The real invasion was supposedly to come between Boulogne and the Somme estuary during the second half of July. A notional ‘1st US Army Group’ under General George S. Patton Jr, the commander the Germans feared the most, boasted eleven divisions in south-east England. Dummy aircraft and inflatable tanks, together with 250 fake landing ships, all contributed to the illusion. Invented formations, such as a 2nd British Airborne Division, had been created alongside some real ones. To increase the illusion, two fake corps headquarters also maintained a constant radio traffic.

One of the most important double agents to work for British intelligence on Fortitude South was a Catalan, Juan Pujol, who had the codename ‘Garbo’. With his security service handler, he constructed a network of twenty-seven completely fabricated sub-agents and bombarded the German intelligence station in Madrid with information carefully prepared in London. Some 500 radio messages were sent in the months leading up to D-Day. These provided details which together gradually made up the mosaic which the Double Cross Committee was assembling to convince the Germans that the main attack was to come later in the Pas-de-Calais.

Subsidiary deceptions to prevent the Germans moving troops to Normandy from other parts of France were also dreamed up. Plan Ironside conveyed the impression that two weeks after the first landings a second invasion would be launched on the west coast of France directly from the United States and the Azores. To keep the Germans guessing, and to prevent them moving the 11th Panzer-Division near Bordeaux north into Normandy, a controlled agent in Britain, known as ‘Bronx’, sent a coded message to her German controller in the Banco Espirito Santo in Lisbon: ‘Envoyez vite cinquante livres. J’ai besoin pour mon dentiste.’ This indicated ‘that a landing would be made in the Bay of Biscay on about the 15th June’. The Luftwaffe, clearly fearful of a landing in Brittany, ordered the immediate destruction of four airfields close to the coast. Another diversion, Operation Copperhead, was mounted in late May when an actor resembling General Montgomery visited Gibraltar and Algiers to suggest an attack on the Mediterranean coast.