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Japanese intelligence relating to Pearl Harbor was collected by members of the Japanese Consulate in Honolulu. The principal agent was Ensign Takeo Yoshikawa (Yoshikawa, Takeo) who was sent to Hawaii in April 1941 using the name ""Tadushi Morimasu"". He communicated, as did agents in other ports around the world, details of ships and their movements to his chiefs in Tokyo. To do this he used the low-grade consular code called J-19 by the Americans and TSU by the Japanese. This had been broken by the Americans, the British and the Dutch, but as consular traffic was considered unimportant, the fact that the messages were being sent and the nature of their content was not shared with Admiral Husband E. Kimmel (Kimmel, Husband E.) by the intelligence officers in Washington, D.C. Thus the ""bomb plot"" message of 24 September, which instructed Yoshikawa to report ships' presence in each of five zones of Pearl Harbor, was not known to Kimmel, and the obvious inference of intended attack could not be drawn. On 15 November Yoshikawa was ordered to report twice a week and on 29 November to report even if there were no ship movements.

Kimmel had, under his own command at CinCPAC, the Communications Intelligence Unit (CIU) but that was concentrating on the Japanese Naval code, JN-25, which had allegedly been broken by the Australian, Eric Nave, in late 1939. This was not shared outside the British-run Far Eastern Combined Bureau (FECB). CIU did not attempt to intercept or read messages originating from the Japanese Consulate.

The Japanese also sent agent Lieutenant Commander Suguru Suzuki on the liner Taiyo Maru which arrived in Honolulu on 1 November. Suzuki could not have seen much from his ship, but he did have a questionnaire completed by Yoshikawa giving extensive detail about ships at Pearl Harbor. A further provision was the presence of a ""sleeper"", and inactive agent for later use by the Japanese, a German ex-Naval officer Bernard (Otto) Kuehn. He was already known to American intelligence, having been identified by FECB in 1936, was painfully indiscreet, and was arrested immediately after the war broke out.

The Japanese task force under Vice Admiral Chuichi Nagumo (Nagumo, Chuichi) preserved radio silence before the attack on Pearl Harbor, but it was necessary to send messages to them using JN-25. It has been suggested that, because the British could intercept and read such messages that they did so, understood them and purposely concealed their knowledge from the Americans in order to thrust the USA into the war. Such speculation, although it could be true, is unsupported by evidence.

Japanese diplomatic messages were conveyed using the code Purple and read using the MAGIC decoding machine. This enabled the Americans to have the full text of the instructions conveyed to the Japanese Ambassador to the USA, Kichisaburo Nomura (Nomura, Kichisaburo) before the intended recipient had it himself.

An important message of which the Americans were aware was #2353, known as the Winds message. This gave codes to be used in short-wave newscasts from Tokyo in order to issue instructions to their missions to destroy codes and machines; a necessary precursor to war.

See also: Central Intelligence Agency; CinCPAC; CIU; FECB; J-19; JN-25; Kimmel, Husband E.; MAGIC; Nagumo, Chuichi; Namuro, Kichisaburo; Pearl Harbor; Purple; Winds