did not occur; if the Kido Butai was detected, it was not tracked.

The original records of Lurline surrendered to Lt. Cmdr. George W. Pease, 14th Naval District in Honolulu, have disappeared. Neither Lurline's log, nor the reports to the Navy or Coast Guard by Grogan in Hawaii have been found. Thus no contemporaneously written evidence of what was recorded aboard Lurline is now available. 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.[84][85] However, the directions given by Grogan in a recreation of the logbook for the Matson Line were 18 and 44° off from known strike force positions and instead pointed towards Japan. According to author Jacobsen, Japanese commercial shipping vessels are the likely source. A re-discovered personal report written by Grogan after the radio log had been passed to the 13th Naval District, dated 10 December 1941 and titled "Record for Posterity", also does not support claims of Kido Butai broadcasting.[86]

The contention that "low-powered" radio (such as VHF or what the U.S. Navy called TBS, or talk between ships), might have been used, and detected, is contradicted as impossible due to the tremendous distances involved[87] and when contact was lost, it was routinely presumed it was because low-powered radio and land line were being used.[88] Freedom of Information Act (FOIA) requests for specific RDF reports remain wanting.[89] "A more critical analysis of the source documentation shows that not one single radio direction finder bearing, much less any locating "fix," was obtained on any Kido Butai unit or command during its transit from Saeki Bay, Kyushu to Hitokappu Bay and thence on to Hawaii. By removing this fallacious lynchpin propping up such claims of Kido Butai radio transmissions, the attendant suspected conspiracy tumbles down like a house of cards."[90]

One suggested example of a Kido Butai transmission is the November 30, 1941, COMSUM14 report in which Rochefort mentioned a "tactical" circuit heard calling "marus".[91] (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 the other vessels directly 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."[92]

According to a 1942 Japanese after action report,[93] "In order to keep strict radio silence, steps such as taking off fuses in the circuit, and 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 two weeks before the attack, the ships of Kido Butai used flag and light signals (semaphore and blinker), which were sufficient since task force members remained in line of sight for the entire transit time. 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 commenced.[94] Lieutenant Commander Chuichi Yoshoka, communications officer of the flagship, Akagi, said he did not recall any ship sending a radio message before the attack.[95] Furthermore, Captain Kijiro, in charge of the Kido Butai's three screening submarines, stated nothing of interest happened on the way to Hawaii, presumably including signals received from the supposedly radio silent Kido Butai.[96] Vice Admiral Ryūnosuke 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 indicating that radio silence was broken until after the attack. Kusaka worried about this when it was briefly broken on the way home.[97]

The appendix to the war-initiating operational order is also often debated. The message of 25 November 1941 from CinC Combined Fleet (Yamamoto) to All Flagships 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... In paragraph four of the appendix to that document, 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.'"[98][99] In addition, Genda recalled, in a 1947 interview, Kido Butai's communications officer issuing this order, with the task force to rely (as might be expected) on flag and blinker.[100]

Several authors have controversially claimed that Winston Churchill had significant advance knowledge of the attack on Pearl Harbor but intentionally chose not to share this information with the Americans in order to secure their participation in the war. These authors allege that Churchill knew that the Japanese were planning an imminent attack against the United States by mid-November 1941. They furthermore claim that Churchill knew that the Japanese fleet was leaving port on November 26, 1941 to an unknown destination. Finally, they claim that on December 2, British intelligence intercepted Admiral Yamamoto's signal indicating December 7 as the day of an attack.[115][116][117]

One story from author Constantine Fitzgibbon claimed that a letter received from Victor Cavendish-Bentinck stated that Britain's JIC met and discussed at length the impending Japanese attack on Pearl Harbor. From a Joint Intelligence Sub-Committee session of 5 December 1941[118] it was stated "We knew that they changed course. I remember presiding over a J.I.C. meeting and being told that a Japanese fleet was sailing in the direction of Hawaii, asking 'Have we informed our transatlantic brethren?' and receiving an affirmative reply." However the author was incorrect. There was no session on 5 December nor was Pearl Harbor discussed when they did meet on 3 December.[119][120][121]

A purported transcript of a conversation between Roosevelt and Churchill in late November 1941 was analyzed and determined to be fake.[138] There are claims about these conversations; much of this is based on fictional documents, often cited as "Roll T-175" at the National Archives. There is no Roll T-175; NARA does not use that terminology.[139]