Could Japan Read Allied Signal Traffic? Japanese Codebreaking and the Advance into French Indo-China, September 1940

The extent of Japanese codebreaking activities before Pearl Harbor has been unclear and many believed that the Japanese Army and Navy destroyed all documents on codebreaking in August 1945. Recently, however, the Japanese Navy records from 1940 were found at the archives of the Defense Agency of Japan. These documents show that the Japanese Navy's codebreaking team was able to read the British, US, French and Chinese diplomatic traffic in 1940. This essay will focus on how the Japanese decision makers exploited the available signal intelligence when Japan advanced into French Indo-China in September 1940.