Signal Intelligence Service, US Armed Forces; Far Eastern Section, Military Intelligence Service, US Armed Forces; Special Branch, Military Intelligence Service, US Armed Forces

MAGIC was the codeword used by the United States to identify deciphered Japanese diplomatic communications immediately prior to and throughout World War Two. During the war, the term MAGIC was also used for deciphered Japanese military communications, as was the term TOP SECRET ULTRA. The documents in this collection are restricted to diplomatic communications. MAGIC included all decrypted messages in Japanese diplomatic codes and ciphers, the most valuable of which were those encoded by the cipher machine known to the US as PURPLE. The ability to break into PURPLE meant that the Americans were able to read the most secret of Japanese diplomatic communications from before the Japanese attack on Pearl Harbor, 7 Dec 1941, to the end of the war in the Pacific. By way of the Japanese ambassador to Berlin, Lt Gen Hiroshi Oshima, MAGIC intelligence also provided information throughout the war about German plans and operations against the Soviet Union and the Allies. The PURPLE cipher machine was used by Japanese diplomatic and military personnel and operated by substituting ordinary typewriter keys, through a series of stepping switches and electrical matrices, into substitute letters. Theoretically, the possible substitutions by the machine cipher were endless and thus difficult to crack. Through MAGIC, however, American cryptanalysts found beachheads into Japanese ciphers from phrases used regularly and repeatedly and available in plain text. Leading the US attempt to break PURPLE was William F Friedman, a cryptanalyst who successfully broke German codes during World War One. Friedman was an expert in statistics an probability and, aided by a cryptanalyst from the US Navy, Harry L Clark, and a team of mathematicians, he successfully cracked the PURPLE code on 25 Sep 1940. Once the Freidman group enciphered PURPLE, they constructed four machines to duplicate its functions and distributed them to Washington, DC, the Philippines, and Bletchley Park, Great Britain. Upon receipt of the PURPLE machine, the British began decrypting diplomatic messages to and from Japanese embassies in Europe, the Far East and the Middle East and, by Jun 1941, had received a second machine for Singapore. Although it revealed the imminence of the war, MAGIC had little operational value. It did not reveal Pearl Harbor as a target of attack, as Japanese diplomats were often not briefed on military plans. MAGIC did, however, reveal Japanese intentions in 1941 of breaking off negotiations with Washington and London, hence indicating plans for war. Through the coded traffic of Japanese ambassador to Berlin, Hiroshi, the Allies were notified of a possible German invasion of the Soviet Union in the summer of 1941, German apprehensions of waging war on more than one front, and German troop dispositions against the Allied invasion of France in Jun 1944. MAGIC's final operation of the war was its revelation to the Allies of Japan's desperate effort to secure Soviet mediation of the war in the Pacific.