Shadows Wearing Enemy Names

By the spring of 1944, British intelligence had achieved something without precedent in the history of warfare: it controlled every single German spy operating in the United Kingdom. Not one had escaped detection. Some had been captured, others had turned themselves in, and one — the sole exception — had taken his own life rather than be turned. Into this total dominion over enemy intelligence, the Twenty (XX) Committee — established in January 1941 by MI5 officer Thomas Robertson and Oxford historian John Cecil Masterman — built the most elaborate deception apparatus in modern military history. Its instrument was the Double Cross System: a network of turned agents feeding a continuous diet of false intelligence to the Abwehr, Germanys military intelligence service.
The deception that enabled D-Day, Operation Fortitude, did not work through grand theatrical lies. It worked through minutiae. Double agents reported the insignia on soldiers uniforms, the unit markings on vehicles, the presence of a vast fictional American army — the First United States Army Group (FUSAG), supposedly commanded by General George S. Patton — massing in southeast England for a landing at Pas-de-Calais. German reconnaissance confirmed what double agents reported because Allied planners had seeded the landscape with inflatable tanks, dummy aircraft on mock airfields, and fake radio traffic to match. Each layer of fabrication reinforced the others, and Ultra — the intelligence drawn from decrypted German Enigma messages — allowed the British to verify in real time that Berlin believed every word.