Seventy Years Ago This Month at Bletchley Park         

March 1941

The Pause before the Storm.

On land there is little fighting in March, except in Eritrea where Allied troops take Keren after bitter fighting.  At the end of the month in the Western desert General Rommel launches a limited attack with his Africa Corps, and decides to turn his local success into a campaign.  This is some weeks before Allied HQ in Cairo thinks he will be ready to attack, and indeed quite contrary to his instructions from German GHQ not to risk his small force.  From BP decrypts, the Allies know the Germans intend to invade Greece soon, and on 7th March the British start landing there to forestall this. The USA and Sweden warn the USSR that Hitler plans to attack them, their brilliant spy in Tokyo, Richard Sorge, even giving them the expected date as the second half of June. The Russian new Chief of Staff, General Zhukov, issues a secret directive instructing the frontier regions to make appropriate preparations.  From the air on the night of 13th March the Luftwaffe attacks Glasgow and Clydebank, causing massive devastation.  At sea on 28th/29th March the Royal Navy surprises an Italian fleet off Cape Matapan in Southern Greece. This was the first major triumph that can now be seen to have resulted directly from the reading of Enigma decrypts by BP.  But perhaps even more important for eventual victory, on 13th March BP is allowed to transmit its intelligence information directly to the Allied HQ in the Middle East, avoiding the delays and interference of the Intelligence services in Whitehall.

Intelligence Information for the Balkans Campaign. 

With the coming of the spring, and no diminution in the night-time bombing campaign, the country is on renewed invasion alert. For the British Intelligence services, if not the Prime Minister, continue to be ambivalent about the threat of the invasion of the UK, and so are reluctant to accept that Germany is preparing to invade Russia. This is despite clear decrypt evidence that the German invasion forces are being dismantled and moving East. By now even the Intelligence Services in Whitehall are accepting that Hitler has turned his attention to the Balkans and the invasion of Greece, but perhaps the Germans have the resources to mount campaigns on both fronts?  Much of the information comes from BP where they are regularly reading the Luftwaffe Red and the German railways Rocket Enigma keys. The German advance over the Danube into Bulgaria on 2nd March comes as no surprise to the Intelligence sections in Whitehall.  The Cabinet decides, after considerable hesitation, to proceed with the planned deployment of troops to Greece that starts on 4th March, building up to four divisions and 60,000 men.  This results in Hitler deciding to build up the scale of his operation in Greece, and to bring it forward to 1st April to minimise the delay the British intervention may cause to his plans for the subsequent invasion of Russia.  This happens to bring his plans into line with the British Intelligence current view on when the assault will begin. The Whitehall estimate of the strength of the Luftwaffe forces in the Balkans is very accurate, due to the steady stream of decrypts, but considerably over-estimates the number of divisions the Germans intend to deploy in Greece.  Then the coup in Yugoslavia on 27th March, brought off with the help of the British Special Operations Executive, puts an anti-German faction into power, causing Hitler to increase to 29 divisions the force he intends to deploy in the campaign, about in-line with the Whitehall predictions.

The Battle of Cape Matapan and Italian Naval Enigma.

