D-Day June 6th, 1944
The deception campaign mounted by the British and the Americans successfully kept an entire German Army out of the fighting until it was too late.
The dilemma for both the Allies and the Germans was deciding where in France, the invasion would land. The two options considered were the Pas de Calais, and, Normandy. Pas de Calais offered both the shortest sea crossing and the shortest route to German heartland. Formidable defenses there, however, caused the Allies to select Normandy. Early on, the Allies built a deception campaign to assure the Germans that Pas de Calais would be the landing point.
One such deception took place just as the invasion of Normandy was started. Two days into the invasion, Garbo, a double agent spy, (German agent actually working for the Allies), reported to the Germans that allied forces were on route to Normandy. Very soon the Germans would be seeing the approach anyway, and this strongly increased the agent’s credibility.
Just at the moment that the Germans were thinking of moving seven army divisions at Pas de Calais to Normandy, Garbo sent another message which explained that Normandy was not the real invasion point but was being invaded falsely to lure the German army away from Pas de Calais, where the real invasion would take place.
The trick worked beautifully. The more the Allies attacked Normandy, the more the Germans were sure that they should keep their armies in Pas de Calais.