When F. W. Winterbottom wrote the first book on Ultra ("The Ultra Secret", 1974), he described how Churchill allowed the town of Coventry be destroyed rather than risk divulging the fact that we were reading the German codes. However, others who were also envolved in Ultra at the time have disputed that account, such as "Most Secret War - British Scientific Intelligence 1939-1945" by Dr. R. V. Jones (1978). Dr. Jones was the head of the British electronic countermeasures unit in charge of detecting and disrupting German aircraft navigation signals used for precision night bombing attacks.

From page 200-205:

"Versions of the story of the Coventry raid have recently appeared, in which Churchill is said to have been presented with the dilemma of evacuating Coventry because of our prior knowledge of the raid, or of doing nothing and so preserving the security of our Enigma work. To the best of my knowledge, this is not true, and I will give my account of what happened.

By 6th November I was back in London after the visit to Swanage, an on the afternoon of, I think, Sunday 10th November I received a teleprint of a decoded Enigma signal to the X-beam stations. It had been sent the previous day and it was most unusual in telling the stations to prepare for operations against three targets numbers 51, 52, and 53. It gave the beam settings for the three targets, and it was only a few minutes work to find that 51 was Wolverhampton, 52 Birmingham, and 53 Coventry. Instructions for three targets had never been sent out at the same time before; it might be that KG100 [Luftwaffe pathfinder unit] was getting even more ambitious and intending to attack three targets in one night, but never before had any target instructions been sent out more than a few hours in advance of the attack. Moreover, whereas for the previous month or so all the beam settings had been specified to the nearest second of arc, for the three new targets the settings were merely speficied to the nearest minute, implying a coarsening of the accuracy by a factor of 60. Were the seconds to be specified later, or was it that whoever had drawn up the instructions had been as well trained as myself in not specifying a higher degree of accuracy than the operation required? I inclined to the latter alternative: remembering that we had already found KG100 dropping flares, it seemed likely that unless the beam orders were to be made more precise subsequently, the Gruppe was expecting to drop something less accurate than ordinary bombs, which could be either flares or incendiaries. I alerted the proper authorities, for it seemed that the foreshadowed change in German policy was now imminent, but I could not say when or in what order the three targets would be attacked.

There were other signs that a change was in the offing. On 3rd November we had some indication that preparations were being made for an abnormal amount of signals traffic between Luftflotte 2, commanded by Kesselring, and its subordinate formations. And on 11th November a long Enigma decode of a signal sent on the 9th November contained orders for what was evidently to be a very major operation under the code name "Moonlight Sonata". KG100 was to be involved, and among its tasks was to check the positions of the Knickebein beams. Four target areas, A, B, C, and D were mentioned, and there was one inflexible word which, had we been able to intrepret it, could have given us a clue: 'KORN'. It meant 'Corn', and I wondered whether it might be a code name for the appearance of radar screens when jamming was present, which we ourselves often called 'Grass' or when spurious radar reflectors were to be dropped, which although we were subsequently to call them 'Window' received the American code name 'Chaff.' What we did not guess was that it was an alliterative code name for Coventry, which the Germans spelt with a 'K'.

Where were targets A, B, C, and D? Since a map from a crashed aircraft had shown some larger areas thus lettered in the south of England, Wing Commander 'Tubby' Grant of Air Intelligence made a dubious correlation, for the map areas were too large to be suitable as bombing targets; they would have made much more sense as dropping areas for a large airborne invasion. Grant's intrepretation was circulated by teleprinter, but it was hard to believe. I would have much more likely believed something that Felkin reported, had I seen it. This was that a prisoner from KG100 had said that the heaviest possible attacks were to be made between 15th and 20th November on Birmingham and Coventry. Grant's intrepretation, however, tended to hold that the day as far as the Air Staff was concerned, and a series of countersteps was planned on the assumption that the main objectives of Moonlight Sonata were somewhere in the south of England. For some of our proposed counters, these misjudgements hardly mattered: these were the offensive measures against German bomber bases, and against beam stations, which were to be attacked by flying specialist bombing aircraft down their own beams, as I had previously suggested.

