One of the vital elements of the invasion was the erroneous German expectation of landings in the Pas-de-Calais. Believing that a number of Allied divisions in the United Kingdom belonged to “Army Group Patton,” the Germans concentrated a strong Fifteenth Army in the Pas-de-Calais, the coastline nearest to England and the area in western Europe closest to the classic invasion routes into Germany. The Allies nourished this belief by a gigantic deception plan designed to convince the Germans that Overlord was only part of a larger invasion effort. Naval demonstrations off the Channel coast, false messages, dummy installations, and other signs of impending coastal assault kept the Germans in a continual state of alert and alarm and immobilized the considerable force of the Fifteenth Army.
The Allied hoax continued well beyond the Overlord invasion. Early in July, the designation of the United States First Army Group was changed to the Twelfth in order to retain in England a fictitious headquarters that the Germans might think capable of launching another invasion. Lt. Gen. Lesley J. McNair, commander of the Army Ground Forces, who was visiting the European theater, was placed in command of the fictitious army group. Later, when McNair was killed while observing the battle in Normandy, Lt. Gen. John L. De Witt was rushed to England in order to give continuing verisimilitude to the Allied deception measures. When the Third Army was committed on the Continent, Patton’s name was at first kept secret for the same reason. Eminently successful, the deception maneuvers fooled the Germans for nearly five months. During the invasion and the subsequent battle for Normandy, when the Germans could well have used reinforcements from the Pas-deCalais area, the Fifteenth Army remained untouched and immobile, awaiting an invasion that never came.