In September 1940, U.S. intelligence managed to break Japanese codes used in Japan's encoded transmissions to its consular offices in the United States. Those intercepts, codenamed MAGIC in 1941, enabled U.S. officials, including the president, to listen in on secret Japanese diplomatic and military messages both before and after the Pearl Harbor attack. The MAGIC intercepts were valuable in helping President Franklin Roosevelt and his military strategists in the conduct of the war with Japan and, through Japan's diplomatic communications, with Germany.
The Japanese government relied on an encrypting system called RED by U.S. intelligence during the 1930s, and its successor called PURPLE, again, by U.S. intelligence beginning in 1939. To send messages, a clerk typed the text with an electric typewriter that conveyed the signal to a machine cipher called by the Japanese, the machine, which enciphered the text and transmitted it. The machine was the premier encrypting device of the 14 separate crypt systems used by the Japanese foreign ministry to communicate with its embassies abroad. The MAGIC complex encompassed all of those 14 systems.
When PURPLE made its appearance in a March 1939 a, a special team was assembled from the army's Signal Intelligence Service (SIS) to break into the new system. The Japanese made an error by using the same message in both the RED and PURPLE systems for a time during the transition from RED to PURPLE, and that mistake gave the army team an entry into the replacement system having already deciphered the RED system. Still, the team required 18 months of exhausting labor to break the PURPLE system and build an analog machine, which duplicated Japan's the machine, to decipher intercepted Japanese transmissions. That success meant all messages in the network of the Japanese foreign ministry, called MAGIC, could be read, and that remained the case throughout the war.
No historian, having examined the MAGIC evidence and documents relevant to the decision to forcibly remove and confine some 120,000 Japanese Americans, believes that the MAGIC intercepts support the government's claim of “military necessity.” A few nonhistorians have maintained that MAGIC shows the United States had cause for alarm over Japanese American espionage and sabotage. In fact, the MAGIC transmissions implicate Japan's consular offices with collecting information and having been instructed to advance Japan's interests among Japanese and African Americans. Those efforts were fleeting and ineffectual, and failed to generate action from sympathy or agreement. As all of the U.S. government's
agencies concluded, not a single Japanese (or African) American was ever convicted of having served Japan in espionage or sabotage during World War II. In fact, the general charged with the Western Defense Command and with executing EO 9066 could point to no evidence in support of his exclusion orders other than “racial affinities” and “the Japanese race is an enemy race …” Japanese Americans, in his view, were potential enemies, and the very fact that “no sabotage has taken place to date  is a disturbing and confirming indication that such action will be taken” (CWRIC, 6).
The MAGIC intercepts were immensely helpful in the U.S. conduct of the war, both in Europe and the Pacific, but they bear little relevance to nor help explain the Japanese American experience during World War II.
Gary Y. Okihiro
Personal Justice Denied. Commission on Wartime Relocation and Internment of Civilians (CWRIC). Washington, D.C.: U.S. Government Printing Office, 1982.