Operation Fortitude and the Double Cross system

Operation Fortitude was the Allied deception plan meant to hide where and when the invasion of northwest Europe would come in 1944. Its northern half suggested an attack on Norway; its southern half, the more important, persuaded the Germans that the main landing would hit the Pas-de-Calais, the narrowest crossing of the Channel, and that any landing in Normandy would be a diversion. The Allies created a fictional First United States Army Group, or FUSAG, in southeast England, commanded by General George Patton, whom the Germans considered the most dangerous Allied general.²³

The deception depended on the Double Cross system run by MI5. Almost every agent Germany sent to Britain during the war had been caught, often with help from decoded German messages from Bletchley Park, and many were turned into double agents who sent their German handlers a blend of truth and fiction. The most famous, the Spaniard Juan Pujol Garcia, codenamed Garbo, invented a network of 27 imaginary sub-agents. Radio operators broadcast fake traffic for FUSAG's units, and some dummy equipment was displayed, though German air reconnaissance over England was rare by then.¹²⁵⁶

The landings in Normandy began on June 6, 1944. Garbo then warned Berlin that Normandy was a feint and the real blow would fall at Calais, and German commanders held their Fifteenth Army there for weeks while the Allies built up their beachhead. Garbo was awarded the Iron Cross by Germany and an MBE by Britain. Historians agree Fortitude was one of the most successful deceptions in military history, while debating how much of the German hesitation came from it rather than from their own assumptions.¹³⁴