In a report dated 22 July 1931 to the Director of Naval Communications, Lieutenant Joseph Wenger commented that most of the information about the beginning of the 1930 maneuvers – that the Japanese Fleet set sail for the South Seas on 15 May and returned on 19 June 1930-was based on observations by the American naval attaché in Tokyo.

Up until this time, the Japanese had regularly informed our attaché about the fleet maneuvers, but no movement report was issued on the 1930 exercises.

The only other source of information about the maneuvers, in addition to the SIGINT described in the following paragraphs, was a letter from an unidentified person on Saipan who saw a squadron of 35 Japanese vessels (cruisers, carriers, and destroyers) engaged in maneuvers for three days. Reportedly over 6,000 naval personnel went ashore on Saipan for a short visit

Only meager information about this major Japanese fleet exercise was available to the Navy until the Commandant of the U.S. naval station at Guam sent intercepted traffic to CNO Washington on 15 September. He also passed on a report of radio activities for the period covering the Japanese fleet cruise. Two chief radiomen prepared the report and Wenger observed that the report from the station on Guam demonstrated “great possibilities inherent in a mere study of radio traffic as a means of gaining valuable information where ordinary methods fail.”

Based on OP-20-G possession of the Japanese Red Code, most of the messages intercepted by Guam were eventually decoded in Washington. It was a revelation! The Japanese maneuver simulated a defense of the western Pacific against the U.S. fleet and involved a complete mobilization of the Japanese fleet, local defense forces, and the shore establishment. The OP-20-G decrypts also revealed Japan’s capture of Guam and the Philippines and a mock air raid by American aircraft carriers on Tokyo.  Most importantly, however, analysis of the decrypted messages disclosed Japan’s knowledge of Plan Orange – the American war plan for the Pacific.

Wenger’s report concluded that Japanese Direction Finding (DF) was very accurate and that they had solved the problem of accurate DF from moving ships. In addition, the Japanese had established DF stations in the Pacific at Bako, Ashizuri, and Hamamatsu. Their military force afloat, combined with a COMINT and DF capability, made the Japanese navy a formidable force in the Pacific.

Even without effective U.S. Navy DF, Wenger wrote that through analysis of the traffic one could identify the flagships and shore stations. He also deduced that radio traffic analysis could determine the enemy’s order-of-battle and communications structure (a). Some of the early and unsophisticated techniques featured identifying increased volumes of communications, signal strength readings, and communications compromises.

Although the maneuvers concluded in mid-1930, the OP-20-G report was not forwarded to Director of Naval Communications (DNC) until a year later. Wenger explained that the Research Desk was tasked by Representative Hamilton Fish of New York to solve Soviet encoded telegrams. During this period, American authorities became aware of the extensive Soviet intelligence operations conducted in America by the Amtorg Trading Corporation based in New York. Fish subpoenaed over 3,000 of their telegrams and gave them first to the Navy cryptanalysts and then to the War Department. Not one encrypted telegram was ever read, however.

Part 3 of 6 Hamilton Fish
Representative Hamilton Fish of New York

In addition to the problem occasioned by Representative Fish’s tasking, Wenger became aware that the Japanese were using a new kind of cipher with eight new keys. The cipher was more complex and considerably different from previous Japanese cipher systems. About 15 percent of the messages were never solved. The U.S. Asiatic Fleet could not afford the luxury of waiting until Washington received the traffic, attempted to decrypt it, and reported the results back to the fleet. New methods were essential if the U.S. were to provide more timely tactical intelligence to its Navy operating in the Pacific. At the same time, the Navy realized that the Japanese Red Code had been superseded by the end of 1930. This new code, called “Blue” by the cryptanalysts, was not recognized by OP-20-G until the fall of 1931. Wenger and Lieutenant Thomas H. Dyer, apprentice cryptanalysts, worked on the code until Wenger was reassigned to the Pacific to take over the radio intelligence organization of the Asiatic Command. It was during this period in the Pacific that Wenger developed the concept of radio traffic analysis and conceived the idea of a major SIGINT exploitation effort against the 1933 Japanese maneuvers to demonstrate the intrinsic value of radio traffic analysis.

 A Successful Experiment-The Turning Point (and example of tactical time-sensitive information)

The primary objective of the U.S. Navy’s SIGINT effort in 1933 was a demonstration of the kind and quantity of intelligence that can be obtained by intercept units in the field through analysis of callsigns, addresses, frequencies, and length of messages without ever mounting any attack on enemy codes and ciphers.  The testing ground for this effort was the Japanese Grand Fleet Maneuvers of 1933. Lieutenant Wenger’s plan was to collect information and produce intelligence in the field. He hoped to demonstrate that the fleet didn’t have to wait for Washington to decrypt the messages and report on the maneuvers – which was usually several weeks after the event.

The plan was ambitious but its success proved the value of radio traffic analysis. Wenger’s plan was to intercept Japanese communications from a number of shore stations and the USS Goldstar, to perform on-the-spot analysis by personnel at the stations, and for the analysts/reporters to forward the “hot” information by radio to CINCAF. Message logs were sent by courier to Wenger aboard the USS Augusta, the fleet flagship, which was anchored off Tsingtao, China, during the exercise. Wenger analyzed the traffic to determine how much information could be obtained by methods short of cryptanalysis and the accuracy of this information.

Ground truth was established by sending a copy of the traffic to Washington for Decryption of the messages and verification of the traffic analysis performed in the field. The grand success of Wenger’s experiment convinced CINCAF of the value of communications intelligence. The CINCAF, Admiral Frank B. Upham, took the lessons seriously and recommended to Washington the creation of a major survivable intercept unit in the Manila area which would operate without interruption in the Ultimate Defense Area during a war in the Pacific.

Part 3 of 6 Admiral Frank B. Upham as a captain commanding the USS Pueblo
Admiral Frank B. Upham as a captain commanding the USS Pueblo

The maneuvers of 1933 and their successful exploitation by the Navy was the turning point in the fortunes of communications intelligence and determined the fate of traffic analysis in the field for many years to come. The Navy was sold on the concept which it nurtured and developed throughout the period between the wars. Signals intelligence, cryptanalysis, and traffic analysis became the bulwarks of the Navy’s intelligence effort against the Japanese during the Second World War. Without the experiment of 1933, the Navy would have been much less prepared to take on the Japanese in 1941 and 1942 when its back was against the wall in the Pacific.

 Source: NSA

(a) Station HYPO note: Also known as network reconstruction, or patterns of life.