The Masterplan for D-Day

As demonstrated by the failure of tanks and equipment to get ashore during the ill-fated Dieppe raid in August 1942, the geology of the battlefield was shown to be just as important as knowing where the bunkers and barbed wire were located. And what these two naval officers did was just one small – but crucial – part of the complex steps taken to plan D-Day.

Indeed, successfully carrying out the largest amphibious operation in history did not happen by accident. It took years of planning, preparation, research, development and thinking beyond the norm to make the invasion possible. Operation Overlord was fought and won not just by men with bombs and bayonets, but also by ‘back-room boys’. It was the truly the boffins’ war, and scientists, engineers and planners were at the heart of it all.

Listen | Giles Milton on lesser-known stories of soldiers and civilians who were involved in the Normandy landings

Finding a way in

When Allied commanders were coming up with the best strategies to defeat Nazi Germany, the American view was that the quickest route to the heart of the Reich was to land in France, take Paris, and then advance through the Low Countries and into the Rhineland. However, such a proposition was not possible when the North Africa campaign came to an end in May 1943, as not enough men, specialist equipment or landing craft were available for an operation on that scale. Instead, the war dragged on in the Mediterranean, with the capture of Sicily in August 1943 and then invasion of mainland Italy at Salerno that September.

While some leaders like Winston Churchill hoped that Italy would prove to be the ‘soft underbelly’ of the Third Reich, in the end it became what veterans called ‘the tough old gut’. Despite drawing German troops away from France and Russia, it soon became apparent that victory would only become possible with an invasion of France.

But where to land? In the summer of 1940, the German high command had made plans for Operation Sea Lion, the invasion of Britain, intending to use the narrow English Channel as their route. This certainly offered the quickest way to France, but with the construction of a huge screen of beach defences known as the ‘Atlantic Wall’, this section of coastline boasted some of the strongest German positions, making any Allied landing potentially costly.

As a result, plans were put in place to scour the French coast and find an alternative site. RAF reconnaissance aircraft began by snapping thousands of aerial photos and carrying out low-level sweeps (not without suffering losses), while maps were produced to identify locations that had good road networks to allow an invasion force to move inland. The British government even made a public appeal for postcards of towns and villages on the French coast that could be used for intelligence purposes.

However, valuable input also came from members of the French Resistance, who helped create a record of German construction of Atlantic Wall defences, especially as they were beefed up following the appointment of Field Marshal Erwin Rommel to oversee them. Gathering such information was highly dangerous and many Resistance members paid the price with their lives.

Along with the geological data gathered by submarine teams, the Allied commanders gradually managed to build a picture of which areas offered the best chance of success.

Gathering pace

The initial decision to land in Normandy was made by the chief of staff to the supreme allied commander (COSSAC) in 1943, Lieutenant-General Frederick Morgan. His team ruled out the Pas-de-Calais region and saw a landing between the Cotentin peninsula and near to Caen as being the most suitable.

At this stage in the war, due to lack of men and equipment, Morgan recommended landing on three beaches along the Normandy coastline, but this was later expanded to five. The work of the Resistance had indicated that there were fewer defences in Normandy than in the Pas-de-Calais, with many bunkers containing antiquated firepower from the First World War. Indeed, some of the bunker complexes were only partially completed. The mapping had also demonstrated good roads to get landing troops off the beach area and inland, and to take them beyond on the long road to liberation.

With plans gathering pace, Churchill and President Roosevelt met in Quebec in August 1943 and confirmed these decisions in a secretive conference known as ‘Quadrant’. After some small changes, a provisional date of May 1944 was decided upon and Operation Overlord was born.

At this point it was evident that a supreme commander of allied forces needed to be appointed to oversee the operation. Churchill may have preferred a British commander such as Frederick Morgan, Harold Alexander or even Bernard Montgomery, but Roosevelt instead proposed General Dwight D Eisenhower. As the Americans were in many ways the more powerful of the two partners, the president’s recommendation was approved at a conference in Cairo in December 1943.

Catherine wheels and funnies

Having decided where to land, the means to affect the invasion now became a pressing concern. Eisenhower oversaw the build-up of British and Canadian troops, with some of the latter having been in England as early as 1939. By spring 1944, more than one million Americans had also arrived in the country, and along with the rest of the Allied forces, they too needed to be trained for the invasion.

Units like the US 29th Division in the West Country were drilled to such a level of efficiency they became the Allied armies’ experts on amphibious warfare. Mock bunkers and sections of the Atlantic Wall were built to help with their training, while along the coast of Devon, concrete landing craft were constructed so that GIs could train exiting them and hitting the beach time and time again.

But to break through Rommel’s Atlantic Wall, it became clear that manpower and firepower alone would not be enough. It seemed easy just to bomb the beach areas where the landings would take place, but it was realised this would create a ‘crater zone’ across the landing areas that could in fact impede progress. Precision bombing did not exist, so to get through the complex and varied defences that protected the D-Day beaches, specialist equipment would be needed.

