F. W. Winterbotham - The Ultra Secret

Ultra remained strictly secret even after the war. Then in 1974, Winterbotham's book, The Ultra Secret, was published. This was the first book in English about Ultra, and it explained what Ultra was, and revealed Winterbotham's role, particularly with regard to the dissemination and use of Ultra.

There had been mentions of Enigma decryption in earlier books by Władysław Kozaczuk, Ladislas Farago, and Gustave Bertrand. However, it was Winterbotham's book was the first extensive account of the uses to which the massive volumes of Enigma-derived intelligence were put by the Allies, on the western and eastern European fronts, in the Mediterranean, North Africa, and perhaps most crucially, in the Battle of the Atlantic.

Winterbotham's account has been criticized for inaccuracies and self-aggrandizement. Winterbotham was evidently no cryptologist and had only slight understanding of the cryptologic side of the multi-faceted and strictly compartmentalized Ultra operation. His description of the pioneering work done by Poland's Cipher Bureau before the war is minimal and incorrect. Winterbotham later responded that he had simply passed on the story that he had been given at the time.

He erroneously suggested that Japan's PURPLE cipher machine was a version of the German Enigma and confused "Dilly" Knox with a different person.

Perhaps the worst flaw in the book is the myth of Winston Churchill and the Coventry Blitz. During The Blitz of 1940–1941, Coventry, a British industrial city, was severely bombed by the Luftwaffe on the night of 14–15 November. There was heavy damage and numerous civilian casualties. Winterbotham asserted that Enigma decrypts had provided clear advance warning of the raid but that Churchill personally decided not to take any special countermeasures that might alert the Germans that the British were reading Enigma. This story has been widely repeated, even though it has been thoroughly refuted by other historians and memoirists.

Peter Calvocoressi was head of the Air Section at Bletchley Park that translated and analysed all deciphered Luftwaffe messages. He wrote "Ultra never mentioned Coventry... Churchill, so far from pondering whether to save Coventry or safeguard Ultra, was under the impression that the raid was to be on London."

Nevertheless, Winterbotham's book is a vivid first-hand account by one of the key figures in the Ultra story, and much of the book still retains interest and validity. Winterbotham's conclusion was that the war's outcome "was, in fact, a very narrow shave, and the reader may like to ponder whether or not we might have won had we not had Ultra."