"CATCHING THE FOX UNAWARE": Japanese Radio Denial and Deception and the Attack on Pearl Harbor
Hanyok, Robert J., Naval War College Review
The attack on the U.S. Pacific Fleet by the aircraft of the Japanese Striking Force (Kido Butai) at Pearl Harbor on the morning of 7 December 1941 was a total surprise to the American commands in Hawaii and Washington. The completeness of the operational surprise-the Imperial Japanese Navy had gathered the force, trained it, concentrated it, and sent it to the launch point without discovery by American intelligence, especially its radio component-was due largely to the success of the Japanese cover plan of radio denial and deception in hiding the existence, makeup, purpose, and timing of the attack. The Japanese navy's denial and deception plan left American radio intelligence, known also as "communications intelligence," with only scraps of information about the Japanese fleet's movements during the weeks prior to the attack.
Even these wisps were intentionally misleading. Planners from Tokyo's Naval General Staff and on the Combined Fleet (Kaigun) staff had developed a synchronized plan for the Pearl Harbor Striking Force that combined the three elements of radio Silence, active radio deception, and radio intelligence in a way that assured Tokyo that the U.S. Pacific Fleet was unaware of the approaching Kido Butai. Furthermore (and this is the subtle part of the Japanese planning) that the attack remained a complete surprise owed much to Admiral Isoroku Yamamoto's Scheme of supplanting the traditional strategic decisive engagement"-a mid-ocean surface battle with the Pacific Fleet-with a preemptive strike. The measure of the plan's success was simply that Admiral Husband E. Kimmel, Commander in Chief, Pacific Fleet, and his command and intelligence staffs expected no attack, despite projections of air assaults on Hawaii as well as suspicious activity that morning.1
Technical parts of the radio deception and silence plan (the latter known as "denial" in modern military parlance) were executed in such a manner as to leave the American naval radio and fleet intelligence officers swaying between uncertainty as to the location of the Imperial Japanese Navy's carriers and conviction that these ships had remained in home waters in accordance with traditional Japanese doctrine and decades of exercises. The possibility that the Americans might have been victims of "self-deception," the tendency of intelligence analysts to rely on assumptions in accommodating new data, as later claimed, does not mitigate the fact that the Japanese fed the Americans false data that the latter accepted as valid intelligence.
Some claim that the Striking Force did not maintain complete radio silence or that Tokyo's radio deception failed to fool the Americans. This dissent comes from two quarters: recent writings on the subject of Pearl Harbor intelligence and the statements of certain intelligence officers assigned to Hawaii at the time. The first group's claim can be dismissed easily. Its thesis is that the Kido Butai transmitted radio messages as it crossed the Pacific and was tracked by the U.S. Navy.2 Its evidence has been expertly dismantled in books and articles.3
The second dissenting group consists of Lieutenant Commander Joseph Rochefort, the chief of Station H (or "Hypo"), the U.S. 14th Naval District's radio intelligence center, and Commander Edwin Layton, the fleet intelligence officer to Admiral Kimmel at the time. In statements and writings after Pearl Harbor, both officers insisted that the Japanese, though sailing in complete radio silence, could not have pulled off a successful radio deception against the U.S. Navy's Pacific area radio intelligence centers at Pearl Harbor and at Cavite (Station C, or "Cast"), in the Philippines.4 Because of the prominent roles of both men in the events leading up to the Japanese attack, their claims will be considered against the evidence presented later in this article.
This article is based largely on extant Japanese and American records. While the Japanese destroyed the majority of their wartime records, some material relevant to Pearl Harbor was captured during the conflict. …