'Fortitude' was the Allied deception plan within 'Bodyguard' designed to persuade the Germans that the main Allied invasion of continental Europe would target the Pas de Calais and/or Norway rather than Normandy (December 1943/June 1944).

'Fortitude' proper was divided into 'Fortitude North' suggesting the imminence of an Allied invasion of German-occupied Norway, and 'Fortitude South' designed to persuade the Germans that the main invasion of France would indeed occur in northern France, but in the Pas de Calais rather than in Normandy.

'Fortitude' was one of the most successful deception operations of the war and arguably the most important. The overall object of 'Fortitude' was to ensure that the Germans did not undertake any reinforcement of their forces in Normandy because of the possibility, if not actually the probability, of a descent or descents on other parts of the German 'empire'. Equally important in the planning and implementation of 'Fortitude' was the importance of delaying the movement of German reserves against the Normandy lodgement and of preventing a major German counter-offensive against the lodgement. The plan therefore aimed to convince the Germans that additional assaults were planned, specifically in Norway and in the Pas de Calais.

The Allies' overall strategic deception plan for 1944 was schemed from December 1943 by Colonel John Bevan’s London Controlling Section within 'Bodyguard', while the implementation of the such deceptions was entrusted to the commanders in the theatres in which the deceptions were to occur.

The planning and execution of the deception plan for 'Overlord' was therefore the responsibility of General Dwight D. Eisenhower’s Supreme Headquarters, Allied Expeditionary Force, which established a special Operations (B) section, under Colonel Noel Wild with Colonel J. A. Jervis-Read as his deputy, to handle deception.

It was first conceived that deception would be implemented via five primary channels: first, physical deception to mislead the Germans with non-existent units by means of fake infrastructure and equipment, such as inflatable rubber tanks and plywood artillery; second, controlled 'leaks' of information (i.e. disinformation) through diplomatic channels for onward passage via neutral countries to the Germans; third, wireless traffic to suggest the creation of actually non-existent formations and units through simulation of the wireless traffic that such formations and units would generate, and which would be detected by the Germans; fourth, use of German agents controlled by the Allies through the MI.5 counter-intelligence service’s 'Double Cross System' (otherwise 'XX System') to send false information to the German intelligence services; and fifth, public presence of notable officers associated with phantom groups, such as FUSAG (1^st US Army Group), most notably Lieutenant General George S. Patton, one of the best-known senior Allied combat commanders.

During the course of 'Fortitude', the Germans' almost complete lack of any air reconnaissance capability over the UK, together with the absence of uncontrolled German agents in the UK, made physical deception almost irrelevant, and the unreliability of the 'diplomatic leaks' resulted in the discontinuance of this aspect of the deception.

Thus most of the deception work was effected via the agency of false wireless traffic and through the exploitation of German agents who had been captured (or surrendered themselves) and been 'turned' to become double agents. The latter proved to be by far the more significant of the two.

In overall terms, 'Fortitude' was so successful that Adolf Hitler regarded the threat and later the implementation of the Normandy invasion as a feint: the Germans leader therefore ordered major armoured formations to be retained in the areas in which he expected 'real' invasions to be made, and thus distant from Normandy, until the Battle of Normandy had been effectively decided in favour of the Allies.

Although 'Fortitude' was controlled by SHAEF, the London Controlling Section retained responsibility for 'special means', namely the use of diplomatic channels and double agents. The Germans had about 50 agents in the UK at this time, but B.1A (the Counter-Intelligence Division of MI.5) had caught all of them but one, who died in unclear circumstances. Many of the captured agents were recruited as double agents, who were used throughout rest of the war to feed German intelligence misleading information, particularly in the pre-invasion period, about the Allied invasion preparations. Reports sent by these agents were carefully composed and co-ordinated to support the particular view of the forces in the UK which the Allied deception planners wished to present.

Supported by the 'Skye' radio deception undertaking, 'Fortitude North' was the plan devised to suggest the likelihood of an Allied invasion of German-occupied Scandinavia, and comprised a component to reoccupy any parts of German-occupied Scandinavia that might be weakened by any withdrawal of German troops, and also a component suggesting an assault on Norway.

Within the same context, British diplomats also began 'Graffham' negotiations with officials in 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 from and to Swedish airfields and the right to refuel aircraft forced to land. These negotiations were made not in the hope of obtaining the concessions requested, but with the intention that news of the negotiations would reach the Germans.

The formation supposedly assigned to this operation was the fictitious British 4^th Army located in Scotland. Since it was highly unlikely that any German photo-reconnaissance aircraft could reach Scotland and return 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 4^th Army was simulated by radio operators.

