The deceptions used in Fortitude South were manifold. These ranged from the building of artificial airfields with papier-mâché aircraft in East Anglia to radio traffic deception by a specially briefed outfit which drove around the southern coast of England simulating an army maneuvering, to the broadcasting of misleading messages from secret agents who had effectively been 'turned' by the Double Cross System, such as Garbo. The Germans had about 50 agents in England at the time, but all of them had been caught due to Ultra and many of the captives became double agents. The British were so desperate to maintain their cover as real agents to feed the Germans false information that they bombed vacant public buildings to demonstrate their "activities".
In Operation Quicksilver the Allies created an entire fake army. FUSAG, the First United States Army Group, was completely fake except for its leader, General George Patton. Patton was unpopular with the Allied high command, but he was regarded by leaders on both sides as one of the Allies' best mechanized warfare experts.
The Allies were able to easily judge the effectiveness of these strategies. Since the Ultra program had cracked the German Enigma code-system early in the war, the Allies were thus able to decrypt the German high command's responses to their actions. They maintained the pretense of a staged landing at the Pas de Calais for some considerable time after D-Day, possibly even as late as September 1944. This was vital to the success of the Allied plan since it forced the Germans to keep most of their reserves bottled up waiting for an attack on Calais which never came, thereby allowing the Allies to maintain and build upon their marginal foothold in Normandy.