This was a Franco/US deception plan for a notional offensive by Général d’Armée Jean Joseph Marie Gabriel de Lattre de Tassigny’s French 1st Army in the Mulhouse and Vesoul area of eastern France in order to contain as great a German strength as possible in the area to the east of the Rhine river and to the south of Lauterbourg (December 1944).
The scenario for 'Knifedge' was set by the continued advance of Lieutenant General Jacob L. Devers’s Allied 6th Army Group during November. This army group’s right wing, de Lattre de Tassigny’s French 1st Army, liberated all of Alsace but a pocket around Colmar, became the first Allied major formation to reach the Rhine river, broke into Belfort on 20 November, liberated Mulhouse on 21 November, and by 26 November had cleared the symbolically important city of Strasbourg.
A recent convert to the notion that carefully planned and deftly executed deception could be a powerful military tool, Devers next planned an offensive with its main weight on the left wing by Lieutenant General Alexander McC. Patch’s US 7th Army, while de Lattre de Tassigny’s 1st Army was to reduce the Colmar pocket and close up to the Rhine river. Eugene J. Sweeney was directed to produce a deception plan for this offensive, with the object of containing the maximum number of German formations in the area to the east of the Rhine river and as far south as possible, preferably all the way to the Swiss border.
Sweeney’s response was 'Knifedge', whose 'story' was that after the great success of his 1st Army de Lattre de Tassigny had been allocated the task of breaking through the German defence in the Mulhouse sector, while Patch’s attack farther to the north would be a diversion for this. Implementation of 'Knifedge' was to be by radio and administrative means of what were now the standard type; diplomatic representations to Switzerland seeking use of a railway through Swiss territory near Basle and requesting the Swiss to intensify their measures to prevent the escape of German war criminals into Switzerland; propaganda designed to arouse public opinion in Switzerland and other neutral countries against the harbouring of war criminals; canvassing by the French around Mulhouse for guides familiar with the northern Swiss/German border and railway personnel familiar with German railway lines along the border; and 'special means'.
'Knifedge' was quickly approved by the 6th Army Group on 30 November and by Colonel Noel Wild of Ops (B) on 2 December. An enthusiast of deception measures, de Lattre de Tassigny immediately ordered the implementation of 'Knifedge' by the 1st Army, which grouped real troops, began physical displays and sought guides and railway personnel, and at the same time instituted the agreed radio and security measures. Even before the attack on Belfort, de Lattre de Tassigny’s had made use of a French irregular warfare officer to organise a line-crossing double agent to plant on the Germans a false order suggesting that the 1st Army was to bypass Belfort, and another double agent had provided similar material to the Abwehr in Switzerland.
For 'Knifedge', No. 2 Tactical Headquarters prepared a fake document directing that priority in railway reconstruction be given to lines in the southern sector, and passed it through the 1st Army’s irregular warfare channel together with some genuine but harmless papers. Perhaps more importantly, the 'Gilbert' double agent in Paris sent reports throughout the first half of December tailored to the 'story' promulgated by 'Knifedge'.
The German 'Wacht an Rhein' offensive in the Ardennes and the 'Nordwind' (iii) offensive against the 6th Army Group combined to change Devers’s plans, and 'Knifedge' was effectively ended before it could have made any impact on the Germans.