Seventy Years Ago This Month at Bletchley Park

April 1940

The War

At last after eight long months the phoney war is over! On 9th April, the Germans occupy Copenhagen and the Danes surrender. The Germans land at five points on the Norwegian coast, and occupy Oslo but the Norwegians fight on for eight weeks, assisted by an Allied task force until it is evacuated in May and early June after the Germans attack in France. But the Germans gravely miss the heavy naval losses that they sustain at Narvik when they come to plan the invasion of Britain for the autumn with few destroyers left.

The Invasion of Norway and Denmark. The Germans achieve total surprise in their invasion of Norway, despite various warnings. On 5th April British ships set off across the North Sea on the long planned operation to lay mines in Norwegian territorial waters; that day Neville Chamberlain, the British Prime Minister, declares “Hitler has missed the bus”. In a brilliantly conceived combined operation the Germans land in Denmark and Norway in the early hours of 9th April 1940, using only some 15,000 troops. They occupy Copenhagen, and King Christian X, recognising that his army is in no condition to resist, immediately surrenders Denmark. The Germans land in Oslo from the air, forcing its evacuation. They land from the sea at five points along the Norwegian coast, including Narvik where the local commander, a fascist sympathiser, orders the local garrison not to oppose the 2,000 German troops who had been carried there in ten destroyers. But elsewhere the Norwegians fight back. The British had withdrawn their naval force from the Narvik area only the day before, but in two naval operations on 10th and 13th April the Royal Navy sinks the ten German destroyers. The British lose two destroyers, and their commander, Captain Warburton-Lee is killed, posthumously receiving the first Victoria Cross of the war. On 9th April HMS Renown has damaged the German pocket battle-ship Gneisnau. Then fifteen Fleet Air Arm Skuas, flying from the Orkneys at the limit of their range, sink the cruiser Königsberg in Bergen harbour, the first demonstration, for those that have eyes to see, of the vulnerability of surface vessels to air attack.

The British, supported by French and Polish troops, land a task force at four points along the Norwegian coast in mid April but these are withdrawn early in May, except for around Narvik in the far north. The weather is atrocious and chaos largely reigns supreme. At first it looks as if the Germans will be forced to evacuate Narvik, but their command of the air proves decisive, and their reading of the British naval codes enables them to inflict heavy casualties on the Allied shipping. The Germans are flying in reinforcements through Oslo. On April 29th the British start their withdrawal but, in an operation that will do something to restore the British reputation, they are to land near Narvik in force on May 28th and recapture the port after desperate fighting. The task force did not know that, three days before they landed, the decision had been taken by the Cabinet to withdraw completely from Norway, because of the desperate situation in France.

The brilliant short German campaign in Norway has been a triumph for Hitler. Almost all his senior military staff had opposed the campaign, both for its extreme audacity and for the short time window before the planned major operation in France. So Hitler had taken personal supervision both of the planning and of the execution of the operation. The result was that such personal control by Hitler of major military operations became the norm, at first to the benefit of Germany but eventually much to the benefit of the Allies. The capture of the Norwegian ports and securing the iron ore supply routes from Sweden were major advantages for the Germans. But the heavy naval losses they sustained had a direct impact on the decision, taken shortly after, not to invade the UK until the British Air Force had been rendered ineffective.

Intelligence Performance before the Norway Campaign. There have been numerous warnings that the Germans are going to invade Norway and Denmark. General Oster, the second-in-command of the Abwehr, the German military intelligence service, gives warnings to the Dutch and the Vatican that Germany will attack Norway on 9th April. Similar warnings reach the Foreign Office from Danish, Norwegian, and Swedish authorities and from British attachés in various capitals. Air reconnaissance photographs of Kiel are at last obtained from a long range Spitfire on 7th April showing heavy activity, and there are British air and submarine reports of the German expedition sailing towards Narvik.

In March 1940, Bletchley Park had made its first contribution to the debate on what the Germans intend to do. As so often was to happen in the future, whenever possible Germany is careful to use land-lines when planning operations, so the BP breaks into the Luftwaffe Enigma, the Red key, provide few clues. Following information given by a double agent “Snow”, the Y service monitors the signals from a German spy ship in Norwegian waters. And in the second half of March BP reads the minor cyphers being used by her to radio back Intelligence information. This was the work of a special section under Oliver Strachey set up to tackle the Abwehr hand codes. (Oliver, a brother of Lytton Strachey, was an old hand from Room 40 in the Admiralty during the Great War. In December 1941 Dilly Knox, another veteran from Room 40, was to break the machine codes of the Abwehr who use a special version of Enigma). The Naval staff is informed on 30th March; the Admiralty orders that the spy ship should not be disturbed because of the “cryptanalytical value” of her transmissions. And then on 7th April, as a result of the signal Traffic Analysis work of the young Harry Hinsley, Hut 4 sends warnings to the Admiralty that there is exceptional German naval wireless activity in the mouth of the Baltic. The warnings are dismissed and the Navy continues with its mine-laying as planned. No Admiralty Intelligence section dissents from the view of the First Sea Lord, Winston Churchill, that a landing in Scandinavia is beyond German powers, and that the German activity simply portends another break-out by heavy ships into the Atlantic or against the British convoys to Norway. The British Air Intelligence branch is still advising that Hitler’s spring offensive would be limited to seizing Holland – to provide a base for an all-out attack on Britain. “A German descent on Narvik is surely out of the question” minutes the Norway desk officer in the Foreign Office. “While there was no confirmation of an unambiguous kind” comments Harry Hinsley, the Official Historian, writing long after the event, “the separate British intelligence branches might have found considerable indirect support if they had collated all the evidence that was available to them and if they had jointly considered it carefully”.

