From The Wall Street Journal:
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.”
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It all began in the summer of 1938, as war with Nazi Germany loomed. British intelligence purchased the country estate called Bletchley Park and relocated the Government Code and Cipher School there from its headquarters some 50 miles to the southeast in central London. Polish and French efforts to crack the Enigma codes had already begun. But in 1939 the British, laboring in sparsely furnished huts amid the quiet and secluded surroundings of Bletchley Park, began working around the clock on the problem.
Hut 6, for example, was responsible for deciphering German army and air force Enigma codes. The intensive analysis of enemy communications traffic carried out in Hut 6 helped produce a map of German networks, army movements and formulations. Decrypted messages helped locate the headquarters of the elite SS Panzer-Korps divisions and informed the Allied commanders of the locations and strengths of SS units in France.
Link to the rest at The Wall Street Journal