As early as 1942, the Allies had started thinking about the best place for attempting a landing on mainland Europe, once the time had come. Every hypothesis was considered, from the fjords of Norway to Brittany and the Bay of Biscay. However, the choice was quickly whittled down to either the Pas-de-Calais or the bay of the Seine.

On the surface, the Pas-de-Calais was the more attractive option, offering a brief Channel crossing and therefore optimum air cover for the Allies and a rapid turnaround of supply ships. In addition, it was the shortest route to the heart of the Reich, the ultimate target of Operation Overlord. Equally aware of these advantages, the Germans were therefore convinced that that was where the attack would come and had strengthened the Atlantic Wall fortifications accordingly, building huge coastal artillery batteries. The German High Command had also deployed its finest troops there ‑ General von Salmuth’s Fifteenth Army ‑ along a double line of defence that would be difficult to break through.

For all these reasons, the Allied strategists rejected the Pas-de-Calais solution as being too obvious to the Germans. In July 1943, COSSAC (the headquarters directed by Lieutenant-General Morgan and charged with preparing the landings) produced a trenchant report in favour of the Normandy coastline. “The chances of successfully bringing off a landing in this sector rather than another are so much greater that, in our opinion, the advantages outweigh the disadvantages”. These disadvantages included a considerably longer crossing (roughly 150 km) and a more complex task for the fighter aircraft charged with providing support for the operations on the ground. On the other hand, the Seine Bay offered wide, easily accessible beaches, sheltered from the westerly winds by the Cotentin Peninsula and therefore suitable for an amphibious assault. Moreover, the German defences were far less impressive along this particular stretch of coastline.

The final decision was taken at the Quadrant conference in Quebec, in August 1943, which was attended by President Roosevelt and Winston Churchill in person.

Having decided to do the exact opposite of what the Germans, relying on purely military logic, could imagine, the Allies now had to do everything in their power to prevent the truth from being discovered. And that was the aim of the Fortitude plan of disinformation.