This was a US deception undertaking designed to persuade the Germans to maintain large forces in the Franco/Italian border area and thereby prevent their use to reinforce other theatres (3 October 1944/29 April 1945).
Only a few days alter landing in France, Eugene J. Sweeney of the US No. 2 Tactical Headquarters was allocated an important task when he reached the headquarters of Lieutenant General Alexander McC. Patch’s US 7th Army in Grenoble. As the forces of this army, which had landed in southern France in 'Dragoon' (i), advanced to the north up the valley of the Rhône river, its lines of communication back to the Mediterranean were steadily more exposed to the threat of an attack by the forces of Generalfeldmarschall Albert Kesselring, the Oberbefehlshaber ‘Südost’, from north-western Italy in an advance through the Alpine passes. 'Ultra' intelligence confirmed that at this moment the Germans had no such plan, but Patch nonetheless felt its only sensible to deter such a move. Sweeney accordingly created a plan to convince the Germans that there was a major Allied build-up of strength along the Franco/Italian border, and this plan was based on the use of spectrum of means including radio, 'special means' and administrative factors.
The implementation of this plan, which was later named 'Jessica', was deferred while it was decided whether or not there would actually be a No. 2 Tactical Headquarters deception organisation, and it was agreed in September that it should be created. By this time the Allied leadership on the Italian front also wanted the Supreme Headquarters Allied Expeditionary Force to launch a deception plan to force the Germans to maintain as many formations and units as possible along the Franco/Italian border and therefore out of the primary combat arena in northern Italy. This accorded well with 'Jessica', which was officially launched on 3 October 1944 and lasted to 29 April 1945.
During this period, the 'Jessica' plan faced a difficult problem of balance: on the one hand it had to suggest that Allied forces on the Franco/Italian border were of a strength which demanded that the Germans hold substantial forces opposite them, and on the other it had to deter a German offensive but without posing a threat sufficient to tempt the Germans into the commitment of a pre-emptive spoiling attack. The problem was rendered that much more tricky by the fact that SHAEF wanted the size of forces joining Lieutenant General Jacob L. Devers’s Allied 6th Army Group in the south of France to be made as small as possible to avoid drawing the Germans to his main front. Moreover, the 'Jessica' plan had to deal with the political fact that French troops were forbidden to operate on the Italian side of the frontier despite the French desire to establish French rule in the Val d’Aosta.
The basic method adopted for 'Jessica' was to ensure that the Germans learned of the arrival of divisions at Marseilles and intended as reinforcements for the 6th Army Group, but at the same time lead the Germans to believe that these divisions had remained in the southern part of Devers’s front. It was also 'leaked' to the Germans the notion that French resistance forces were being retrained as regular units destined for the Alpine front.
Sweeney wished to upgrade the force on the border to a notional corps, and therefore requested the fictional US XXXI Corps from 'Vendetta' and 'Ferdinand' (ii), but this 'formation' was still in play with Brigadier Dudley W. Clarke’s 'A' Force on the Italian front. Sweeney thus requested the creation of a new fictional corps for 'Jessica', but there were strong objections to the use of any further order of battle deceptions on the continent.
For two weeks after the Germans' December breakthrough in 'Wacht am Rhein' in the Ardennes, the 'story' promulgated was that the Allied forces along the Franco/Italian border were being reduced to allow the reinforcement of forces farther to the north. When that threat and the subsequent German offensive on Devers’s front had faded, 'Jessica' was allowed to revert to its original concept, and additional notional forces were provided by doubling, for German consumption, the actual reconversion of superfluous anti-aircraft troops into ground combat units.
At the request of the Allied Force Headquarters in the Mediterranean theatres, in its last phase as the war drew to an end, 'Jessica' was developed into a more aggressive form in order to aid the final offensives in Italy. The genuine forces on the Franco/Italian border were reorganised on 1 March into Général de Corps d’Armée Paul André Doyen’s Détachement de l’Armée des Alpes. 'Jessica' was thus upgraded to provide a realistic possible threat of an Allied trans-border offensive, with the equivalent of two more French divisions added to the notional order of battle, active patrolling on the front, and authority to for the French units to make raids into Italy.
'Jessica' was implemented primarily by double agents and radio deceptions by the 7th Army’s standard signals branches. The primary 'special means' channel was the double agent 'Forest', with the double agent 'Monoplane' making a smaller contribution.