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Seventy Years Ago This Month at Bletchley ParkSeptember 1940The Battle for Britain. Britain daily awaits the invasion as the battle rages in the skies above. The country has now been brought to full invasion alert. But as the days pass, with every day bringing news (however exaggerated) of fresh triumphs in the air, British morale rises. Gradually, towards the end of the long month, the belief spreads that “Hitler has missed the boat”, that he won’t dare to come – at least not this year. Now the focus is on the bombing, the Blitz, with a new sense of unity of purpose developing, a common defiance in adversity; “Britain can take it!” At the beginning of the month the challenge for the Intelligence Services is to give adequate warning of the invasion, but steadily moves to giving firm assurance that the danger is passed for this year. Britain is now fighting on two fronts and the demands of the defence of Egypt and of the developing Atlantic battle must tax our overloaded services even further. For on 13th September Italian forces invade British defended Egypt from Libya along what was to become the famous coastal road, capturing Sidi Barrani but then hesitating to advance much further. And on 23rd-25th September Britain makes things even more difficult for itself by mounting an abortive attack on the French fleet at Dakar. This is called off when open warfare begins to break out between the French warships and the Royal Navy. The Battle of Britain. By the beginning of September, the attacks on the RAF airfields were drawing to their end. Fighter Command has learnt to send standing patrols from 10 and 12 Groups to watch over the airfields, as the fighters from the front-line 11 Group battle it out with the waves of German bombers and their protective screen of fighters. The price to pay for attacking the airfields in daylight is proving too heavy for the Luftwaffe; only seven aircraft were destroyed on our airfields in September. The British Intelligence Services continue to over-estimate the size of the German airforce, but the German Intelligence services are even worse at over-estimating the damage they have done to the RAF. At the beginning of the month German Air Intelligence declares that at least eight airfields have been knocked out, and the rest of the system severely depleted, claiming Fighter Command is reduced to “a mere 100 serviceable fighters”. The reality is that the airfields and radar systems have survived with remarkably little disruption, and the number of British fighters now starts to climb slightly higher than at the beginning of the battle due to the ever increasing rate of replacements coming from the factories. The number of fighters operational has actually gone up from 701 at the beginning of the month to 738 available by the 6th. By a strange irony in reality both sides had lost virtually the same number of fighters during August. The RAF had lost some 444 aircraft, almost all fighters, during August, and the Germans had lost 443 fighters but they had also lost some 450 bombers. Fighter Command had lost 22% of its pilots during August but now emergency measures were providing a steady stream of good replacement pilots, such as by bringing the experienced Polish airmen into the battle. But for the Germans the loss of their experienced pilots was proving much more of a problem. Then on 7th September the Germans mount the first mass daylight raid on London, setting the docks and the East-End ablaze. The 350 German bombers seemed to fly in out of a clear sky, for our fighters were expecting them over the airfields, but on the way back home the Germans suffered severely. Thereafter the battle changed to the clear advantage of the British. In the week from that raid on 7th until 15th September the RAF lost 120 fighters to only 99 Luftwaffe fighters, but the Germans had also lost 199 bombers. They were increasingly forced to change tactics, providing more close support for their bombers and going over to night bombing. On Battle of Britain Sunday, September 15th, more than 200 German bombers set out for London and 158 reach it, but that day they lose 34 bombers with a further 20 most extensively damaged, a loss rate of 25%. This was the last great daylight raid, with the air battle increasingly merging into the “Blitz”. It would be nice to think that this change of tactic was a direct result of Churchill’s order for “revenge” attacks on Berlin, but the evidence is that attacks on the large cities had always been planned as the third stage of the air campaign. However there can be no doubt that the change was very providential for the RAF. If night bombing reduced the ability of the RAF to intercept the bombers, the German night bombing accuracy was poor - even if better than ours at this time, because they had the aid of the navigational beams when we were not manipulating them. And night flying proved to bring a high attrition rate due to accidents. By now the RAF has learnt to harness the available Intelligence especially that from the Y service intercepting the chatter of the pilots about their next missions as they returned to base, and so anticipate some of the coming enemy moves. The Battle moves decisively in our favour. The Invasion Date? Unbeknown to British Intelligence, that Sunday 15th September had originally been set by the Germans for the invasion, “Sea-Lion”. This date seemed to both sides to be about the most suitable because of the tides and moon. From hints derived from the Enigma decrypts, as well as from photo-recce showing the build up of the invasion barges, on 7th September the Intelligence Services had initiated the warning (“Cromwell”) that the invasion was imminent and this was maintained until well into October. (Actually the Official History says that to learn the date the Intelligence Services “resorted to every conceivable source and to some devices that were barely conceivable…The Vice Chief of the Naval Staff was surely being sarcastic when he suggested on 1st October that Naval Intelligence should set up an astrological section”). Hitler, like his naval advisors, had never been keen on what would inevitably have to be a sea-borne invasion. He postponed the date first to 27th September, then to 12th October and finally, as the air war failed to produce evidence of the command of the air that victorious landings would demand, until May 1941. The German navy had suffered severely during the Norwegian campaign, and in mounting an invasion they knew what they would