The secret that stayed secret for thirty years 

The intelligence Bletchley produced was codenamed Ultra, and the rules around it were stricter than the rules around almost anything else in the war. Commanders in the field were told what to expect but rarely told how the knowledge had been obtained. Where possible, a cover story was attached: a lucky reconnaissance flight, a well-placed agent, an intercepted radio call from a scout aircraft. Acting on Ultra without a plausible alternative explanation was forbidden, because a single obvious response could cost the entire source.

That discipline created decisions that look monstrous when stated flatly, and there is a persistent popular claim that Coventry was deliberately left undefended in November 1940 to protect the secret. The evidence does not support it. The warnings available before the raid did not clearly identify Coventry as the target in time, and historians who have gone through the files, including the official historian F. H. Hinsley, reject the story. The genuine dilemmas were quieter and more numerous: which convoy to reroute, which supply ship to sink, how much success was too much success.

Inside Bletchley, compartmentalisation did the same work. Staff signed the Official Secrets Act, were told to say they did clerical work, and generally did not know what happened to the paper that left their hut. Married couples who both worked on the site have described discovering only decades later what the other had been doing.

Then the war ended and the silence did not. The Bombes were dismantled, the huts emptied, and the veterans went back to universities and offices and told nobody. There was a hard strategic reason: captured Enigma machines were sold on to other governments after the war, and Britain had no interest in advertising that such machines could be read.

The public account arrived only in 1974, with Frederick Winterbotham's The Ultra Secret, followed by the official histories and the gradual release of files. By then many of the people involved were dead. Alan Turing had died in 1954, two years after being prosecuted for homosexuality and subjected to hormone treatment, with none of his wartime work publicly known. He received an official apology in 2009 and a royal pardon in 2013.

The cost of thirty years of silence was not only personal. It also distorted the history of technology, because the machines built at Bletchley in the last years of the war were erased from the record at exactly the moment computing was being invented somewhere else.

Those machines deserve their own chapter, because one of them was the first of its kind in the world.