BP breaks the Yellow. In a remarkable demonstration of their growing expertise, on the 15th April BP breaks the Yellow, the German Army-Air Force Norwegian campaign key, shortly after it is introduced on 10th April and reads it almost every day, and often within an hour or so of the messages being transmitted. This was accomplished in Hut 6, under the leadership of Gordon Welchman, by using the Polish Netz, Zygalski sheet, method, as developed at BP by John Jeffreys, though he has now had to leave BP though illness. (John was to die of tuberculosis and diabetes in 1944). After Germany changed the setting up procedures for the other Enigma keys of their Army and Air Force on 1st May, the Yellow retained the old procedures, presumably because it was too difficult to change in the middle of a campaign. So the Netz sheet method could continue to be used for this key, and the large increase of radio traffic incumbent on a military campaign proves a great help to the cryptographers.

The First Naval Enigma breaks. On 26th April the German patrol boat or armed trawler Schiff, VP2623, was captured by the Royal Navy as she sailed from Germany to Narvik, calling herself the Polaris. Though the ship was carelessly ransacked, some invaluable cypher forms were recovered (but not apparently any key sheets). These gave BP some matching plain and cypher text in the Dolphin key. These enabled BP to break the traffic for 22rd to 27th April, some of them in mid May though others were not broken for some weeks. The young Joan Clarke (Joan Murray) working to Alan Turing did much of this work in Hut 8. She made use of the prototype bombe “Victory” installed in Hut 1, which lacked the diagonal board, so many false stops arose. The decrypted messages were without much direct operational value for naval purposes, though they did enable Hut 4, the naval intelligence hut, to build up background information about the German navy. And they were absolutely invaluable for the information that they gave the code-breakers about Dolphin, the main deep-water cypher used by the German navy, including at that time the German submarines. Hut 8 was to break Dolphin regularly from August 1941 onwards, after the capture of documents from the German meteorological trawlers in the North Atlantic.

The Value of the Early Breaks. According to the Poles, BP and the Poles broke some 126 key settings in total from January to mid-June, 83 % of these were being solved at BP. The Poles say that over 5,000 messages were read in this period. (The Official History refers to over 1,000 messages a day being read during the campaign in France once BP had re-entered the Red on about 20th May). Of course establishing a key setting enabled all the messages intercepted in that setting to be decrypted just as a German cypher clerk would do, typically for one day until the key setting changed at midnight. So BP is now reading hundreds of German messages. Unfortunately only the Navy had made special arrangements for the feeding of such information to their forces. But both the Red and Yellow had little information of direct value for the Fleet. (By contrast, the Germans were well informed about our naval operations, since at the time they were reading our navy and merchant navy codes). Such material as does reach the army commanders in Norway is ascribed to “Agent Boniface”, which does not add to its air of authenticity – but does result in some delightful requests for the agent to be asked to obtain information! And BP has to learn to interpret the copious material they are reading, which is often in pro-forma style or requires an understanding of German military terms that they then lack. That the success of BP with Enigma had little, if any, direct impact on the conduct of the fighting in Norway nor indeed for many months to come until ways were devised for handling this valuable information, is one of the tragedies of the Intelligence war.

Attack on Enigma by the “Bombe” method. Alan Turing has been working on the design of his “bombe” for finding the Enigma key settings. Alan rejects the technique that the Poles and Hut 6 are using, believing that the Germans will soon abandon the repeat sending of the code wheel settings. (In this he was proved right, for they abandoned the repeat on 1st May 1940, but strangely it was to reappear in due course on some naval keys). Instead he relies on the “crib” approach where the code-breaker “guesses” what part of the message may say. The development of the prototype “bombe”, a complex electro-mechanical machine, has been in progress under “Doc” Harold Keen, the research director of the British Tabulating Machine Co at Letchworth. The first prototype bombe had been installed in Hut 1 at BP on 18th March. Other than the Schiff related breaks this bombe, Victory, did not achieve many results due to the high number of false stops it generated, and it was not until August when a second bombe, Agnus, was installed in the “bombe room”, Hut 11, with a “diagonal board” relay plate, that the bombes became fully effective.

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