suffer against the British navy provided with good air cover. The German airforce continued to persuade themselves that their assault was bringing the RAF to its last gasp, but even they had lost their enthusiasm for the gamble in the face of their mounting losses of experienced pilots. And, in any case, Hitler’s attention was now focussed on the East; the attrition of night-time bombing might yet persuade the stubborn British that peace was desirable! The Value of Intelligence Information during the Battle of Britain. While there can be no doubt that the accumulation of information about the German air force from the flow of “Red” key Enigma decrypts provides very considerable important background, the work of BP cannot be of much direct help in the air battle. The continuing decrypts of that Luftwaffe key has brought about a scaling down of the official, vastly exaggerated, estimates of the likely number of bombers available to the Germans. But on the crucial questions of air strategy here the Enigma decrypts can be of little help. The British air intelligence teams are suspicious of the Air Ministries official kill figures, but continue to over-estimate the Luftwaffe strength, and do not realise that the Germans are suffering from the low serviceability of their aircraft. Because the Germans are largely using landlines for orders on strategic policy, during the battle advanced warnings of policy changes are rarely obtained. But in the day-to-day fighting, the Enigma decrypts provide an ever-increasing amount of Intelligence information, though short notice changes in the Luftwaffe plans can invalidate the Enigma-based forecasts. Lower-grade information from the Y service, especially plain language radio traffic, is of some considerable value but at first tends to report the situation as it emerges rather than providing a forecast. Information from the Home Defence Units round the coast, using German-speaking WAAF and WRNS staff on high frequency radio intercepts from the Luftwaffe pilots and their ground controllers, is sent direct to the local RAF commands as well as to HQ Fighter Command, where it can be correlated with the invaluable radar information and that from the Observer Corps. The simple code for these transmissions had been broken by BP and was read by the RAF Y station control centre at Cheadle. With growing regularity and accuracy as the battle proceeded, the organisations exploiting the Luftwaffe signals traffic are able to give advanced information about the enemy’s attacks. BP breaks the “Brown” Navigation Beams Key. The second bombe, the first to be equipped with the vital diagonal board, had been delivered to BP on 8th August and had been installed in Hut 1 in place of the first bombe, Victory, which was taken back to the BTM factory at Welwyn for fitting the diagonal board and general refurbishment. Victory comes back into service during September, and it seems very probable that it is re-installed not at Bletchley Park, but in the stables at the nearby Wavendon House. Because of the threat of the air bombing, steps had been taken to open an alternative site, staffed by BP and under its complete control but sufficiently far away that it might escape an air assault on the Park. (By the year-end there were some 650 GC&CS staff of whom 550 were at BP and 56 at Wavendon including Leading Seaman Fordham who was seconded from BTM, the firm who built the bombes). In due course there would be five bombe outstations, all run from BP, maintained by Eric Jones and his team, and from March 1941 manned by the WRNS, who in the end came to number some 1600 girls working on over 200 bombes. In September when there are just two bombes, the demand for machine time is enormous; inevitably during the invasion threat the demands of the naval team in Hut 8, who had so far failed to break into the German naval Enigma, have to take a back seat. Hut 6 is concentrating both on breaking the general purpose Luftwaffe key, the “Red”, and now on the Luftwaffe “Brown” key used by the German regiment who run the navigation “beams” (IV L N Versuchs regiment). The Brown key is first broken by Hut 6 on 2nd September 1940, by the use of Cillies. Thereafter for some time the Cillies continued to be the primary method of Enigma code-breaking. But gradually, as experience built up, the bombes took over for, once decent cribs were available, the bombes were likely to do the job faster and with less manual effort. At this time a bombe might have to be deployed on one job for some 18 or more hours before the correct key was found, typically producing a result in the evening after the key change made by the enemy in the middle of the night. It is Stuart Milner-Barry who now builds up the team that provided the vital cribs, based on a study of the decrypts. Gradually as experience of the decrypts grew it became customary for the Brown to be broken most days, though never as regularly as for the Red – and this continued until the end of the war. (The German Brown Enigma operators came to provide much entertainment for Hut 6 due to their remarkable incompetence). The brilliant Crib Room team became a vital part of Hut 6; indeed Stuart was made head of Hut when Gordon Welchman was promoted A D. (Mech) in Oct. 1943. The Detection of Lorenz. BP (Oliver Strachey and the few who were to grow into his ISOS team) had been reading some of the minor cyphers used by the Abwehr, the German military secret service, for some months. BP went on to break the standard cypher of the spies in Dec.1940 and the Abwehr Enigma in 1941. So it is perhaps not so surprising that all but one of the 21 very inept spies who were sent to the UK during the run up to “Sea-Lion” were picked up within a fortnight. Maybe searching for their transmissions explains why it was the Southampton police (or so it is said) who during September 1940 first detected the high-speed teleprinter-type transmissions that came to be recognised at BP as carrying the “Fish” codes. These used the Lorenz SZ42, often called the Geheimschreiber (though that was really a rather similar machine made by Siemens). In due course BP established a special team at the Y station at Knockholt to intercept this traffic. These Fish codes were used for the very top army communication channels and the breaking of some of these messages from 1942 onwards proved to be of the utmost value, the crowning glory of the work of BP. The Bletchley Park Trust welcomes the preparation of these notes, but the authors are responsible for the statements and the views expressed. |
