Double-Cross system

In the World War II Double-Cross system of the United Kingdom, all German spies captured in the British Isles were given a choice between execution and becoming a Counterintelligence#Double agent | double agent. The British-controlled double agents were part of a larger strategic Deception | deception operation, the principal intention of which was to make the Germans believe that the main Allied invasion would come at some place other than the actual beaches of Normandy. There is every indication that the German staff believed the deception, which had the main invasion coming at the Pas de Calais. Even after the Normandy invasion, Hitler refused permission to release reserves against the beachhead, saving them for what he insisted would be the "real" invasion.

Only those German reconnaissance aircraft that would pass over appropriately arranged decoys were allowed to complete their missions, and the Allies generated dummy signals corresponding to imaginary units whose organization and position was supportive of the overall deception plans. Clandestine human-source intelligence (HUMINT), imagery intelligence (photoreconnaissance) (IMINT), and signals intelligence (SIGINT) were all collecting plausible data and returning it to the Germans. The Germans were unable to use collection techniques that revealed the real Allied plan. This was part of the larger strategic deception program, Plan Bodyguard, directed by the London Controlling Section (LCS).

The Offensive counterintelligence|counterespionage section of the U.S. Office of Strategic Services, which worked closely with the Double-Cross committee, was named X-2 in humorous respect for "XX".