Seventy Years Ago This Month at Bletchley Park   

May 1940

The Fighting in Norway. 

 Following the German assault on Denmark and Norway on 9th April, Allied forces have been fighting alongside the Norwegian forces near Trondheim and Andalsnes. Soon most of the fighting in Norway draws to its end, with the evacuation of the Allied troops from all but the north of Norway early in May. On 1st May the Norwegian government gives up, but their army fights on. Further Allied landings take place near Narvik on 13th May and the Allies capture that port on 28th May, with the Germans falling back to the Swedish frontier. But even before these landings have taken place the British government has taken the decision to totally evacuate Norway, and this is carried out from 31st May until 8th June with the loss of the aircraft carrier Glorious, two destroyers, and two squadrons of precious fighters

The brilliant short German campaign in Norway has been a triumph for Hitler. He had taken personal supervision both of the planning and of the execution of the operation. The result was that such personal control by Hitler became the norm. The capture of the ports and securing the Swedish iron ore supply routes 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.

The Invasion of Holland, Belgium, Luxembourg and France. 

The long-expected German assault in the West takes place in the early hours of 10th May with the German invasion of the Netherlands and Belgium. In a masterly display of detailed planning, the Germans bypass the Maginot line to its north, encircle the Dutch and Belgium armies and pin the British Expeditionary Force and the French First Army Group against the Channel. As planned, the British move forward into Belgium, but the Germans use airborne assaults to capture key fortresses and river crossings. The Belgium army fights bravely, but the capture of their fortress at Eben-Emael on 11th May by glider-borne troops leads to a hole developing in their line through which a Panzer division streams. It is led by a little-known General Erwin Rommel who had impressed Hitler by his performance in Poland. The French engage Rommel after he crosses the Meuse but loose sixty-five tanks, though they cause Rommel to loose thirty of his. The bombing of Rotterdam on 14th May results in many casualties and the Dutch surrender the next day after delaying the Germans longer than they had planned. 

Meanwhile on 13th May, totally unexpectedly, the main German armoured assault led by General Heinz Guderian bursts through the “impenetrable” Forest of Ardennes, crosses the Meuse near Sedan, and swings in a broad semi-circle to head for the Channel coast to cut off the Allied forces in Belgium. The British bomber offensive to destroy the pontoon bridges over the Meuse is a costly failure, loosing in one day forty of the seventy-one planes involved. The attempts by the French to hold the line at Sedan are a failure, and on 15th May panic breaks out in Paris where it is still thought the city is the target of this thrust.  The one impressive armoured counter-attack is led by Colonel Charles de Gaulle near St Quentin, but the Germans sweep on, reaching the Channel near Abbeville on 21st May. The British and the French had fallen back to the Scheldt on 16th May. On 19th May Winston Churchill, who had become Prime Minister of a coalition government on 10th May, orders the Admiralty to prepare for an evacuation from the Channel ports.  On the 21st the British mount a counter-attack at Arras to break through the encircling armies. It causes near-panic in Rommel’s Panzer division, but is thwarted by German dive bombers and the lack of any accompanying French movement. On 23rd May General Gerd von Rundstedt orders a pause in the advance on the Channel ports to give his troops time to regroup. Next day Hitler backs this order to conserve his prized Panzers, for his eyes are now set on the coming battle for France against the forces that stand along the Somme and before Paris. And when the advance is restarted on 26th May the Panzer generals are also looking to conserve their armour for that fight.  The surrender of King Leopold of the Belgians on 27th May leaves a huge gap in the perimeter defence near the coast, but an as yet unknown junior general, Bernard Montgomery, in a brilliantly executed manoeuvre rapidly fills the gap. The remaining escape corridor round Dunkirk steadily contracts but the perimeter holds, despite the unremitting air onslaught. In the rapid campaign, numbers on each side had been roughly equal, but the Germans were far superior in the air and they used their tanks in armoured Panzer formations, with the Allied tanks dispersed in penny numbers throughout their forces.

The Dunkirk Evacuation.

