Intercept of the Yamamoto Message of 20 May 1942 by Bill Price

Radio Intelligence or Signals Intelligence (SIGINT) played a key role in supporting the United States forces in WWII. However this was not a new method of capturing intelligence. The United States started signal intelligence during the Civil War when both sides trained their telescopes on the other’s flag signals and copied messages being transmitted. Letters were represented by a “one” or a “two” corresponding to movements of the signal flag to the right or to the left of the flag signaller. The characters were represented by a combination of ones or twos which was similar to the dots and dashes used in Morse Code. Later these combinations were shown on a disc with the alphabet being on one ring and the one and two code combinations being on a facing disc. Signal officers agreed that the letter R was to be used to set up the “cipher disc”. Before a message was transmitted by flag, the sending officer would transmit a number combination, for example 2122. The receiving signalman would rotate the disc so that the letter R would be opposite this number.

Both sides were adept at copying signals and breaking the substitution code used. To complicate this analysis, the code could be changed from message to message or several times in a message. Knowing that the other side could read their flag signals, signalling was used to plant false information on the enemy. The code was simple substitution, so both sides used frequency analysis to analyze the code and make it readable. As years passed the United States became more adept at developing new and more complicated codes and cipher systems. Today we have the National Security Agency as well as the Navy Security Group, Army security Agency and Air force security service.

The WWII attack on Japanese Navy codes was carried out by the Navy Security Group. The stations were referred to as Fleet Radio Unit. The major station in the Pacific was at Pearl Harbor under the code name of HYPO. This group advised Admiral Nimitz who was Commander in Chief, Pacific Fleet. The first group in the Pacific was at Corregidor in the Philippine Islands. Referred to as CAST, this group was evacuated to Melbourne, Australia and was called FRUMEL. The headquarters was OP-20-G in Washington, DC

The Imperial Japanese Navy Combined Fleet, commanded by Admiral Isoroku Yamamoto, was based at Hashirajima Anchorage of the western inland sea. Hiroshima overlooked this bay of water. Admiral Yamamoto’s flagship, Yamato, was anchored here and had a direct land line to Tokyo and the IJN General Staff. Plans for the attack and invasion of Midway were made by officers on the Yamato, at this inland sea headquarters. Admiral Yamamoto had planned the attack of Pearl Harbor and the carriers that made this attack sailed from this bay. Now five months after this December surprise attack a new attack was being planned. The targets were the U.S. carriers in the Pacific and Midway Island as well as the Aleutians.

Admiral Yamamoto was determined to capture Midway as a base for his navy and force the remaining United States carriers to respond to what would be certain destruction by a seasoned, superior and well-trained Japanese force. Yamamoto was obsessed with protecting Tokyo, and Col. James Doolittle's raid on the Japanese homeland made securing Midway essential to deny the Americans this forward base.

Plans for the Midway operation were begun in April 1942. However, the principal officers of the Navy were not in agreement on this operation. They did not think that the American carriers would respond and their destruction would be unlikely. Yamamoto was facing the possibility of being the only proponent of this plan, one that would grow and become overly complicated.

Development of this plan was supervised by Yamamoto’s Chief of Staff, Rear Admiral Ugakii. Destruction of the American carriers and capture of Midway would clearly shift the balance of power in the Pacific and perhaps worldwide for the Axis forces. But most of the Japanese Admirals questioned whether attacking Midway would force the Americans to respond with their three carriers. Soon Yamamoto would become the sole proponent of such an operation. He felt that occupying Midway would force the United States to negotiate.

At Yamamoto’s direction the Combined Fleet Headquarters started developing the plan:-

The Combined Fleet had seven large and four light carriers. The United States had only three carriers in the Pacific. By the end of April most of the Japanese ships were anchored in the inland sea near Hiroshima.

On the first day of May a four-day series of war games was launched on the Yamato. Preliminary plans had been developed for the attacks on Midway and these games would test their operational effectiveness. However two large carriers were lost in the game; but this loss was ignored. On May 4 the games were completed. Changes were debated, but a number of officers were still not pleased with the plan. Despite the fact that many problems had been addressed, several remained unsolved.

While these plans were being revised, the battle of Coral Sea began to occupy the Japanese forces which were extending their holdings to the South Pacific. It highlighted the inability of their carrier force to protect the planned invasion of New Guinea. In the fight with American carriers, 77 Japanese planes were lost. The U.S.S. Lexington had been severely damaged and was evacuated for the coup de gras. The U.S.S. Yorktown was also hit, and the Japanese believed it had been sunk.

So on May 25 at the Hashirajima anchorage, the Midway and Aleutians attacks were conducted in tabletop maneuvers aboard the Yamato. The war games were completed during the first week of May. Although the Japanese could not read the American messages, traffic analysis of the signals yielded some useful radio intelligence.