On 26th March, pressed by the Germans, the Italian fleet sails for its first large-scale offensive of the war; it was to be the last of any importance. They intend to strike against the British convoys carrying troops from Alexandria to Greece.  On 25th March reading the Luftwaffe Red and Light Blue keys provides a general warning that German fighters from Libya are to move to Palermo “for special operations”.  Then late that day comes a message from the decryption by BP of the rarely used Italian naval Enigma, warning that this day is D-3 for an operation in the Island of Rhodes area.  Dilly Knox had first broken the Italian naval Enigma in April 1937.  This was an earlier version of the machine and, unlike that in use by the Germans, it has no plug-board so that it was, in principle, simpler to break. But unfortunately it was only used for a few messages a day so the reading of these occasional messages remained a major feat of cryptanalysis. Now the veteran Dilly Knox with the help of his twelve girls in his ISK team in the Cottage, notably Mavis Lever and Margaret Rock, are performing the feat.  On 26th March two further Enigma messages are decrypted pointing to an Italian naval thrust into the Aegean or Eastern Mediterranean.  Admiral Andrew Cunningham, C-in-C Mediterranean Fleet, immediately orders his forces to converge on the waters south of Crete.  But the Official Historian, Harry Hinsley, for once allows a good story, perhaps because as a young man he was personally involved in the Naval section at BP “Knowing that the Japanese Consul General in Alexandria regularly reported on the movements of the Fleet, and as regularly played a round of golf, the C-in-C visited the club house with his clubs and an overnight bag, and let himself be sighted” before he embarked on HMS Warspite. (Decrypts confirm that the Italians believed the fleet was still in Alexandria on the 27th).  On the evening of the 28th the Fleet catch the Italians completely unawares, and sink three cruisers and two destroyers for the loss of only one aircraft. The experience resulted in the Italian Navy leaving the British to control the eastern Mediterranean, which was crucial in the evacuation of Greece and Crete.

Intelligence Information from BP Goes Direct to the Field.

Ever since GC&CS was created back in 1919 it had been understood that their role was limited to cryptography, not deriving Intelligence information – at least as far as the military side of their work was concerned.  Because it was recognised that the cryptographers at BP needed to understand the traffic they were trying to break it was accepted that they should have translators and some limited Intelligence capability to interpret the material and refine the cryptographic process. Their output to the service Intelligence branches in Whitehall would take the form of the raw decrypted and translated material, though they could add explanatory comments such as suggesting what a corrupted place name might be.  And indeed this was what BP did provide throughout the war – though that was far from all that BP came to provide!  The BP staff would not be shown the other material that the Intelligence branches were gathering, nor provided with advance notice of our own plans, except in very exceptional circumstances. 

But by the spring of 1941concern about the delays and miss-appreciations that were coming out of the military Intelligence branches came to a head when BP broke the Light Blue and it was realised that this could give information of immediate operational value to the commanders in the Middle East if only it could be got to them in time.  On 13th March BP is authorised to send paraphrased decrypted material, with its origin disguised, direct to the Middle East, without going through Whitehall.  (It would seem that this was after a long battle and probably one can detect the heavy and impatient hand of Winston Churchill in the decision).  In the first place it went to the Director of the Combined Intelligence staff at GCHQ, Cairo, for distribution but on 27th March it started to go direct to the army and air commanders in Greece.  Of course the Whitehall Intelligence branches retained the right to make their comments and re-interpretations, but those would have to follow the BP material.  There was a precedent from the “agents reports” sent direct from BP to the British GHQ in France in May 1940, but it was certainly a revolutionary step forward!   The Intelligence branches were furious and fought a long battle to regain their authority, which culminated in the management troubles in Hut 3 during the winter of 1941/42. The historian and Hut 3 Intelligence Officer Ralph Bennett writes long after: “Armoured at first by civilian ignorance of military conventions, accustomed to giving reason and logic precedence over hierarchical authority, and unencumbered by inherited prejudices, the ‘amateurs’ marched forward into the almost unknown with a boldness hide-bound Whitehall Officers would not have displayed.”  As time went by and the value of BP became all too apparent, the Intelligence Branches gradually came to work in harmony with these strange but wise people who inhabited the huts at BP.  

The Krebs Capture and the Breaking of German Naval Enigma. 

