The first the Germans learned of the FUSAG forces coming their way was through a spy working in New York under the alias Albert van Loop.
The Abwehr had prying eyes, too: reconnaissance planes that flew at 33,000 feet over the English countryside trying to spot FUSAG units and record their activities and movements.
On the ground, the real units earmarked for Overlord but temporarily assigned to FUSAG had no trouble appearing as though they meant business.
This British intelligence group had a stable of double agents sending specially crafted intelligence about FUSAG to the Abwehr.
He told the Germans he had been appointed as a liaison between Free Polish forces and Patton's FUSAG headquarters.
Another secret agent, code-named Tricycle (a Yugoslav named Dusko Popov), sent a detailed report in February 1944 on the FUSAG order of battle.
The messages showed that the Nazis were buying the FUSAG deception.
Other Allied officers also let slip bits of seemingly sensitive information about FUSAG and the Pas de Calais.
If the Germans ceased to believe in the FUSAG threat, then the substantial German forces guarding the Pas de Calais would be sent to Normandy.
The tempo of activity in the FUSAG area quickened after June 6.