On May 21 the Nagumo force sailed out of the inland sea through the Bungo Straits to train the crews with fleet maneuvers.

After revision of Midway and Aleutians plans, an operational order in a detailed 12-part message was sent to all participating forces on 20 May as Operational Order 14. The length of this message revealed how complex the plan was in coordinating the Aleutians invasion and the invasion of Midway with troops from Saipan. Two carrier forces, submarines, and troop transports also had to be coordinated.

In the morning of May 27, the Nagumo force, Kido Butai, sailed out of the inland sea. Admiral Yamamoto followed with a support force on May 29. Japanese radio intelligence aboard the Yamato reported American submarines outside the bay. The radio intelligence staff could not read the American Navy code, but traffic analysis of the various transmissions and message headers, which were not enciphered, yielded ample intelligence of the forces -- six American submarines lay outside the bay. The Japanese radio intelligence crew continued to observe the activity. But the U.S. Navy radio intelligence was reading Japanese Navy messages and the American Navy knew of the planned attack before the Japanese ships left the inland sea near Hiroshima. The May 20 message, with the detailed plan, had been intercepted by both FRUMEL and HYPO. Although garbled, it gave ample intelligence of the planned attacks and for the first time it revealed 4 June as the date for the attack.

Now the Midway plan began to unravel. Japanese patrol planes and submarines were scheduled to refuel at French Frigate shoals on May 30. But on approach, two American ships were anchored there and the patrol planes could not be refueled. The American Navy was using the shoals as a seaplane base.

Radio intelligence operators aboard the Yamato, and Kaga now received an increase in American radio traffic out of Hawaii. Out of 180 messages, 72 were sent as urgent. Although the Japanese carrier force was superior, the Naval General Staff would not accept the Midway plan. However, the staff sent a message to Yamamoto and Nagumo that the U.S. carriers were near the Solomon Islands. Since the Japanese submarines were late in arriving on station to watch for departure of American ships from Pearl Harbor, Admiral Spruance with Task Force 16 had already sailed undetected.

These events took place leading up to the attack on Midway. The use of multiple carrier forces, submarines, and infantry transports required detailed instructions for execution. Operational Order 14, in 12-parts, had precise instructions. At 2330 on May 20, this message, although garbled, was identified by Yeoman Bill Tremblay at FRUMEL. The Duty Officer was awakened and HYPO and OP-20-G were notified through COPEC. The intercepted text followed. HYPO located its intercept which was also garbled and had also been put aside as secondary importance for processing.

Fortunately their copy contained some parts not available to FRUMEL. FRUMEL and HYPO kept the COPEC circuit busy with missing parts and decoding solutions. Time was of the essence since this intercept identified 4 June as the attack date for Midway.

HYPO provided direct support to Admiral Nimitz, Commander in Chief of the Pacific Fleet. As the intercept was decoded and translated, Rochefort, commander of the HYPO radio intelligence staff, was kept apprised of the results. On May 25, he met with Admiral Nimitz and gave him this surprising intelligence from the lengthy intercept:-

A search of records in the National Archives has not found this intercept. A query to the NSA archive has produced negative results as well as a disturbing reply, NSA does not keep copies of intercepts. A review of Special Research Histories (SRH) did not find a study of the Japanese intercept. As a result, many of those interested in the Battle of Midway conclude that this message never existed. However Rochefort discusses it in his oral history and Holmes, also at HYPO, describes it in his Double Edged Secrets.

At this time Admiral Mac Showers was an Ensign at HYPO. He recalls being given a copy of his intercept to use in posting the Japanese ships on the HYPO plot board. Admiral Ralph Cook was an Ensign at FRUMEL, and in an NSA publication, he describes how Yeoman Bill Tremblay discovered this message at 2330 on 20 May. Ralph went on to become an Admiral and command the Naval Security Group. It is doubtful that either of these officers would fabricate the 20 May intercept.

It is clear, without exception, that the 20 May 1942 message, was intercepted by both FRUMEL and HYPO The subject line was Operational Order 14.

In discussions with Admiral Showers, the lack of information addressing this intercept is puzzling considering its importance. I would conclude that the intelligence derived from this intercept was perhaps the best obtained during World II. This intercept allowed us to defeat a superior Japanese force and save Midway Island. Four of the largest Japanese carriers were destroyed with a substantial number of experienced pilots and all their planes. This victory reversed the Japanese advances in the Pacific.

Considering the value of information in this intercept and the results in saving Midway and reversing the Japanese successes in the Pacific war, this may have been the most important intercept during World War II. It clearly underscores the importance and necessity for radio intelligence, the leverage for success.

I'd like to thank Bill Price for allowing me to reproduce his above work on the Yamamoto Intercept.

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