Invasion Intelligence

The Allied invasion of Northern France which took place on D-Day, 6th June 1944, demanded land, naval and air forces cooperate effectively to overcome the strong German coastal defences. Knowledge of the enemy’s forces and dispositions would be invaluable, and therefore the invasion was supported at all stages by the Codebreakers of Bletchley Park. By this stage in the war, each stage of decrypting and analysing enemy messages had been split up and assigned to specialists. Far from merely decrypting messages, Bletchley Park had mastered the entire intelligence process from interception to dissemination: it had become a true intelligence factory.

This Collections uncovered album was first published on 10 April 2019.

The Western Front Committee was established at Bletchley Park in November 1942. It existed to coordinate all intelligence from across the Park which might be relevant to a forthcoming invasion of Northwest Europe and included senior representatives from the various departments. From February 1943 the committee began to produce reports according to a set format. Held in the archives of the Bletchley Park Trust and amounting to over 450 pages, these provide a unique insight into the sheer depth of the Codebreakers’ knowledge of the enemy in the run up to D-Day.

The committee’s first and most important task was to build up a comprehensive order of battle of German forces in the West, recording every unit, its location and its strength. Each report included up-to-date intelligence about German forces in France. By D-Day Bletchley Park had identified all 58 German divisions in France and the Low Countries. This extract of the report for 19th May 1944 mentions the 21st Panzer Division, which would be encountered by British troops on D-Day, and 1st and 12th SS Panzer Divisions, which would both be heavily involved in the ensuing battle for Normandy.

The comprehensive overall picture of enemy forces built up by Bletchley Park was vital for the D-Day planners, but sometimes individual pieces of intelligence could also be of great importance. Along with information about German armoured formations, the report of 26th May mentions the move of 91st Luftlande (Air Landing) Division to the Cotentin Peninsula, directly into the planned drop zone of the US 82nd Airborne Division. This intelligence came from a single intercepted message, and led to the invasion plan being altered on 30th May, only a week before D-Day.

The reports also record the origin of each piece of intelligence, demonstrating the breadth of the Codebreakers’ sources. This list from the 26th May report reveals that knowledge of 91st Luftlande Division’s movements derived from Jellyfish, the teleprinter link between Field Marshal von Rundstedt in Paris and the German High Command in Berlin enciphered on the Lorenz machine and decrypted in the Testery at Bletchley Park. By 1944 Hut 3’s sources included Enigma, teleprinter ciphers, low-grade non-machine ciphers, various Japanese diplomatic ciphers, traffic analysis, aerial reconnaissance, and reports from Allied agents and resistance organisations in occupied Europe.

The Western Front Committee also reported on German beliefs about Allied plans. Particularly useful were messages of the German military intelligence service, the Abwehr, which Bletchley Park had been reading since 1940. As this report from March 1944 shows, these demonstrated the chaotic state of German intelligence, which routinely produced wildly inaccurate and conflicting assessments of Allied forces and intentions. The Allies used such intelligence to monitor the effectiveness of their deception efforts, which encouraged the Germans to greatly overestimate the size of the invasion forces and hold divisions back from Normandy to counter further landings expected near Calais.

Much intelligence before D-Day came not direct from German sources, but via Germany’s ally Japan. The Japanese ambassador in Berlin, and military and naval attachés across Europe, sent reports on the German forces back to Tokyo – which were read by Bletchley Park. The thousands of Japanese Military Attaché messages intercepted and decrypted were indexed by subject on over 6000 cards which reveal the range of information they contained. This is one of numerous cards recording messages relating to the German army in France – though a later indexer has amended it to include forces in Russia.

Prominent individuals who appeared in the messages had their own index cards. This card is for Field Marshal Erwin Rommel. Earlier messages refer to his command in North Africa in 1942, but by November 1943 he had taken over responsibility for German troops in Northern France and was overseeing the strengthening of the coastal defences there.

The reports of the Western Front Committee and the Japanese Military Attaché decrypts represent just two sources of intelligence among many which were fed into Hut 3, the section responsible for producing ULTRA intelligence concerning the German army and air force. The most important messages were summarised into ‘headlines’. This one refers to a message listing the names and locations of no fewer than seven Panzer divisions due to be visited by General Guderian.

From 1941 the Prime Minister, Winston Churchill, received daily intelligence reports from Bletchley Park, including full copies of important messages. The headlines indicated which messages from the previous 24 hours to select from the mass of decrypts for inclusion in the red box which was placed on his desk each morning. As well as gathering intelligence about German ground forces, it was also important to monitor the progress of the air campaign to disrupt road and rail communications in France and the Low Countries, known as the Transportation Plan.

Doreen Tabor (later Nicholson) worked in Hut 3 from 1941 to 1945. By D-Day the section had moved from its namesake wooden hut into a new brick building, Block D. Nicholson recalls ‘this great enormous building which had a long corridor where we all had to hang our coats; it was very damp and smelt of raincoats. The place was very dark with the black-out, we kept the windows closed so as not to let out even a peep of light and the electric light didn’t give very good illumination at all. There was no romance in it, I liked the old huts. We knew little about Enigma, Hut 6 or where the information was coming from, but you knew what to do and you passed it on, and although we had no overall understanding, it worked.’

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Invasion Intelligence

This Collections uncovered album was first published on 10 April 2019.

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Invasion Intelligence

This Collections uncovered album was first published on 10 April 2019.

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