A History of U.S. Communications Intelligence during World War II: Policy and Administration

Chapter 2. The First Year of War

THE IMPACT OF WAR

With the outbreak of war in the Pacific on 7 December 1941, the most important U.S. COMINT facilities became the Navy's Cast unit in Corregidor and the Hawaii unit, soon to be known as Hypo, They were also the most exposed, and Cast had to be evacuated within a few months. The Army would also lose its only station then capable of monitoring Japanese army traffic - MS-6, Fort Mills, Philippine Islands.

MS-6, which had copied Japanese army traffic in 1939–40 (though SIS managed no solutions), had during 1941 worked almost exclusively against Japanese diplomatic communications. These intercepts were laboriously reenciphered, often by the chief of MS-6, Major Joseph Scherr, and then radioed to Washington. ...