This was an Allied deception plan suggesting an offensive by Lieutenant General Jacob L. Devers’s Allied 6th Army Group to break through the German defences of the 'Siegfried-Linie' and Rhine river in that part of Germany to the east of the Rhine river and to the south of Karlsruhe, and thereby persuade the Germans to maintain or even reinforce their forces in the sector and thereby facilitate the real 'Undertone' assault across the Rhine river by Lieutenant General Alexander McC. Patch’s US 7th Army (March/18 April 1945).
By the beginning of March, the Allied plans for the 7th Army to close up to the Rhine river and then cross had become sufficiently definite for a revived and version of the 'Knifedge' deception and diversion plan to be approved as 'Accordion' (ii). The object of the revised scheme was to persuade the Germans that the operational situation demanded they retain their maximum possible strength in the area to the east of the Rhine river and to the south of Karlsruhe.
The 'story' promulgated by 'Accordion' (ii) was that Général d’Armée Jean Joseph Marie Gabriel de Lattre de Tassigny’s French 1ère Armée was preparing to launch an offensive to breach the Rhine river and the 'Siegfried-Linie' defences close to the Swiss border during the last half of March, after which Patch’s 7th Army would undertake a similar offensive at Strasbourg, farther to the north.
In addition to the usual methods of implementation for this 'Accordion' (ii) undertaking, it was requested that the Supreme Headquarters Allied Expeditionary Force arrange diplomatic negotiations in which permission was sought from the neutral Swiss for the use of a railway passing through their country near the border with Germany, and heavy bomber operations were also requested for the area bounded by Basle, Karlsruhe and Ulm: the planners were soon advised that neither of these was possible.
The resources currently available to the French 1st Army were too limited to make feasible much in the way of any physical implementation of the 'Accordion' (ii) scheme. A plan was formulated for the use of the fictional US 23rd Division, but this had to be abandoned after it was decided that this 'formation' would be better employed in supporting deception operations for Lieutenant General Omar N. Bradley’s US 12th Army Group. More significant was traffic disseminated via 'Gilbert', one of the war’s most successful double-agent channels. 'Gilbert' reported to the Germans that Général de Brigade Charles de Gaulle, the Free French leader, was adamant that the French be the first to cross the Rhine river, that French forces were being strengthened by several notional or near-notional formations (the III Corps, IV Corps, and Corps Expéditionnaire Français) and that recruitment within France was providing more men than was actually the case. When the 7th Army launched its real 'Undertone' offensive late in the month, 'Gilbert' told the Germans the excuse that the unforeseen seizure of the Remagen bridge and unexpected German weakness in the Saar had paved the way to a change in plan and a postponement of the French attack.
The Rhine river was crossed by the 7th Army on 26 March and by the French 1ère Armée five days later at the end of the same month. Although it was proposed that 'Accordion' (ii) should be continued in a revised form, designed to persuade the Germans to hold major forces in the Schwarzwald area to the south-west of Stuttgart, the pace of the Allied progress was so rapid that there was little more need for it, and 'Accordion' (ii) was officially terminated on 18 April. Even so, Germans retained five to seven divisions in the Schwarzwald area facing only two French divisions along the southern reaches of the Rhine river, though any contribution by 'Accordion' (ii) cannot be determined.