The Fortitude South story would be that FUSAG was being prepared to invade Pas-de-Calais some weeks after an initial diversionary invasion. That would allow Operation Neptune's landings to be passed off as a distraction from the later main invasion. Pas-de-Calais offered a number of advantages over the real invasion site, such as by being the shortest crossing of the English Channel and the quickest route into Germany. As a result, Erwin Rommel had taken steps to fortify that area of coastline heavily. Strangeways felt that would help the deception seem realistic in the minds of German high command.
A deception of such a size required significant organisation and input from many organisations, including MI5, MI6, SHAEF via Ops B, and the armed services. Information from the various deception agencies was organised by and channelled through the London Controlling Section. To help keep the approach well-organised, Strangeways divided the implementation stages into six subplans, codenamed Quicksilver.
| Plan | Special Means | Description |
|---|---|---|
| Quicksilver I | Leaks | The basic "story" of Fortitude South was to be leaked, under Quicksilver I, largely through the double agent network and some diplomatic channels. |
| Quicksilver II | Wireless traffic | Radio deception was used to simulate the movement of troops across the south of England, with German listening posts expected to pick up the traffic. |
| Quicksilver III | Physical deception | A display of dummy landing craft, including associated simulated wireless traffic, road signs, and restricted areas. |
| Quicksilver IV | Physical preparations | Any invasion target would have been prepared with attacks in advance of landings and so Quicksilver IV covered a number of air activities including bombing of the Pas-de-Calais beach area and tactical railway bombing immediately before D-Day. |
| Quicksilver V | Physical Deception | Overall increased activity around Dover (such as by giving impression of extra tunnelling and additional wireless stations) to suggest embarkation preparations. |
| Quicksilver VI | Physical Deception | Night lighting deception to simulate activity at night in places that dummy landing craft were situated. |
The FUSAG deception was not implemented primarily with dummy tanks, aeroplanes, or other vehicles. At that stage of the war, the Germans were unable to fly reconnaissance planes over England and so Strangeways felt that such effort would have been wasted. However, temporary buildings were constructed and dummy landing craft were stationed at likely embarkation point in the East and the South-East of England. As the FUSAG commander, Patton paid many of them a visit, along with a photographer, to ensure that their location was noted. The landing craft, built from wood and canvas and nicknamed Bigbob's, suffered from being too light. Wind and rain flipped many of them over or ran them to ground during the operation.
Instead of extensive physical measures, most of Strangeway's plan relied on radio signals and leaks through double agents. Managing that information flow had to be done with caution since leaking supposed top-secret invasion plans would have been very obvious. Instead, the deceivers used tactics developed by Clarke in Cairo. Agents were allowed to report minutiae such as insignia on soldiers' uniforms and unit markings on vehicles to allow the Germans to build up a picture. The observations in the south-central areas largely gave accurate information about the real invasion forces since Clarke had stressed that using as much real information as possible led to better outcomes. Reports from the South-West of England indicated few troop sightings, but in reality, many units were housed there in preparation for D-Day. Reports from the South-East depicted largely-notional Quicksilver forces. That approach aimed to convince German intelligence services of an order of battle for the Allied forces that placed the centre of gravity of the invasion force opposite Pas-de-Calais.