Seventy Years Ago This Month at Bletchley Park  

June 1940

Britain Stands Alone. 

The Dunkirk evacuation ends on 4th June, and the evacuation of the British forces from Norway on 8th June. Italy declares war on 10th June, but the incursion of their troops is held by the French at the frontier. Following the collapse of the front in Northern France under the renewed German assault of 5th June and swift advances of the German Panzers, France surrenders on 22nd June.  Britain is now alone, awaiting the inevitable German onslaught. Winston Churchill warns the country “The whole fury and might of the enemy must very soon be turned on us. Hitler knows that he will have to break us in this island or lose the war….. Let us therefore brace ourselves to our duty and so bear ourselves that if the British Empire and its Commonwealth lasts for a thousand years men will still say: This was their finest hour”.

The Dunkirk Balance-sheet.

By the time Admiral Ramsey had stood down the operation on 4th June a total of 224,000 British troops had been evacuated from Dunkirk, together with 114,000 French troops. More than 34,000 British soldiers were captured in the Dunkirk perimeter. There was a wave of relief sweeping the UK at the “Miracle of Dunkirk”. Churchill both saluted the remarkable achievement and tried to get the country to face the harsh reality of our position; the disaster was hardly a cause for celebration.  The Allied forces evacuated to the UK had lost a vast armament, 475 tanks, 38,000 vehicles, 1,000 heavy guns, 90,000 rifles and 7,000 tons of ammunition. Only 500 heavy guns remained in the UK.  In all, 222 naval vessels and 665 civilian craft had joined in the brilliantly improvised evacuation. Of these Britain had lost six destroyers and 24 smaller naval vessels, largely to the air assault. Numerous of the civilian craft never returned from their ferrying work off the beaches.  Britain had lost about a quarter of its total fighter force during the campaign and the evacuation, though on some days over Dunkirk the young and inexperienced RAF pilots were shooting down considerably more German aircraft than they were losing. 

It was the Y organisation in the UK who made the major contribution to Intelligence during the evacuation, by jamming the communications of the German dive-bombers, and by providing, from Luftwaffe intercepts, Intelligence that helped the navy to control the shipping off the beaches. On 1st June, through their decrypts of the Enigma Red key which Hut 6 had broken back into on 22nd May and now read regularly every day, BP make their first substantial contribution to strategy by revealing that, even during the evacuation, the Luftwaffe has eyes set on the continuing campaign in France, not on any plans for the invasion of the UK. This enables Churchill to override the objections of his Air Staff and answer the French pleas by sending more fighter aircraft to France.  During the next few weeks of June, the British Government could follow the progress of the Panzer divisions, observing the collapse of the French defence through the eyes of the Luftwaffe and its army liaison, as BP became increasingly expert at interpreting its many decrypts.

The Renewed Assault in France. 

It is often forgotten that after Dunkirk there remained in Western France some 136,000 British and 200,000 Polish troops, with most of their equipment still intact, supporting the battered 64 French divisions. They faced the 143 German divisions who were flushed by their success but hastily reforming, having suffered considerable disruption during the race to the sea. It is said that Rommel’s armour was reduced to 50% serviceability by this time.  After bombing Paris on 3rd June, the Germans renew the assault, first from the Somme towards Paris and the Seine on 5th June, and then towards Rheims on 9th June. The British were holding the left of the line from Abbeville to the sea.  Within two days both German attacks are through the Allied defensive line and within a week 35 French divisions have been lost, the weak ones pulling down the strong. On 12th June 38,000 French and 8,000 British troops waiting to be evacuated from St.Valery-en-Caux have to surrender when the beaches are heavily shelled.  That day Paris is declared an open city and the French government is evacuated to Tours, on to Bordeaux and finally back to Vichy. On 16th June Paul Reynaud, resigns and the 84-year-old Marshall Philippe Pétain, the hero of Verdun in the First World War, becomes Prime Minister. That evening he makes contact with the Germans to seek an armistice, blaming the UK for the disaster. General de Gaulle, now in London, had wanted to form an Anglo-French redoubt in Brittany. Now the evacuation begins of 163,000 British, Canadian and Polish troops from the French Atlantic ports. The armistice is finally signed on the 22nd June, after a silent Adolph Hitler had left, in the very railway carriage in the Forest of Compiègne where the Germans had signed the surrender in 1918.

Churchill had made five trips to persuade the French to fight on, but Britain is well aware , from the Enigma decrypts, of just how fast French resistance is collapsing.  Contrary to the persistent French myth, the British had made a major sacrifice to help them; of the 261 British fighters sent to France between 5th and 15th June, 75 had been shot down or destroyed, and a further 120 were unserviceable or lacked fuel. In those ten days the RAF had lost a further quarter of its remaining strength, now vitally needed for the defence of Britain.

Naval Intelligence and the War at Sea. 

