By the fall of 1940 United States Army cryptanalysts had solved some of the Japanese Foreign Office's highest grade cryptographic systems. The interception, decryption and translation, on a current basis, of secret Japanese world-wide diplomatic messages then began. The information the United States derived from this source, designated MAGIC, was highly classified and closely guarded. It went to only a few of the highest-level United States officials. It is important to note that MAGIC could only read Japan's diplomatic code, not its military code.
In 1977, The Department of Defense released to the public this multi-volume study of the "MAGIC" or communications intelligence background related to the 1941 Pearl Harbor attack. In its review of classified records pursuant to E.O. 11652, the Department of Defense decided that it was in the public interest to declassify the intelligence which the U.S. obtained from the communications of its World War II enemies. This study contains a major part of the communications intelligence which the U.S. derived from intercepted Japanese communications during 1941.
The volumes contain intelligence concerning Japanese secret plans, policies, and activities which U.S. cryptologic specialists produced. Attention should be given to particular important material in this study that correlates the intelligence with the discussions of Secretary of State Hull and Japanese Ambassador Nomura in the critical months before Pearl Harbor.