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Historical Events
Historical Events | Aug. 20, 2021

Pearl Harbor Review - Following the Fleets

One major contribution of U.S. Navy cryptologists in the period between the wars was to give American naval and civilian policy makers information derived from studies of Japanese Navy maneuvers. This capability required all the many skills and facilities the U.S. Navy had been developing since the inception of its cryptologic service after World War I.

Initially, the Navy's leadership looked on these intercept activities only as practical training for cryptologic personnel, but, as it happened, this "training" resulted in invaluable information for American Navy commanders.

The Japanese Navy conducted annual maneuvers of at least part of their fleet. Every third year, they would hold "Grand Maneuvers," involving all or a very large percentage of their combatants.

Some intercept was proposed for the maneuvers of 1927 by Commander Ellis M. Zacharias, later author of the book Secret Missions about World War II intelligence; he suggested a "radio ambush" of the Japanese fleet as it conducted maneuvers. By his own unconfirmed account, Zacharias was authorized to sail a ship outfitted with intercept equipment through the maneuver area and succeeded in getting messages to and from the Japanese flagship. No official records exist of this effort, however.

The first class of U.S. Navy intercept operators with classroom training began their intercept activities with the Japanese maneuvers of 1929. This proved good experience in preparing them for the Grand Maneuvers of 1930, when the U.S. Navy emphasized radio intercept operations.

The Japanese mobilized in May and June for maneuvers in the Mariana Islands. Official announcements minimized the size of the forces involved; ostensibly, the fleet was testing the feasibility of operations in the South Pacific.

The U.S. Navy COMINT site on Guam intercepted and forwarded to Washington about 500 encrypted messages from the Japanese fleet during these maneuvers. Cryptologic personnel in the Pacific region reconstructed a fairly complete Order of Battle for the Japanese fleet through Traffic Analysis.

Later, in Washington, cryptanalysts worked on the messages; the first break was found by Ms. Agnes Driscoll -- as Captain Laurance Safford drolly noted, "as usual." Once decrypted, the messages revealed that the entire Japanese fleet, with its shore establishment, was involved. The decrypted messages also showed that the Japanese had made an "excellent estimate" of the U.S. War Plan ORANGE, America's contingency plan for operations against Japan.

U.S. Navy cryptologists again developed important information from intensive study of what the U.S. Navy called the "Orange Grand Maneuvers 1933," ORANGE being the cover name for Japan in U.S. war plans. Official Japanese announcements advised interested parties that maneuvers would be held in August in the area of the Bonin Islands. Intercept was obtained from several fixed and several temporary collection sites.

The study of the 1933 maneuvers, conducted by Lieutenant Joseph Wenger, took about six months. Wenger, principally basing his conclusions on Traffic Analysis, produced a 115-page report that laid out the Japanese Order of Battle in considerable detail. The study also revealed that the Japanese were aware of American intercept activity and had taken new Communications Security precautions.

In 1933, in addition to gathering intelligence on a potential enemy, the U.S. Navy studied the study, that is, evaluated its cryptologic performance in the wake of the exercise.

The after-action evaluation noted several specific problems in the Navy intercept organization in the Pacific: there was a shortage of trained personnel, the lack of personnel made it difficult or impossible to maintain a constant D/F watch, and there were delays in forwarding intercepts from the site to a decrypting center.

The commander of the Asiatic Fleet was impressed with Wenger's report and made recommendations for establishment of a cryptanalytic center in the Pacific. This eventually resulted in the creation of Station CAST on Corregidor.

Over the next three years, Navy cryptanalysts in Washington solved more of the codes used by the Japanese in the 1933 maneuvers. Their solutions added details, but confirmed the validity of Wenger's original report.

U.S. Navy cryptologic personnel monitored Japanese fleet maneuvers in 1934 and 1935 also. The results were much the same. Intercept personnel produced good information, but the same problems in organization and process were observed. As the Commander-in-Chief of the Asiatic Fleet wrote to the CNO in December 1934, he was "impressed by the immense task confronting a relatively small number of U.S. naval intercept operators."

The efforts in monitoring the "ORANGE" maneuvers paid off in three ways. The intercepted messages contained a considerable amount of detailed information about the structure and operations of the Japanese Navy; this data became ever more important as the two countries moved toward war. Second, the monitoring activity and subsequent exploitation of traffic provided excellent training for American COMINT personnel. Finally, the summary products prepared after the maneuvers demonstrated to the U.S. Navy's senior leadership what COMINT could do for them, and led them to support, in fact, expand, the Navy's COMINT program.



  • Captain J. S. Holtwick, Jr., USN (Ret), Naval Security Group History to World War II, June 1971, SHR-355, CCH Files.
  • Carl Jensen, "Traffic Analysis," NCVA (Paducah, Ky.: Turner Publishing Company, 1996), pp. 26-27.
  • LT J. N. Wenger, Japanese Naval Maneuvers 1930, SRH-222, CCH Files.
  • Anon., Various Reports on Japanese Grand Fleet Maneuvers (June-August 1933), SRH-223, CCH Files.
  • Anon., Various Reports on Japanese Grand Fleet Maneuvers (August-October 1934), SRH-224, CCH Files.
  • Anon., Various Reports on Japanese Grand Fleet Maneuvers (1935), SRH-225, CCH Files.