Operation Fortitude was the collective codename for a number of the deception operations used by the Allied forces during World War II prior to and following the Normandy landings. It was part of the overall deception plan for 1944, Operation Bodyguard. Fortitude had two forms: Fortitude North, which was to instill in Hitler and his generals fear of an amphibious landing in Norway, and Fortitude South, which was to trick the German high command into believing that the landings would take place in the Pas de Calais rather than on the Normandy beaches.

The deceptions used in Fortitude South were manifold, but carefully orchestrated. 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 the work of B Division of MI5 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 largely fake except for its leader, General George Patton, some token units and a complete signals outfit sending made up traffic. 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.

At no point were the Germans fed false documents describing the invasion plans. Instead they were allowed to construct a misleading order of battle for the allied forces. To mount a massive invasion of Europe from England, military planners had little choice but to stage units around the country with those that would land first nearest to the embarkation point. By placing FUSAG in the south-east, German intellegence would (and did) deduce that the center of gravity of the invasion force was opposite Calais, the point on the French coast closest to England and therefore a likely landing point.

The Allies were able to easily judge the effectiveness of these strategies. ULTRA intelligence — that gained from the breaking of German codes and ciphers, such as the Enigma machine — was able to provide an indication of the German high command's responses to their actions. They maintained the pretense of FUSAG and other forces threatening 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.

Quotes

"So how successful was (Operation) Fortitude? It couldn't have been more successful" --Stephen Ambrose