D-Day and Intelligence

Hugh ‘Quex’ Sinclair, Head of MI6
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It was the 1st Duke of Marlborough (1715) who once said: ‘No war can be conducted successfully without early and good intelligence.’ That was also the belief of Hugh ‘Quex’ Sinclair (the head of MI6) in 1938 as Britain faced the escalating threat of war from Nazi Germany. Sinclair believed whoever would win the intelligence game would win the war – and with that in mind, he purchased Bletchley Park in Buckinghamshire and moved the Government Code & Cipher School (GC&CS) out there from Broadways Buildings (then MI6 HQ in London).

In the years leading up to D-Day, quietly in the Buckinghamshire countryside teams of men and women were busy in sparsely furnished huts seeking to break the German enigma codes at Bletchley Park. On the surface, this work looked anything but high tech but their work was often very mundane and they had little real understanding of the bigger picture or the impact of what they were doing for the war effort. These were ordinary people, doing extraordinary things. One estimate places the number of personnel at 7,825 at Bletchley and its outstations by 4th June 1944 – just 2 days before D-Day.

Research by Bletchley Park’s official historian, David Kenyon, has clearly demonstrated that Bletchley Park became ‘an intelligence factory, with ancillary operations conducted all over the UK.’ But, as Kenyon writes, the achievements of the cryptanalysts at Bletchley ‘would have been entirely redundant if it had transpired that the Germans were encrypting their laundry lists.’ Prior to D-Day, for example, Hut 6 was involved in intensive traffic analysis that enabled the mapping of German networks and army movement / formulations and enabling an outline of order of battle. Decrypted messages, for example, located the headquarters of an SS panzer-Korps and aided the Allied commanders with knowledge of locations and strengths of SS units in France. One of the most valuable sources of intelligence came from the decrypted FISH messages. These were encrypted messages passed along 2 networks known at Bletchley as BREAM and JELLYFISH. BREAM was the communication link between Berlin and commander Field Marshal Kesselring in Italy. JELLYFISH was the teleprinter connection between Field Marshal Gerd von Rundstedt (Commander in Chief, West) and his masters. The first JELLYFISH breaks came in April 1944 and it meant the Allies were ‘reading’ the top secret messages between Hitler’s military command and his Commander in Chief in the West. Without the multi-faceted code-breaking and decrypts at Bletchley, as Kenyon says: ‘OVERLORD (D-Day) without ULTRA would have been much riskier.’

Bletchley Park was at its peak just before D-Day. In that respect it had a parallel impact to the intelligence gained by MI9, a branch of military intelligence operating from three clandestine sites of Trent Park in North London, and Latimer House and Wilton Park in Buckinghamshire. The unit was commanded by Colonel Thomas Joseph Kendrick under the obscure name of Combined Services Detailed Interrogation Centre (CSDIC). It processed prisoners of war for intelligence by  surreptitiously bugged their conversations after interrogation from a specially equipped ‘M Room’, manned by secret listeners. Working closely with Bletchley since 1939, CSDIC was an example of the successful partnership of HUMINT and Tech. Ahead of D-Day, CSDIC gained confirmation of the V-1 and V-2 development site at Peenemünde which led to the RAF bombing of the site in mid-August, rendering it unoperational. It bought extra time for the Allies and delayed Hitler’s first launch of a V-1 on London until 13 June 1944, a week after the successful D-Day landings. Without the intelligence from the M Room, it is doubtful that the Allies would have realised the significance of Peenemünde before it was too late. Germany could have won the tech war which would have made it difficult to mount the D-Day landings the following year. This intelligence coup was a major landmark in thwarting Germany’s race for weapon superiority.

With preparations underway for the D-Day landings, the intelligence at Kendrick’s sites was gathering was gaining pace. Across the three CSDIC sites, interrogations produced new intelligence on the location of displaced German war industries, electrical sub-stations, and a large synthetic oil plant at Auschwitz, as well as the Normandy defences and bunkers along the Atlantic Wall. Of particular value was information from prisoners on underground factories in Germany and Czechoslovakia that could not be identified from aerial reconnaissance missions. An example of this was the Mittelwerk Niedersachsenwerfen, near Nordhausen, which manufactured V-2 rockets, Junkers aero engines and jet propulsion units

But the war could not ultimately be won solely by intelligence. At some point there had to be ‘boots on the ground’ and the full-scale invasion of Normandy. Battles were hard won in Normandy, and gains slow –the Allies had the expectation to take Caen on the first day of the invasion – but it would in fact be just over 6 weeks before Caen was taken on 20 July 1944. By then, the bombing of the city had been so intense that scenes that confronted Allied troops looked like something from the Apocalypse. The job of the intelligence workers at Bletchley, Trent Park, Wilton Park and Latimer House, as also with RAF Medmenham, was not over with the D-Day landings. They would continue to provide vital intelligence ahead of Arnhem, the Ardennes and during the invasion of Germany.