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 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.
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