During World War II, signals intelligence played an important role in strategic decision making. Its best-known successes were in COMINT and included breaking the Japanese and German codes in magic and ultra. The Axis powers also had some success in breaking Allied codes, most especially those regarding the Atlantic convoys. Signals intelligence activities during World War II were not limited to intercepting and reading communications, however. ELINT activities during the war included monitoring radar signals to determine the transmitting power, range, and accuracy of the air defense systems built by both the Allies and the Axis powers. In the Atlantic and Pacific Oceans, Great Britain and the United States used sophisticated radio receivers to approximate the positions of submarines, merchantmen, and surface warships by means of triangulation (interpreting the strength and point of origin of radio transmissions). As the war progressed, ELINT techniques became more sophisticated. Because of the role played by both COMINT and ELINT in the conflict, World War II has sometimes been referred to as the SIGINT war.
Shannon A. Brown
Budiansky, Stephen. Battle of Wits: The Complete Story of Codebreaking in World War II. New York: Simon and Schuster, 2000.; Kahn, David. The Codebreakers: The Comprehensive History of Secret Communication from Ancient Times to the Internet. New York: Scribner's, 1999.; Laqueur, Walter. World of Secrets: The Uses and Limits of Intelligence. New York: Basic Books, 1985.; Prados, John. Combined Fleet Decoded: The Secret History of American Intelligence and the Japanese Navy during World War II. New York: Random House, 1995.