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Signal Intelligence in the Pacific War
by GERHARD KREBS -- abstract

The declassification of a great amount of documents on signal intelligence, particularly in the United States during the last three decades, has widely enlarged our knowledege on this aspect of World War II . This is not only the case for the European and North African theater but also for the Pacific War. While cryptologic activities were reduced in the years after World War I, they were intensified again in the late 1930s. The USA had reached good results in the period immediately before Pearl Harbor though it did not make full use of the contents of the intercepted Japanese messages. During the war, however, the ability to read the telegrams and radio communication of the enemy powers contributed widely to the successes in the strategic and operational field. British, Dutch, Australian, and Canadian cryptologers also had their share. In contrast to the alliied side of the Pacific war the fact is widely unknown to historians that also Japan since the early 1930s was able to read the military and diplomatic ciphers of the United States as well as of Great Britain, though to a lesser degree than their enemies, and exchanged cryptographic information with the Axis partners, including captured code books. Research, however, is hampered by the very limited number of documents which have survived the war.