Swedish Signal Intelligence, 1900-1945 by C. G. McKay and Bengt Beckman. Frank Cass Publishers (http://www.frankcass.com), 5824 NE Hassalo Street, Portland, Oregon 97213-3644, 2002, 310 pages, $49.50 (hardcover).

Signals intelligence (SIGINT) is seldom covered in historical texts, which makes McKay and Beckman's first authoritative account of Sweden's SIGINT both valuable and unique. It is published in English and charts the path of a neutral Sweden as it sought to keep a fine balance between the Russians and Germans in World War I and the Allies and Germans in World War II. The authors are to be commended for their detailed, up-front explanation of SIGINT: how radio and telegraph coding was used between various countries and their diplomatic missions, what kinds of transmissions third parties could intercept, and the numerous tasks involved in decoding that data.

As an integral part of the book, the authors documented Sweden's ties to Finnish independence groups in 1917 and its associations with the independent Baltic states of Estonia, Latvia, and Lithuania prior to their conquest by the Soviet Union in 1939. Those conquests and the 1939 Winter War in Finland brought home to the Swedes the threat they faced from an expanding Soviet Union. The Swedes reorganized their military repeatedly during the interwar years in response to potential adversaries and developed a SIGINT organization that would serve it well during World War II.

The outbreak of World War II and the German occupation of Denmark and Norway made the Swedish position of neutrality more precarious. Soviet codes were difficult at first but were eventually mastered; the German Enigma keys were even harder to break. German landline communications ran through Swedish territory and could be monitored and exploited. That opportunity allowed the Swedes to eventually break the encryption codes used by the German Geheimschreiber teleprinter. That successful work on its simultaneous machine encryption required both an understanding of codes and electro-mechanics. With a staff of fewer than 400 people, the Swedes achieved impressive results: Soviet, German, and American codes were broken, and Swedish code security was monitored and corrected when necessary. Interestingly, while the Germans realized that the integrity of their communications crossing Sweden had been compromised, their own bureaucratic bungling prevented any improvement in their signal security. …