The Secret Normandy Campaign — Inside the Allies’ Shadowy Intelligence War Before D-Day

Allied troops storm the beaches of Normandy, June 6, 1944. For years leading up to the D-Day invasion, a secret army of spies, recon pilots, diplomats and intelligence analysts laid the ground work for Operation Overlord. (Image source: WikiCommons)

“A four-year effort, it involved round-the-clock work by literally thousands of codebreakers, reconnaissance pilots, commandos and even diplomats and envoys. No detail was overlooked.”

By David Abrutat

D-DAY IS RIGHTLY seen as one of the greatest combined military operations ever undertaken. It was ambitious on a size and scale that is unparalleled to this day.

Equally impressive as the Operation Overlord plan itself is the mammoth but top-secret intelligence campaign that preceded the landings of June 6, 1944. A four-year effort, it involved round-the-clock work by literally thousands of codebreakers, reconnaissance pilots, commandos and even diplomats and envoys. No detail was overlooked, from the positions and composition of coastal batteries and shore defences to enemy morale and command, control and communications – it was all painstakingly collected, processed and catalogued by an army of analysts.

By the time the Allied armada put to sea on the eve of Overlord, it would prove to be the most well-prepared invasion force of all time. Here are some details about the secret war the made the Normandy invasion possible.

For years, British cryptanalysts struggled to break the German Enigma code. Their efforts would prove decisive in the lead-up to the Normandy Invasion. (Image source: WikiCommons)

Codebreaking

In the run up to D-Day, some 18,000 encrypted German messages, many from Hitler himself, were being collected and deciphered every single day at Bletchley Park. These signals would provide both tactical and strategic insight into German military intentions and dispositions around Northern France. Bletchley had been successful at breaking the three-rotor wheel Enigma machines, which provided Allied planners with rich streams of German intelligence, up until Feb. 1, 1942. That’s when the Nazis introduced a fourth rotor-wheel. The intelligence output from exploiting Enigma traffic was referred to as ULTRA, and Churchill was one of its key recipients. Prior to D-Day, ULTRA was to provide Allied HQ with a complete German order of battle, and critically knowledge on the locations of the German armoured divisions. From June 6, 1944 onwards, ULTRA material was to provide a huge amount of timely intelligence, with some German signals being intercepted, processed, analysed and reported to deployed commanders within hours. After the war, the Joint Intelligence Committee (JIC) was to report that SIGINT was “by far and away the most valuable of all intelligence sources.”

Allied recon specialists used canoes like these to do up-close surveys of the Normandy coastline. (Image source: WikiCommons)

Scientific breakthroughs

The intelligence collection operations behind D-Day were not just the work of Bletchley Park or the Spitfire or Mosquito aerial reconnaissance missions, it was also the field of scientific intelligence that made a significant difference. Scientists like Dr. Reginald Jones, who was one of the key radar researchers of the time, were critical to Overlord in understanding the German radar systems being deployed to the French coastline. There was also naval hydrographers like Lt. Cdr. Frank Berncastle with their specialist skills that were used to survey the Normandy coast for the Mulberry harbour placements critical to the logistics supply once the beachhead was established. Vital to getting a fuller understanding of the load bearing capacities of the beaches themselves were Special Forces officers like Major Logan Scott-Bowden and Lt. Cdr. Nigel Clogstoun-Wilmott who would get ashore to get sand samples and measures gradients during a series of COPP (Combined Operations Pilotage Parties) reconnaissance operations during 1943 and 1944.

An intercepted report filed by a Japanese envoy touring the Nazi defences on the French coast would provide the Allies with a treasure trove of details about the Hitler’s Atlantic Wall. (Image source: WikiCommons)

The diplomatic front

Intelligence comes in many forms and real breakthroughs often come from the most unlikely of sources. The work the U.S. signals intelligence community did against Japanese ciphers was to have a secondary effect in exposing the traffic of the Japanese Ambassador to Berlin, Lt. Gen. Hiroshi Oshima. In one of his forays to Normandy under the invitation of the Germany military, his 16-page report, covering in explicit detail the nature of the German defences and fortifications was to be a huge coup for Allied intelligence.

Allied recon missions provided thousands, perhaps millions, of photographs of the invasion area. (Image source: Gov.uk)

Eyes in the sky; ears on the ground

World War Two was a total war, and total war requires total intelligence. Operation Overlord was one of the boldest invasions ever mounted. Almost immediately after Dunkirk, the Allies began laying the ground work to return to Europe. A myriad of collection efforts were soon underway: from airborne reconnaissance missions over northern France to commando raids against targets all along Hitler’s Atlantic Wall. Intelligence ranged from the almost industrial scale collection and interpretation of aerial photographs at RAF Benson and RAF Medmenham, small naval teams undertaking hydrographic surveys under the noses of the German defences, to the small recon missions mounted by swimmer/canoeist teams from the COPP, who brought back sand samples from the beaches themselves. More than 3,200 aerial reconnaissance missions took place prior to the invasion to acquire photographs of key installations and defences around the five landing beaches, in the run up to D-Day the planes were undertaking 80 sorties a day over the Normandy coast.

An inflatable rubber tank designed to confuse the enemy. Allied commanders massed an entire make-believe army in southeastern England to fool the Germans into believing the anticipated cross-channel invasion would land at Calais. (Image source: WikiCommons)

Deception

The London Controlling Section (LCS) and its American equivalent, the Joint Security Control (JSC) would coordinate all deception activity across the military and civilian services. The all-encompassing deception plan, Operation Bodyguard was born. The mosaic of deceit for D-Day would include famously creating false military units in southeast England complete with lifelike rubber dummy aircraft, landing craft and tanks. A air and sea operations were launched to disrupt enemy radar, while false lighting operations were mounted in key ports on the south coast to fool enemy observers. The deception was completed by way of a barrage of phony wireless traffic and the creation of a network of double agents. It was a global project involving thousands of military and intelligence staff.

The years of intelligence collection for D-Day would ultimately shape the success of the allied invasion of Normandy. The planning for Overlord warranted a scale of intelligence collection effort that had never been seen before, nor is it ever to likely be repeated. Normandy was an intelligence operation unparalleled for the detail, breadth, accuracy and scale of its collection effort alongside the creativity and guile of its deception strategies.

ABOUT THE AUTHOR: David Abrutat is the author of the upcoming book Vanguard: The True Stories of the Reconnaissance and Intelligence Missions Behind D-Day (available for pre-order HERE). A former Royal Marine Commando and reconnaissance specialist, he has long been fascinated with World War Two and the work of clandestine forces around the world. He is currently an associate fellow and lecturer in the MA Security and Intelligence Studies in the Department of Economics and International Studies at the University of Buckingham. He regularly lectures on D-Day.