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.

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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 German signal traffic throughout the 1930s with varying degrees of success. However, shortly before the Munich conference in September 1938, the Germans made alterations to their enciphering machine–the so-called Enigma machine–and in mid-September, darkness closed over German message traffic. The Poles continued their work, however, and after France and Britain’s guarantee of Polish independence in March 1939, they passed along to the British what they had thus far achieved. Considerable cooperation had also existed earlier between the Poles and the French. Building on what they had learned from their Continental allies, British cryptanalysts finally cracked some of the German codes in April 1940, just before the great offensive against France and the Low Countries.

Other successes soon followed and gave Allied intelligence officers and commanders valuable insights into German intentions and capabilities. Nevertheless, the British were only able to break a small proportion of the specific codes used by the Wehrmacht. At the end of 1943, the Kriegsmarine, for example, used up to forty different ciphers, all requiring different Enigma machine settings. During the Battle of the Atlantic, the transmissions from U-boats to shore and from the commander of submarines to his boats received the highest priorities from cryptanalysts at Bletchley Park, the location of the British decoding efforts in Europe.

Even with the exceptional resources available there and at that time, it took experts several days and in some cases up to a week to find solutions for a particular day’s settings on the Enigma machine. The task of getting invaluable intelligence information out to the field where it could be of direct help was, of course, immensely difficult, especially given fears that if the Germans found out that their codes were being compromised on a daily basis, Ultra intelligence would dry up.

In 1940 during the Battle of Britain, this need for concealment was not great, but as the war spread throughout Europe and the Mediterranean, it became an increasing problem. Accordingly, the British and their American allies evolved a carefully segregated intelligence system that limited the flow of Ultra to a select number of senior officers. The Ultra information dissemination process lay outside normal intelligence channels. For example, the intelligence officers of the Eighth Air Force would not be aware of the existence of Ultra and would therefore not know the duties of the Ultra liaison officers. Those officers, in turn, would forward Ultra intelligence only to the commanders of the Eighth and Ninth Air Forces. The system seems to have worked, for the Germans never caught on to how extensively their ciphers had been compromised.

Unfortunately, there were drawbacks. Intelligence is used only if it reaches those who understand its significance. Three specific incidents underline this point with great clarity. The first occurred in early September 1944, as Allied armies pursued the beaten Wehrmacht to the Third Reich’s frontiers. On September 5, Bletchley Park made the following decryption available to Allied commanders in Western Europe:

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