 24,000 troops are evacuated from Norway from 31st May until 8th June with the loss of the aircraft carrier Glorious, two destroyers, and two squadrons of precious fighters. With the Glorious went down 1,500 officers and men. The young Harry Hinsley in Hut 4 at BP has been trying to warn the Admiralty that signal analysis indicates that the German capital ships are coming out of the Baltic but once again the Admiralty chose to largely ignore the warnings. There is no doubt that at this time the German naval Intelligence service is operating much more effectively that the British. They are reading over 30% of the traffic they intercept in the North Sea, and continue to do so regularly until August 1940. “It was worth more to their side than a score of heroic deeds” wrote a German historian about the accurate information on the movement of British ships the service provided at this time.  The Germans are reading several of the main British naval codes, but it would be the summer of 1941 before BP could read a German naval Enigma key regularly. The German naval Enigma operators use tables to provide security for their code- wheel settings, and they employ three additional code-wheels, as well as the normal 5 in use in the other Services.

But now the tide is beginning to turn.  After the loss of the Glorious the Admiralty takes steps to build up close relations with the naval Intelligence team in BP led by Frank Birch. (One principal go-between, Harry Hinsley became a popular figure in the Navy, becoming known as “the Cardinal”.  He went on to become the famous historian). During the course of the war the German navy used 27 different hand cyphers, 6 of them high grade. In a remarkable success for the BP naval section, almost entirely eclipsed by BP’s subsequent success in reading the naval Enigma, three of these high grade naval cyphers are read by BP for the greater part of their life-span, quite often currently, as well as the traffic in the majority of the lesser cyphers. The Dockyard cypher, Werftschluessel or WS, was first detected in April 1940 and first read some months later with the help of captured documents.  Once naval Enigma keys were being read it became an excellent source of cribs to help to find the Enigma key settings. (A crib derived from some other cypher that was being read, such as WS, was called a “kiss” in the wonderful language that evolved at BP!).  However, the severe German naval losses that they have suffered during the Norwegian campaign ensures that the German navy plays little part in the campaign in France, or even during the evacuation from Dunkirk except for some effective E-boat activity. 

With the entry of the Italians into the war on 10th June the work of the BP Italian naval sub-section under Wilfred Bodsworth becomes important. Since 1937 they had been reading the most secret book code of the Italian navy, but the Italians now changed these, and BP failed to read them after July 1940, except for rare, capture assisted, occasions. BP read the valuable but rarely used Italian naval Enigma from September 1940 until it was withdrawn in the summer of 1941. For in December 1940 the Italian navy introduced a medium grade Swedish cypher machine, the C-38m. This was broken by BP in June 1941 and read almost continuously thereafter with great impact on the war in N. Africa.

Enigma code breaking in Hut 6. 

On 22nd January BP had broken their first Enigma key using the Polish Netz, Zygalski sheet method. Gradually they build up their expertise, and broke some 50 key settings, largely of the Luftwaffe general purpose Red key, before they broke the Yellow Norwegian campaign key. They first broke this on 15th April and then read it every day until it fades out in mid May.  On 1st May Germany changed the setting up procedures for all their Army and Air Force keys, except for the Yellow. (The Yellow retained the repeat sending of the message Indicator, presumably because it was too difficult to make a change in the middle of a campaign). So the methods pioneered by the Poles can no longer be used.  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 Red key on 22nd May for messages of 20th May, and thereafter the Red is broken virtually every day until the end of the war. So during the campaign in France throughout June Hut 6 is reading some 1,000 Luftwaffe messages a day. BP is now in business with a vengeance, and never looks back!  Gradually Hut 3 builds up its expertise at interpreting these messages, the large increase of radio traffic for a military campaign proving a great help.  Though the information is not yet of any great value directly for battlefield purposes, if only because of the chaos at Army HQ in the field, it does begin to have a major impact on strategy. Even as early as 1st June the Luftwaffe decrypts make it clear that the Germans do not intend to invade the UK until France is defeated.  But with the fall of France, now the challenge for BP is to detect the German invasion plans.  On 23rd June Red decrypts establish that some Luftwaffe units are refitting for operations against the UK

The “Cillis”. 

The team in Hut 6, led by Gordon Welchman, is now into one of the most remarkable periods of the whole history of BP. They have lost the use of the methods derived from the pioneering work of the Poles because the Germans have dropped the double sending of the encyphered message setting, as Alan Turing had always said they would sooner or later.  But his first Turing bombe is not yet fit for general use, though Hut 8 is using it to help with the first few naval Enigma decrypts.  So Hut 6 builds on the study by a team led by Stuart Milner-Barry, of the many other Enigma messages that had been decrypted by then.  Astonishingly, they are able to devise methods of working out the Red key settings that depend upon the stupid mistakes sometimes made by the German cypher personnel in setting up their Enigma machines.  These were called “Cillies” 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 start indicator setting, then followed by “LER” for the encrypted message text setting.  If “QWE” is the indicator setting then the encrypted message setting might be “RTZ”, taking advantage of the German keyboard layout. Another clue 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 predicts this habit and so it is always known at BP as the “Herivel Tip”.  There are three watches at work in Hut 6 at this time, led by Gordon Welchman, Dennis Babbage, and Hugh Alexander.

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