The principal objective of Fortitude was to ensure that the Germans would not increase their troop presence in Normandy because of the appearance of attacks against German positions elsewhere. Equally important was to delay the movement of German reserves to the Normandy beachhead and prevent a potentially disastrous counterattack. The plan therefore aimed to convince the Germans that additional assaults were planned—specifically in Scandinavia and in the Pas de Calais.
The overall strategic plan for deception by the Allies in 1944 was planned by London Controlling Section and laid out in Operation Bodyguard. However the actual conduct of such deceptions was the task of the commanders in the theatres in which the deception was to occur. The execution of the deception "cover plan" for Overlord was therefore the responsibility of SHAEF under General. Dwight D. Eisenhower. A special section called "Ops (B)" was established at SHAEF to handle deception.
It was initially envisioned that deception would occur through five main channels:
During the course of Fortitude the almost complete lack of German aerial reconnaissance, together with the absence of uncontrolled German agents in Britain, came to make physical deception almost irrelevant. The unreliability of the "diplomatic leaks" resulted in their discontinuance. The majority of deception was carried out by means of false wireless traffic and through German double agents. The latter proved to be by far the most significant.
In fact, Fortitude was so successful that Hitler regarded the Normandy invasion as a feint: he kept his Panzer units where he expected an attack and away from Normandy, until the battle was decided—in Normandy. Although Fortitude was controlled from SHAEF, London Controlling Section retained responsibility for what was called "Special Means": the use of diplomatic channels and double-agents.
The three most important double agents during the Fortitude operation were:
Fortitude North was the fictitious assault on Scandinavia. It consisted of two parts: first a plan to re-occupy any parts of occupied Scandinavia that might be weakened by withdrawal of German troops; second an assault on Norway.
The (fictional) unit assigned to this operation was the British Fourth Army, which was located in Scotland. Since it was highly unlikely that a German reconnaissance plane could make it to Scotland and back to German-controlled territory without being shot down, the primary means of deception was the use of double agents. In addition, the radio traffic of the imaginary units assigned to the Fourth Army was simulated by radio operators.
British diplomats also began negotiations with neutral Sweden in order to obtain concessions that would be useful in the event of an invasion of Norway, such as the right to fly reconnaissance missions over Sweden and the right to refuel planes that made emergency landings. These negotiations were made not in the hope of obtaining the concessions but with the intention that news of the negotiations would reach the ears of the Germans.
The units making up Fourth Army varied throughout 1944. Some were real units whose actual role and organization was disguised by agent reports; some were entirely fictitious. The order of battle at the peak of the operation was as follows:
British Fourth Army fictional - HQ Edinburgh
Fortitude South was conducted with the intention of convincing the Germans that an invasion would come to the Pas de Calais—a logical strategic choice for an invasion since it was the closest part of France to England and its beaches were not easily defended. While it was hoped that this would reduce the number of troops in the Normandy area at the time of the invasion, even more important was to dissuade the Germans from reinforcing the Normandy battleground in the days immediately after the invasion. To this end the Allies hoped to convince the Germans that the Normandy invasion, when it occurred, was a diversion, and that the main invasion was still to come near Calais.
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 intelligence would (and did) deduce that the center of the invasion force was opposite Calais, the point on the French coast closest to England and therefore a likely landing point.
In order to facilitate this deception additional buildings were constructed; dummy vehicles and landing craft were placed around possible embarkation points. It was originally intended to make many such fakes, but the extremely low level of German aerial reconnaissence and the belief that most German spies were under British control meant that such effort were reduced to a minimum. A huge amount of false radio traffic was transmitted, commensurate with a force of that size.
A deception of such a size required input from many organisations, including MI5, MI6, SHAEF via Ops B, and the armed services. Information from the various deception agencies was organized by and channeled through the London Controlling Section under the direction of Lieutenant-Colonel John Bevan.
The Allies were able to 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 pretence 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.