It remains the archetypal tale about how far governments will go to protect their intelligence secrets, even at great cost to civilians. 

But even though people at the top of our intelligence establishment have told me this story, there is clear and convincing evidence that it's not true. Did Winston Churchill, nervous about the Germans discovering that U.K. cypher-crackers had broken their Enigma codes, fail to act on intelligence warning of a Luftwaffe raid against Coventry in November of 1940?

By that time, the small huts full of men and women and Bletchey Park in England were routinely breaking the cypher that encrypted traffic between the German Air Force HQ and Luftwaffe signal operators who were directing radio beams cross Britain in order to help the pilots hit their targets. Often, the so-called "BROWN" cypher would be broken well in advance of that night's raids, which meant that civil defense officials could order up added protection from the Royal Air Force.

Not so on November 14. The Luftwaffe launched an enormous blitz at Coventry, destroying more than half the city and killing 600 people The legend that Churchill ordered that the city not be warned germinated in the 1970s when Britain finally declassified the fact that it had broken many of the German cyphers which used the Enigma machine.

In his 2001 book, Station X, historian Michael Smith has eyewitness testimony from the codebreakers whose job it was to tackle "BROWN" that on that day they simply failed to break the cypher — something that happened a fair amount of the time. There was some indication from another coded stream that a big German operation was planned for that night, and indeed, Churchill ordered air defenses around London beefed up. But there was absolutely no evidence that Coventry would be the target. Keith Batey wrote in a diary obtained by Smith that although they knew that a big bombing was coming, "we did not know more and to our dismay did not break any key for several days before 14 November: the result was the unhindered and disastrous bombing of Coventry." Batey, of course, had no incentive to throw himself under the bus well after the war when giving his recollection of a technology that had already been declassified.

While it was certainly true that Britain did everything in its power to make sure that Germany did not know that its tactical orders were being read by the British, that did not include the deliberate sacrifice of 600 souls. Indeed, it is doubtful that the German high command knew about the Enigma secret until much, much later in the war. 

There was a time when the codebreakers DID figure out something disastrous was about to happen to the Royal Navy, in June of 1940. Nothing was done. This was not because the Navy wanted its ships to be destroyed — it was a classic problem of intelligence analysis. Since the secret of Enigma was SO tightly held,  the Navy intelligence analysts did not know where these cryptographers were getting their material from, and assumed it was from MI-6's spies (that was the cover story). The Navy didn't trust MI-6 and didn't trust the material and chose to ignore it. 

British officials learned from these and other mistakes in the handling of intelligence, indeed learning that the hardest part to figure out in the whole process was how to disseminate vital intelligence, flag it as such, without exposing the source, which could lead to the cutting off of intelligence. Their methods revolutionized the way that "Special Intelligence," the British term for what we'd call Signals Intelligence or SIGINT, was distributed to commanders in the battlefield. It spurred the development of new technologies that allowed field units to receive intelligence rapidly, and it led, for a time, to a great advance in the way intelligence and operations mixed during World War II.

Lessons for today: Well, there are many, especially in light of the Benghazi tragedy. The best intelligence is often fragmentary. Governments DO go out of their way to protect sources and methods, and there ARE trade-offs. But the best intelligence is intelligence that is used, and is seen as a legitimately collected and seen as such. Intelligence analysis does not work in real life like it does in the movies.