Churchill and Eisenhower spent a lot of time in early 1944 being shown designs and mock-ups of all sorts of inventions to help achieve this. Some of them were rather fanciful, such as the ‘Catherine wheel’ – an explosive device that was meant to roll across the beach taking out the defences, but could easily go into a spin and return just as quickly to the troops who launched it.

Meanwhile, the British developed adapted tanks known as ‘funnies’ that could breach German defences or assist the men who would land on the D-Day beaches. A lot of these designs were based around British-built Churchill tanks, with the Armoured Vehicle Royal Engineer (AVRE) being most common.

Thicker armour would help the vehicles survive anti-tank weapons, while their main guns were replaced by Petard spigot mortars firing what the British called a ‘flying dustbin’ that could demolish concrete structures. The tanks could also carry bundles of brushwood to drop in bomb craters so they could be crossed, scissor bridges to get over obstacles and walls, or bobbins of carpet matting to allow vehicles to cross easily over areas of soft sand.

In addition, the Valentine tank was adapted so it could float ashore using flotation screens, but this was eventually replaced by the Sherman Duplex Drive tank, which was implemented in large numbers by the time D-Day arrived. On the British landing beaches, the arrival of Shermans among the assault infantry often helped tip the balance and enabled men to get off the beach and make their way inland.

Assembling the armada

However, tanks, men and weapons were of no use if they could not be landed on the Normandy coastline. The supreme commanders knew that substantial investment in the naval side of Overlord – code-named Operation Neptune – was essential.

Under the command of Admiral Sir Bertram Ramsay, who had presided over the evacuation of Dunkirk in 1940, Neptune comprised a naval force of nearly 7,000 vessels from eight different navies. Among them were landing craft of every shape and size, which would put tanks and vehicles ashore, launch rockets on the defences and take men in for ‘H-Hour’ – the time of the assault on each beach.

For the British, the most common craft was the Landing Craft Assault (LCA). This 41ft boat had a crew of four and could carry 31 combat troops. With a good speed and rapidly deployable ramp, it was ideal for the amphibious nature of D-Day’s combined operations.

In contrast, the Americans instead chose to deploy the US-made Landing Craft Vehicle Personnel (LCVP) or ‘Higgins boat’. Faster and shorter than the British LCA, it could transport 36 men and had a quick drop ramp at the front that allowed a rapid exit. These proved to be so successful on D-Day that General Eisenhower later said of the boat’s inventor: “Andrew Higgins is the man who won the war for us.”

One aspect that Lieutenant-General Morgan had highlighted during the early planning for Operation Overlord was the use of artificial harbours. No army can advance without fuel, food or ammunition, and all of this needed to be brought over, landed and then distributed. There was no port in the landing area suitable for the purpose, as they were all too small.

As preparations for Overlord entered their final phase, the need  for deception was paramount

Instead the Allies would have to bring their own harbours with them, and thus the Mulberry harbour was devised. Perhaps one of the most amazing scientific achievements of the whole operation, its walls were made from massive floating concrete sections that could be towed across, flooded and then used to make what Churchill had said needed to be a port as “big as Dover”. One was available for the British sector at Arromanches and a second was built at Omaha beach after D-Day. A storm destroyed the American one, but the British harbour was repaired and kept open well into 1945, supplying the invasion and paving the way for -logistical success.

Fooling the enemy

As preparations for Operation Overlord entered their final phase, the need for deception was paramount. It was almost impossible to hide such a mammoth invasion force, so intelligence officers decided to exploit this fact by creating a dummy army made up of inflatable tanks and vehicles as part of an elaborate plan known as Operation Fortitude.

‘Led’ by the renowned US general George S Patton, the ghost army was positioned in the south-east, giving credence to the idea that an invasion force would really land in the Pas-de-Calais and anything else was a diversion. In addition, in the final 24–48 hours before D-Day, the RAF dropped tons of metal strips along the French coast. In great enough density, these would confuse enemy radar and stop the Germans from discovering the airborne armada that would bring in the men in the early hours of D-Day.

Dummy parachutists were dropped far away from the actual intended dropzones, in order to confuse the Germans as to where a main invasion would occur. These methods and many others were all part of arguably the most successful deception plan in military history and helped to ensure Allied success on the Normandy beaches.

On the night of 6 June 1944, Churchill and Eisenhower – both sitting in their war rooms – received messages that D-Day had been a success. More than 150,000 men were ashore, and while some of the landings had been easier than others, it seemed that victory was now finally in sight.

The bravery and tenacity of combat troops on the ground had made it possible, but it was also the scientists, engineers, factory workers, pilots and frogmen that helped make D-Day the crucial moment of the Second World War that it was.