Double-Cross system
In the WWII UK Double-Cross System, all German spies in the British Isles were captured, and given a choice between execution and becoming a double agent.[1] The British-controlled double agents were part of a larger strategic 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.
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. HUMINT, IMINT, and 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 Staff (LCS).[2]
The 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".