Waking at last from their apparent lethargy, the British improvise a remarkable evacuation from the mole and across the beaches round Dunkirk.  Until near the end the French believe we are forming a bastion that will remain to threaten the German rear. Churchill refuses the pleas from the French government for more fighter squadrons to be sent to France.  In practise during the nine-day operation from 27th May until the 4th June of the 338,000 men rescued 120,000 were French.  But the bitterness at the “betrayal” in the minds of the French remains till this day.
Intelligence preceding the Attack on 10th May. Hitler first gave instructions on 28th September 1939 for the attack through Holland and Belgium to be planned at the end of the Polish campaign, though it was then postponed several times by the bad winter weather.  Following the capture by the Allies of documents from a crashed German aircraft in the Netherlands on 12th January, the German plans were altered to include the thrust through the “impenetrable” Ardennes.  Numerous warnings were received by the Allies, including, it is said, those from Admiral Canaris’s German Secret Service staff, the Abwehr, who had sent a Catholic lawyer, Dr Müller, to warn the Vatican. The Pope sent the information on to his nuncios in Brussels and the Hague by coded radio messages that the Germans were reading. Hitler at once instructed Admiral Canaris to investigate this leak. So Canaris promptly sends Dr Müller back to Rome to seek out the source, an act described (surely by Churchill?) as a “stroke of genius equalled only by its wit”.  The assault on Holland and Belgium was expected, if not the date, but the Panzer drive through the Ardennes came as a total surprise, though it is said that warnings were given about the Ardennes part of the offensive by a source in the Abwehr as early as 25th March. It is known that the traffic on a German Secret Service link to the Ardennes area was intercepted and decrypted in the spring by MI5 (by BP?); it carried Intelligence that should have indicated the likelihood of an attack there.  Germany was careful to use land-lines when planning operations, so the BP breaks into the Luftwaffe Enigma Red key provided few clues on the forthcoming attacks.  Before the attack the French believed the Germans could deploy up to 7,500 tanks, the British estimated 5,800 and the Germans actually used 2,445. The loss of the Red until 22nd May meant that BP could provide little useful information for the first two weeks of the campaign.
 
BP overcomes the loss of the Repeat Sending of the Message Key. 

In a remarkable demonstration of their growing expertise, on the 15th April BP had broken the Yellow, the German Army-Air Force Norwegian campaign key shortly after it was 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 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).  On 1st May Germany changed the setting up procedures for all the keys of their Army and Air Force, except for the Yellow. (The Yellow retained the repeat sending of the message setting, presumably because it was too difficult to make a change in the middle of a campaign). So the methods pioneered by the Poles could no longer be used, except for the Yellow.  With the assault in the West, the need for Intelligence information has become crucial. In a great triumph for BP, they break back into the Luftwaffe general purpose Red key on 22nd May for messages of 20th May, and thereafter the Red is broken virtually ever day until the end of the war. The large increase of radio traffic incumbent on a military campaign proves a great help, with Hut 6 reading 1,000 messages on Red each day for the rest of May.  BP now never looks back!  22nd May is sometimes called the “Birthday of Ultra”.

The “Cillis”.  The Turing bombe is not yet fit for general use, but Hut 6 builds on the study by a team led by Stuart Milner-Barry of the many decrypted messages.  They had devised methods of working out the keys that depended upon the stupid mistakes made by the German cypher personnel in the settings for their Enigma machines.  These were called “Cillis” at BP, possibly after a girl whose name was repeatedly used by one German operator.  Some use obvious words, such as “HIT” as the three letters they send in clear to give the rotor ground indicator setting, then followed by “LER” for the encrypted message text setting.  If “QWE” is the ground indicator setting then the encrypted message setting might be “RTY”, taking advantage of the keyboard layout.  An important clue to the wheel ring settings comes from the habit the careless operators have, after setting the rotor alphabet ring, of dropping the rotor back into the machine in the easiest way; John Herivel detects this habit and thereafter it is always known at BP as the “Herivel Tip”.

The Value of the Early Breaks. 

Though BP reads many German messages during the Norwegian campaign following the breaking of the campaign Yellow cypher on the 15th April, this information proved of little value to the UK forces in Norway.  Such material as does reach the army commanders 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!  During the campaign in France, BP has still to learn to interpret the copious material they are reading on the Red key, which is often in pro-forma style or requires an understanding of German military terms that they still lack.  And conditions at the Allied GHQs in France are so chaotic that little of value could be made of this Intelligence.  General Gort is constantly shifting the position of his command post, and is often absent for long periods.  It cannot have helped that on 18th May he sent his Chief Intelligence Officer to form a scratch force to plug one of the many gaps that appeared in the line. This raised havoc in the Intelligence organisation at GHQ, but probably did not matter much since Lord Gort seems to have had little use for Intelligence information.  (By contrast, the Germans were well informed about our operations, since they were reading some of our naval codes, some French codes, and had good field listening posts who picked up our orders to evacuate as early as the 25th May).  However from 24th May onwards an embryo “Signals Liaison Unit” is created at GHQ to get the BP Ultra Intelligence direct to Lord Gort.  But by then it is too late to have much impact, and security restricts the circulation of the Ultra material. Our field listening posts do intercept that famous order to halt the Panzers on 23rd May, since, inexplicably, it is sent out in clear. The retrieval of very important documents from a German staff car on 25th May reveals German plans to exploit the growing gap in the line between the Belgium and British forces. This information leads to the plugging of the line so saving the BEF, but with the only troops available who had been designated to join the French in their counter-attack.  Virtually all the records of the BEF were destroyed during the retreat –which was perhaps as well!

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