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Seventy Years Ago This Month at Bletchley Park April 1941Mixed Fortunes in the Mediterranean. The British cap their remarkable campaign in East Africa by forcing the evacuation of the Italian forces from Addis Ababa on 5th April. But the small German force in North Africa under the dynamic leadership of General Rommel is sweeping the Allies back across Libya throughout the month, finally re-crossing into Egypt on 27th April, leaving the Australians holding out in cut off Tobruk. Now the limited British forces in the Middle East are further stretched by having to divert forces to Iraq to forestall a German take-over. On 6th April German armies attack in the Balkans, the Yugoslavs surrendering on 17th, followed by the Greeks on 23rd April. The British fall back steadily down Greece, finally mounting a successful evacuation in the last week of April, but leaving all their heavy gear behind. Despite the large Luftwaffe activity in Greece they are still able to make large-scale raids on England, attacking Coventry again on 8th, mounting a devastating raid on London on 16th and on Plymouth on 30th April. But decrypts reveal on 24th April that the Luftwaffe is moving from the Channel area to Poland for Operation Barbarossa. The Western Desert War. General Erwin Rommel and his only recently landed Africa Corps attacked the British and Australians in the Libyan Western desert at El Agheila on 24th March. Taking the British by surprise, and easily breaking through the lightly held line, he decided on 31st March to advance across the Cyrenaica desert in Libya, ignoring his instructions to wait until his small force could be built up. He advances along the coastal road and across the desert tracks, following in reverse the route pioneered by the British just a few weeks before, capturing General O’Connor in the process. He bottles up some 24,000 men, largely of the Australian 9th Division, in Tobruk on 11th April, and advances to cross the frontier into Egypt. But on the night of 30th April he turns on the thorn-in-his-side at Tobruk, where the Australians repulse his assault, inflicting heavy casualties on his tanks. The Allied forces had been severely weakened by the move of troops and aircraft to Greece and now Iraq. The RAF in the Desert is reduced to only 30 aircraft, though luckily the Luftwaffe is not strong there either. The Axis Africa Corps consists largely of units of the 5th Motorised and the 15th Panzer division together with what remains of the Italian desert army. With the threat of invasion still hanging over Britain, her troops in N. Africa have been bolstered by troops from the Commonwealth, India, Australia, New Zealand and South Africa. Then on 21st April, learning from an Ultra intercept that Rommel is to be reinforced with a further armoured division, Churchill persuades the War Cabinet to agree to the risky step of despatching 240 tanks direct from the UK to the Desert Army. During this campaign the Germans are well informed of the activities of their enemy by a steady flow of field signals interception, due to the lax signals security of the British desert forces – as Enigma decrypts reveal! BP is reading two sets of Enigma, both the regularly broken Luftwaffe general service Red key, and the new Luftwaffe North Africa key, the Light Blue, which had been first broken on 28th February 1941. But at this time tactical field intelligence, as distinct from strategic level information, is virtually non-existent for the British in the Western Desert. This is because the Italians have at last tightened up their communications security and in any case rarely refer to German movements. The British Signals Interception “Y” unit, though well practised at dealing with the Italian traffic, is untrained in the interception and exploitation of German radio traffic. On 2nd March, at the time of the decision to transfer troops from North Africa to form the British Expeditionary Force in Greece, General Wavell had advised the Cabinet that he discounted the possibility that Rommel could constitute a serious threat before the summer, because of the slow rate of build up possible in view of the sea-crossing and because of the lack of experience in the desert of the German forces. At the time it was known from decrypts that the Germans were preparing an operation for the invasion of Greece that would be larger than any they might attempt in North Africa. Though the estimate of the rate of build-up of the Africa Corps proved correct, the belief that Rommel would not attack with a small, inexperienced force was incorrect. Even when the attack started on 24th March it was believed in GHQ, Cairo, that Rommel would not be ready to try to retake Benghazi until about mid-May, though there was some concern because of the Enigma evidence of the strengthening of the Luftwaffe in North Africa. “During this advance British intelligence was scarcely more successful in influencing the fighting than it had been in foreseeing it. Nor had it provided in advance any warning of the superiority of Germany’s equipment” says the Official Historian. On 13th March a direct flow of Ultra decrypts had been established from BP to GHQ Cairo, and on to the top British Commanders in the field, though at this time they were only told that it came from “a completely reliable source”. During the first few weeks of the service BP were sending no more than one signal a day to Cairo, and the selection of intelligence by Hut 3 at this time has been described as “patchy and capricious”. Unfortunately both BP and Cairo failed to send to the field commanders some important information they had obtained from decrypts, presumably through inexperience of the type of information really valued in the field, and because BP had still to learn to work very fast. And some of the information BP did forward proved to be incorrect, perhaps due to corruption of the intercepts. But BP did send a steady stream of valuable strategic information. “When all this has been said, however, the Enigma traffic was still invaluable for its strategic information, if not on the tactical level, and it had been so from an early stage in the German advance”. By the end of April the volume of BP Enigma decrypts relating to North Africa and the Middle East was increasing and information became available on the daily intentions of the Luftwaffe, with regular reports on the German fuel and rations situation. Steps were taken to strengthen the distribution process and reduce the delays in Cairo with two Army officers who were “indoctrinated” as to the source. But it was not until August 1941 that the Special Liaison Unit distribution system was introduced; the information then went direct to the field commanders who at last were told about the source. The Campaign in Greece. On 6th April the Germans, supported by the Italians and Hungarians, launch three armies into Greece and Yugoslavia, with both overwhelming air superiority and far greater armoured