From wikipedia "Bletchley Park" on the influence of the whole deciphering activity:
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The intelligence produced from decrypts at Bletchley was code-named "Ultra". It contributed greatly to Allied success in defeating the U-boats in the Battle of the Atlantic, and to the British naval victories in the Battle of Cape Matapan and the Battle of North Cape. In 1941, Ultra exerted a powerful effect on the North African desert campaign, against the German army, under General Erwin Rommel. General Sir Claude Auchinleck stated that, but for Ultra - "Rommel would have certainly got through to Cairo". Prior to the Normandy landings on D-Day in June 1944, the Allies knew the locations of all but two of the 58 German divisions on the Western front. Churchill referred to the Bletchley staff as "The geese that laid the golden eggs and never cackled".