Piercing the Fog - Air Force Historical Studies Office

The Pacific and Far East
watchers, photoreconnaissance, agent reports, and radio intelligence units, using
radio methods throughout the war zone. Japanese deception involved quite good
attempts at using English speakers on tactical radio nets to confuse or mislead
Allied pilots and ground forces. In addition, skilled Japanese interrogators
extracted valuable data from POWs, many of whom, because of the widespread
aerial fighting, were airmen. Although some Japanese units, like the Southern
Army, could dismiss intelligence studies, others pursued it. Even Southern Army
realized its error and recreated an intelligence section early in 1944. By then,
however, it was too late for such a move to influence the war’s direction.”
For radio intercept work, each major Japanese army headquarters had
assigned to it their version of a Y-Service unit that dealt with coded or
encrypted British, American, or Chinese message traffic. The larger and more
important the headquarters, the larger and better equipped the intercept section
was. These field units proliferated, but they were by no means as successful as
were the comparable Allied organizations. Allied ULTRA could occasionally
trace Japan’s sources and use of radio intercept data. An early March 1944
decryption of Japanese radio messages confirmed that their surveillance and
study of Allied air operations involved direction finding and traffic analysis
done as CB did in Brisbane and as the radio intercept squadron did at Fifth Air
Force ADVON’s operational headquarters in New Guinea. Allied interpreters
who studied the messages concluded, however, that no evidence suggested the
Japanese were successful in breaking important, high-level codes or ciphers.‘*
This meant that the Japanese, despite some monumental American blunders in
keeping secrets, remained unaware of the true nature and extent of Allied
ULTRA operations. Many of Japan’s most valuable intelligence accomplishments
came from their efforts to counter Allied air power. Japanese cryptographers
often broke low-level air operations codes. For example, they were able
to predict B-29 actions in China by reading messages directing the turning on
of aircraft homing beacons.
In the SWPA, reports of Japanese intelligence activities unearthed by
ULTRA appeared in Willoughby’s daily SIBS. As much as possible, the Allies
tracked the location of Japanese radio intercept units, noting that the removal
of one from Rabaul to Truk early in 1944 possibly indicated the decline in the
Rabaul unit’s relative importance in Japanese plans and the perception that
Nimitz’s carrier and amphibious forces posed the greater threat to the Pacific
islands. On the other hand, the move could have resulted simply from Japan’s
wish to avoid risking the valuable unit to the heavy and continuing air attacks
by Admiral Halsey’s carrier aircraft and by Kenney’s bombers and fighters.
Despite the apparent move of the radio intelligence unit, some capability
remained at Rabaul, for it was there that the Japanese gained some of their best
radio intercept data on B-29 operations from the Marianas. Throughout the war,
knowledge of Japanese radio intelligence activities remained fragmentary, with
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