The success of the British code-breakers at Bletchley Park during World War II has become legendary. The technological challenges they faced were huge. Racing against the clock, men and women like Mavis Batey, Dilly Knox and Alan Turing took messages intercepted by Allied intelligence and looked for ways to decrypt the German Enigma codes—which changed daily and Adolf Hitler believed to be unbreakable. Many books have been written about Bletchley, but none has focused exclusively on the significance of its work for D-Day.
As Allied nations commemorate the 75th anniversary of the largest amphibious landing in military history, historian David Kenyon reveals in “Bletchley Park and D-Day: The Untold Story of How the Battle of Normandy Was Won” that the British signals intelligence operation, by then known as “Ultra,” reached its peak performance only immediately prior to the beginning of Operation Overlord, the codename given for the invasion. By the day the first troops landed, Mr. Kenyon writes, Bletchley Park had become “an intelligence factory, with ancillary operations conducted all around the UK.”
As of June 4, 1944, he reveals, drawing on declassified files and new research, the outfit had an estimated combined personnel of 7,825, many of them brought on shortly before. Mr. Kenyon shows us how the work of the site extended beyond Enigma (the German encryption machine), the “bombe” (the machine invented to decipher Enigma messages) and Colossus (the early computer used to decode ciphers). Its support of D-Day, the author argues, should be considered one of Bletchley's most consequential achievements.
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