The formations of the supposed 4^th Army varied throughout 1944. Some were real formations whose actual role and organisation was disguised by agent reports, and others were entirely fictitious. The order of battle at the peak of 'Fortitude North' was as follows: the 4^th Army, headquartered in Edinburgh, comprised the fictional II Corps, headquartered in Stirling, with the real 55^th Division based in Northern Ireland, the fictional 58^th Division based at Aberlour) and the real 113^th Independent Brigade that was the garrison of the Orkney and Shetland islands; the fictional VII Corps, headquartered at Dundee, with the real 52^nd Division based at Dundee, the fictional US 55^th Division based in Iceland and three fictional US Ranger battalions based in Iceland; and the real US XV Corps, headquartered in Northern Ireland, with the real 2^nd, 5^th and 8^th Divisions.

'Fortitude South' was designed to convince the Germans that the invasion would land in the Pas de Calais, which was a logical strategic choice for an invasion as 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 German formations in Normandy at the time of the invasion, even more important was its task of dissuading the Germans from detaching formations from Generaloberst Hans von Salmuth’s 15^th Army in the Pas de Calais area to reinforce Generaloberst Friedrich Dollmann’s 7^th Army in the Normandy battleground during the two weeks 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 in the Pas de Calais with its advantages, so far as the Allies were concerned, of a shorter sea crossing from south-eastern England, its less easily defended beaches and its greater proximity to the bases from which tactical warplanes would operate.

The key element of 'Fortitude South' was 'Quicksilver' (ii), which was posited on the creation of a fictitious army group. The Germans were therefore to be persuaded that the Allies had the real Allied 21^st Army Group under General Sir Bernard Montgomery (the genuine Normandy invasion force), and the fictitious US 1^st Army Group under Patton located in the south-eastern England for a descent on the Pas de Calais.

The order of battle suggested in 'Quicksilver' (ii) for the US 1^st Army Group included a major airborne component (the fictitious British 2^nd Airborne Division and fictitious US 9^th and 21^st Airborne Divisions), the fictitious US 9^th Army (which became 'real' later in 1944), the fictitious US 14^th Army and the fictitious British 4^th Army.

The wholly fictitious order of battle for the US 14^th Army at Little Waltham comprised the 9^th Airborne Division at Leicester, 21^st Airborne Division at Fulbeck, XXXIII Corps at Bury St Edmonds with the 11^th Division also at Bury St Edmonds, 48^th Division at Woodbridge and 25^th Armored Division at East Dereham, and XXXVII Corps at Chelmsford with the 17^th Division at Hatfield and Peverel, and 56^th Division at Ipswich.

The order of battle for the British 4^th Army at Hatfield comprised the fictional 2^nd Airborne Division at Bulford, fictional II Corps at Tunbridge Wells with the real 35^th Armoured Brigade at Maresfield, real 55^th Division at Three Bridges and fictional 58^th Division at Gravesend, and fictional VII Corps at Folkestone with the real 61^st Division at Wye, fictional 80^th Division at Canterbury and fictional 5^th Armoured Division at Newmarket.

At no point were the Germans fed false documents describing the invasion plans. Instead they were fed misinformation which allowed to construct a misleading order of battle for the Allied forces. To mount a huge invasion of the coast of German-occupied northern 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. The placement of the 1^st Army Group in south-eastern England persuaded German intelligence to come to the conclusion that the main weight of the invasion force was based in the area 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, and dummy vehicles and landing craft were placed around possible embarkation points. It was originally intended to construct many such faked items, but the extreme paucity of the German photo-reconnaissance capability and the belief that most German spies were under British control meant that such effort were reduced to a minimum. However, a huge volume of false radio traffic was transmitted, commensurate with a formation of army group size.

A deception of so great an extent required the co-operation of many organisations, these including MI.5, MI.6, SHAEF via Ops (B) and the armed forces. Information from the various deception agencies was organised by and channelled through the London Controlling Section. The Allies were well able to judge the efficacy of their strategies, most especially as the 'Ultra' intelligence system provided a timely and accurate indication of the German high command’s responses to the Allies' actions.

The Allies maintained the pretence of the 1^st Army Group and other forces threatening the Pas de Calais for some considerable time after D-Day, possibly to a time as late as September 1944. This was vital to the success of the Allied plan since it forced the Germans to keep a sizeable proportion of their reserves bottled up waiting for an attack on the Pas de Calais, which never came, and thereby allowing the Allies to maintain and build upon their initially marginal beach-heads in Normandy into a sizeable lodgement from which they finally surged into the main body northern France late in July 1944.

Some of the primary reasons for the success of 'Fortitude' were the long-term view taken by British intelligence to cultivate captured and surrendered agents as channels of disinformation to the Germans; the use of 'Ultra' decrypts to read Enigma-coded messages between the Abwehr and the German high command, which quickly informed them the effectiveness of the deception tactics; the insistence of R. V. Jones, the Assistant Director Intelligence (Science) at the British Air Ministry, that for every radar station attacked within the real invasion area, two were to be attacked in the areas outside it; the extensive but diffuse nature of the German intelligence apparatus and the pernicious and ultimately self-defeating rivalry among its various elements; and the high degree of German respect for Patton as a field commander who, the Germans believed, would be the Allies' natural choice to lead the invasion.