by Gerald Boerner
“The blow was heavier than he had hoped it would necessarily be. … But the risks paid off; even the loss was worth the price.”
— Franklin D. Roosevelt’s Reaction to the severity of the attack
“… just prior to World War II, [the US] had some 700 people engaged in the effort and [was], in fact, obviously having some successes.”
— on the problem of retaining cryptographers
“If by these means Japan could be led to commit an overt act of war, so much the better," of the eight ‘plans’ (of the actions to be taken)…
— Memos from the Defense Department
“…[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.”
— Mark Parillo, in his essay The United States in the Pacific
“Ten days before the attack on Pearl Harbor 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.’ ”
— Henry L. Stimson, United States Secretary of War
“…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…”
— Stark, CNO, and Marshall, Army Chief of Staff
“Any strength that we might send to the Far East would…reduce the force of our blows against Germany…”
— Stark’s Plan Dog
“… 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.”
— Oliver Lyttelton, the British Minister of War Production
Pearl Harbor: Conspiracy Theories about the Japanese Attack
The attack on Pearl Harbor (or Hawaii Operation, Operation Z, as it was called by the Japanese Imperial General Headquarters) was an unannounced military strike conducted by the Japanese navy against the United States’ naval base at Pearl Harbor, Hawaii, on the morning of Sunday, December 7, 1941 (Hawaiian time, December 8 by Japan Standard Time), later resulting in the United States becoming militarily involved in World War II. It was intended as a preventive action to keep the U.S. Pacific Fleet from influencing the war the Empire of Japan was planning to wage in Southeast Asia against Britain, the Netherlands, and the United States. The attack consisted of two aerial attack waves totaling 353 aircraft, launched from six Japanese aircraft carriers.
The Pearl Harbor advance-knowledge debate is a dispute over what, if any, advance knowledge American officials had of Japan’s December 7, 1941 attack on Pearl Harbor.
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.
American warships burning
out of control following
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 intelligence information available to U.S. and other nations’ officials. Rather than attribute the lack of preparedness at the base to failure-to-process, some have argued that the U.S. must have had some degree of advanced knowledge of the attack.
Assertions that Japanese codes had already been broken
U.S. signals intelligence in 1941 was both impressively advanced and uneven. In the past, the U.S. MI-8 cryptographic operation in New York City had been shut down by Henry Stimson (Hoover’s newly appointed Secretary of State), citing "ethical considerations", which inspired its now broke former 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, 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 Naval Code D (called JN-25 by the U.S. after March 1942)
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, [the US] had some 700 people engaged in the effort and [was], 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. Also see the Harris article for more detail and a broader perspective.)
PURPLE… The break into PURPLE was a considerable cryptographic triumph, and proved quite useful later in the War. It was the highest security Japanese Foreign Office system, but prior to Pearl Harbor carried little information about Japanese plans; the military, who were essentially determining foreign policy for Japan, distrusted the Foreign Office and left it "out of the loop". Unfortunately for the U.S., the two U.S. crypto groups generally competed rather than cooperated, and distribution of intelligence from the military to U.S. civilian policy-level officials was poorly done (eg, capriciously selected for distribution) by both the Army and Navy who handled the traffic on alternate days, and furthermore in a way preventing any of its recipients from developing a larger sense of the meaning of the decrypts. Along with the obsession with security, there was little or no analysis done for recipients. Decrypts were typically provided raw, completely without context, and without much taking into account the needs of the recipients. As well, recipients were not permitted to retain them, or notes made from them, again for security reasons.
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, premature.
JN-25… The JN-25 superencrypted code is one of the most debated portions of Pearl Harbor lore. JN-25 is the U.S. Navy’s final term for the cryptosystem the Imperial Japanese Navy sometimes referred to as Naval Code D. 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. It was an example of the then state of the art in crypto systems and was quite different than modern forms of message encryption in being a code (e.g., battleship = 63982) and further being superenciphered with an additive cypher, taken from a large book. So, for example, 63982 + 12345 = 75227 (using modulo arithmetic, non-carrying addition and non-borrowing subtraction, also called Fibonacci or "Chinese" addition), giving the actually transmitted group (75227); on receipt the additive was subtracted (75227 – 12345 = 63982 (modulo arithmetic again)) and the code group looked up in the current JN-25 code book.
The worth of the additive step is that the next time anyone mentioned ‘battleship’, a different additive would be used. It was based upon the Japanese syllabary (kana), due to the difficulties in using kanji in telegraphy and the fact that electric teletype printers were more or less easily converted (e.g., more characters in the syllabary) to kana from the Roman alphabet.
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.
Detection of Japanese Transmissions
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 which persisted for several days, and came from signal source(s) moving in an easterly direction, not shore stations – presumably 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 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.
The Kido Butai was constantly receiving intelligence and diplomatic updates. Regardless of whether or not the Fleet broke radio silence and transmitted, there was a great deal of radio traffic picked up by its antennas. In that time period, it was not unknown for a radio antenna to reflect the energy of an incoming signal back to the ionosphere, where ionospheric skip could result in its reception hundreds or even thousands of miles away. Note that the Fleet, considered as a whole, contained a large array of such possible reflectors. Thus it is conceivable that the Kido Butai did not break radio silence but was detected anyway.
Such detection would not have helped the Americans track the Japanese fleet. A direction finder from that time period reported compass direction without reference to distance. 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 Fleet’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.
Statements by High-Ranking Officials
One perspective is given by Vice Admiral Frank E. Beatty, 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, with perspicuous remarks as:
"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."
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.’"
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, in Day of Deceit Stinnett claims all action items were implemented. 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."
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…." 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." He concluded, "[I]t is of grave importance to our war effort in Europe…" 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" 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…" 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 and with Churchill’s to "quiet" Japan.
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." 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. 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. In addition, Roosevelt authorized a so-called Neutrality Patrol, which would protect the merchantmen of one hostile nation, Britain, from attack by another, Germany. Furthermore, Roosevelt ordered U.S. destroyers to report U-boats, then later authorized them to "shoot on sight". None of these is the act of a disinterested neutral, while all are unquestionably of assistance to Britain.
Background and biographical information is from Wikipedia articles on:
Advanced Knowledge Debate (Pearl Harbor) that can be found at…