There is increasing frustration both in the Admiralty and at BP at the delay in breaking naval Enigma.  Of course those few acquainted with the detail understand that it is a much more difficult task than breaking the Luftwaffe Enigma, both because the naval service coded their short messages before encrypting them using their Enigma machines, and because through-out the war they showed a great sense of signals security.  It is true that at this time in early 1941 no Army Enigma is being broken either, but the Army could always keep its traffic to land-lines except when the fighting was active whereas the Navy had no choice but to use radio communication with its ships.  Long ago, by the end of 1939 Alan Turing had worked out the way in which the German navy encoded its indicators and messages, using short signals to minimise the chance of giving away their location by radio direction finding.  By mid-1940 he and his team in Hut 8 knew that the German Navy used three extra code-wheels in addition to the standard five wheels used by the other services from which to select the three to put into their Enigma machines on any particular day, and Hut 8 probably knew the wiring of all eight of them.  But the extra wheels mean that very considerably more precious bombe time is required to break a naval message.  And, as always with Enigma, without experience of the traffic it is difficult to find satisfactory cribs with which to prime the bombes.  Turing and those in the picture recognise that they are unlikely to make progress without the capture of some material.  A small team under the Director of Naval Intelligence has been planning this for some months.  In September 1940 Ian Fleming had proposed a scheme, worthy of 007, for the capture of a ship using a restored German aircraft that would crash land in the sea alongside as bait.  More surprisingly, apparently this hair-brained scheme had been accepted and Ian Fleming and his volunteer team were only frustrated by the failure of a suitable German ship to appear in the Channel in daylight.  Now a commando expedition to wreck installations in the Lofoten Islands provides the opportunity to capture a German trawler, when on 4th March the signals officer from the destroyer HMS Somali boards the Krebs. Though the crew of the Krebs manage to throw overboard the Enigma machine, three code-wheels and some documents are recovered from the Captain’s cabin.  When these finally reach Hut 8 on 12th March they are found to include an Enigma key-sheet for February.  This enables Alan Turing and his team to read much of the traffic for February in the all-important Home-waters naval key, still used at this time by the U-boats, called Dolphin at BP. But this is achieved with considerable difficulty, as they painstakingly first have to reconstruct the bigram table used by the German navy to encode the message setting. They then start to break some messages cryptographically for April and May but it is so slow going, without good cribs and suitable code-books, so the resulting material is of little use for operational purposes.  But Hut 8 now knows what it needs if the U-boats’ Enigma is to be mastered.  Unfortunately Admiral Dönitz is suspicious about the capture and demands a separate key for his U-boats.

The Wrens Arrive at BP. 

There are now four Turing bombes, located at BP and the nearby outstation at Wavendon.  The small, tri-service, team of technicians under Sergeant E. Jones (RAF) is stretched to the limit providing a round-the-clock service, for the bombes are always in demand.  Eight Wrens arrive at BP on 24th March in an experiment to determine if such unskilled staff can successfully operate the machines – “it was doubted if girls could do the work”!   That the experiment proved a success now cannot be doubted, for by the end of the war there were 1676 bombe-girls (all Wrens) at work in three shifts on 211 machines; by then the team of technicians under Sq. Ld. Jones to maintain the machines had grown to 263 (almost all RAF).  The Wrens were not the first service girls to work at BP, for some 6 WAAFs have been manning the telephone exchange since the early days, in Room 5 in the Mansion. Now 36 WAAFs form the majority of the Communications Section that operate the teleprinters from the Y stations, to the transmitters, etc.  And by this time at work in Hut 5 are at least 10 “Volunteer ATSs” helping the Military section.  In the early days the Wren ratings live in billets in Bletchley that are famed for their lack of baths.  In her end-of-war report Superintendent E. Blagrove wrote: “There were many difficulties in the early days in the struggle to live. Ration cards failed to appear, the bath and laundry situation caused many headaches, medical and dental arrangements had to be organised and the problem of billets was always cropping up. There was a magnificent spirit amongst these pioneers and whenever they turned they found great co-operation and many helping hands. The stimulation was the knowledge of the essential work on which they were employed. Their keenness to do well and their enthusiasm was the inspiration for all who came later. These ratings were destined to be the future Officers and Chief Wrens of their Section”.  The Wrens rapidly spread across the site and by the end of the war the total compliment of Wrens at BP had grown to 2963.

The Bletchley Park Trust welcomes the preparation of these notes, but the authors are responsible for the statements and the views expressed.