My own part int he operational story was the detailed forecasting of KG100's targets on a night-to-night basis and although I had pointed to the unusual threat to Birmingham, Coventry and Wolverhampton, and to KG100's pathfinding role, I had no standing in the making of the general Intelligence assessment. If someone had correlated my warning with that of Felkin, and especially if the KG1 prisoner had mentioned Wolverhampton as well (which he did not) thus making the correlation complete, the Air Staff appreciation might have been less in error. Whether or not this would have made much difference to what happened to Coventry is doubtful, because we did not yet know the exact order in which the towns were to be attacked, and we might reasonably have expected Wolverhampton to be the first; and even if we did know the correct order, both previous and subsequent experience showed that neither our nightfighters nor our guns could seriously damage the Luftwaffe.

On 11th November, while we were speculating about Moonlight Sonata, KG100 was to have attacked Liverpool (target No. 34) and Coventry (target No. 49, different from 53) but these operations were cancelled on account of weather. The Gruppe came out against these two targets on the following night, and then on 13th November it had no operations. So far there was nothing unusual - the target directions were specified with the old accuracy. By this time Charles Frank was with me, and I had introduced him to the Enigma situation, commenting that - thanks to Bletchley - if we could hold our through the winter we had a chance of winning in the end.

Together we braced ourselves for the following night, and for whatever 'Moonlight Sonata' might mean. It happened to be one of those afternoons when the Enigma signals to the X-beams stations were not broken in time, and we were therefore left guessing. There was, of course, evidence that could be gathered by flying our own aircraft along the German beams and establishing where they were pointing. Morever we could listen and obtain the beam frequencies, so that we could set our jammers appropriately. Somewhere between half-past-five and six o'clock Addison telephoned me from his headquarters at 80 Wing asking for my help in deciding the frequencies on to which to set his jammers. He was fairly sure that the target was somewhere in the Midlands, and his problem was to decide which beam was which, and therefore which to try to obliterate with the three or four jammers he had available. He then read out to me the list of radio frequencies as determined by our listening aircraft. I could see at once that the measurements must be wrong, in that they did not match up with the figures that I knew from the 'Anna' code. I therefore made a mental correction of the measurements as far as I could - for example, 68.6 should probably have been 68.5, if our receivers had been properly calibrated, or 70.9 should have been 71.0. But deciding what, for example, 66.8 meant, was more of a lottery.

The one other clue that I had spotted was that there seemed to be a convention that the director beams would generally be on frequencies between 66.5 and 71.5 and the cross beams between 71.5 and 75.0 megacycles per second, the division being presumably due to operational convienance. Remembering that we needed to knock out the main and reserve director beams and at least one of the cross beams, I then made my mental gamble, and suggested a set of frequencies to Addison which he said he would adopt. All this took no more than five minutes on the telephone but I was well aware that in those snap decisions I was probably gambling with hundreds of lives. Sobering though this thought was, the fact remained that someone had to do it, and I was easily in the best position.

Because of the abruptness in his move to London, Charles Frank was staying with Vera and me, and I can recall driving back with him to Richmond and wondering, as we passed in the bright moonlight through Roehampton, where the target really was. I for one did not know, and I do not think that anyone else did, either. Certainly I have no recollection of Coventry being mentioned in an Enigma message in the way that some accounts have stated; the teleprinter room into which the messages came was immediately across the corridor from my own, and no message mentioning Coventry was brought to me, as it certainly should have been if it had existed. As for any argument as to whether or not Coventry might have been forewarned, I knew nothing of it.

The suggestion that Churchill was warned in the afternoon that the target was Coventry, and that he then had to decide whether the city should be warned or not, has been effectively disposed of in a review in The Times Literary Supplement for 28th May 1976 by Sir David Hunt, at one time his Secretary, drawing on evidence of Sir John Colville who was on duty at No. 10 Downing Street on the night in question.

The book goes on to describe how salvaged equipment from wrecked German aircraft showed the British were using the wrong modulation tone in their jamming equipment so that the German equipment wasn't affected. It shows that a series of errors were responsible for the poor showing that night. As he mentioned, at that stage of the war, British anti-aircraft defenses weren't very effective against night bombers, either. It seems the story about Coventry being sacrifaced to protect Ultra is just that - a story. Any spelling errors are my own, not Dr. Jones'.