forces, rapidly advancing from Bulgaria to seize the port of Salonica on 9th April, cutting off a Greek army which promptly surrenders. The Axis advance into Yugoslavia from Hungary is aided by a massive air raid on Belgrade killing 17,000 civilians on Palm Sunday. The city falls on 13th April, and Yugoslavia formally surrenders on 17th April. The Italians advance into Greece from the annexed Albania on 11th April, leaving the four British divisions and the Greek forces on the Aliakmon river line, in the shadow of Mount Olympus, outflanked by the 50 Axis divisions. Step by step the British fall back down Greece, forewarned by the Enigma decrypts of the progress of the Axis forces. The Greeks sometimes fight hard, especially against their old enemy, the Italians, but are weakened by dissention, even treachery, in their leadership. They surrender to their invaders on 23rd April. To the delight of all in the Army who know the precedent, the British make a stand at Thermopylae, giving time and cover for the evacuation of the Allied force. Once again the RAF seems to the hard-pressed infantry to be missing, though their small and overwhelmed force of fighter aircraft has sacrificed itself magnificently if fruitlessly. Over six days from 24th-30th April some 50,000 Allied soldiers are evacuated from the small ports of Southern Greece, under strong naval cover, largely going to Crete. (Unfortunately the Army apparently forgets to pass on to their few remaining Air Force colleagues the evacuation details). The British Expeditionary Force has lost 16,000 men (14,000 of them prisoners) in Greece, and all their heavy equipment, transport and aircraft have to be left behind. Once again the overwhelming and fast moving German forces have won an easy victory, though the partisans in the Balkans soon prove to be a much more formidable problem than in the previous territories that the Germans have so easily overrun. The British field Y unit in Greece was the first fully mobile one in the Middle East, arriving in Egypt in January 1941. They had originally dealt with low-grade Italian cyphers, but the unit had operated in France and Belgium in 1940 so some of the team know the German language and procedures, which enables them to tackle the Luftwaffe tactical signals and codes. At this time BP has yet to make any progress with the German Army high-grade cyphers for lack of traffic. And perhaps because the Y unit is constantly on the move during the retreat the service it provides in Greece is believed to have been of little operational value. But the Intelligence derived from the Luftwaffe Enigma decrypts at BP, sent out direct to the British GHQ in Greece, is of enormous value for warning of the progress of the Axis forces and so reducing the scale of the calamity. For the first time the breaking of Enigma by BP has a significant effect on British military operations, even if it cannot prevent the disaster. And now a new challenge is about to open up in Crete. Hut 8’s struggle with Dolphin. U-boats and aircraft sink 195 British ships in April. Due to a key sheet for the German navy “Home Waters” key, Dolphin, captured from the trawler Krebs, Hut 8 at BP had been able to read the signals on this key for some, but still not all the days of February. Now they struggle to break the signals for April cryptographically, and finally succeed on 22nd April, then going on to break the keys for nine days of April. It is difficult because they have to rebuild the code-book the German navy use for encoding their indicators before encryption in Enigma, and because they are feeling their way to find good cribs, largely from repeated information derived from the Dockyard Cypher, Werft, and then from the naval short weather reports repeated on the land-based transmitter, DAN. The delays at this time in breaking the signals do not make them of operational use, but Hut 8 is finding better cribs and gradually improving its performance. The Leak about Purple. Though it was Hugh Foss and Oliver Strachey of the GC&CS who had broken the Type A Japanese diplomatic cypher machine in November 1934, by 1940 the effort at BP on the Japanese codes had had to be reduced under pressure of the European war. It was the US Army team who first broke the Type B Japanese diplomatic cypher machine in September 1940. The Americans built a machine to simulate what they called the Purple machine, and when a delegation of cryptographers from the, still neutral, US came to BP in February 1941 they brought one of these machines with them, essentially in exchange for information on Enigma. BP gave the US team details of how the work of John Tiltman was enabling the breaking of some of the Japanese army codes, as well as information on Enigma and probably on the Turing bombe. They were given a copy of “Prof’s Book”, written by Alan Turing, which setout in detail how to break the Enigma keys. But they were not given one of BP’s precious Enigma machines, resulting in Captain Laurence Safford, the head of the US Navy cryptographers, demanding that the UK should give the US an Enigma machine in return for the Purple machine. He was always very opposed to co-operation with the British, and did not endear himself to BP by writing this complaint in an unclassified letter. On 29th April the German Minister in Washington sent a signal to Berlin stating that an “absolutely reliable source” had revealed to him that the Americans had broken the Japanese most secret method of communications. Luckily apparently neither the Germans, nor the Japanese when alerted to the “alleged leak”, believed that this could have been done. This was just as well because the British were already benefitting from the “Magic” information derived from Purple. (Indeed this whole story about the leak was revealed from a Purple decrypt from the Japanese Ambassador in Washington to Tokyo of 20th May1941). Particularly valuable were the signals from Berlin of the Ambassador, General Hiroshi Oshima who was very well respected in Berlin; he was able to obtain important information on the Nazi plans, etc, which he sent back to his government - to the great benefit of BP! A current example is the information Oshima sends about the German plans to invade Russia; and over the years to come his messages gave BP much information, such as detailed accounts of the state of the “Atlantic Wall”. At this time there are those in BP who are very opposed to close collaboration with the US cryptographers on the grounds that the Americans are not sufficiently security conscious. This was by no means the only occasion when this fear was proved to be not without some foundation - though there are those who blame a coding mistake in the British Washington Embassy for this leak. A U.S. historian has described the two sides at this time as “… walking around and eying each other like two mongrels who have just met.” The Bletchley Park Trust welcomes the preparation of these notes, but the authors are responsible for the statements and the views expressed. |
