The full contribution of intelligence to the winning of World War II is clear only now, nearly sixty years after that conflict. Over the intervening decades it has been discovered that throughout the war the intelligence services of the Western powers (particularly the British) intercepted, broke, and read significant portions of the German military's top-secret message traffic. That cryptographic intelligence, disseminated to Allied commanders under the code name Ultra, played a significant role in the effort to defeat the Germans and achieve an Allied victory.

The breaking of the high-level German codes began with the efforts of the Polish secret service in the interwar period. By creating a copy of the basic German enciphering machine, the Poles managed to read...