This was an Allied deception plan by the London Controlling Section designed to persuade the Germans that the pace of the Allied build-up in the UK, especially of US forces, was greater than in fact it was (May/August 1943).
The necessity for convincing the Germans that Allied forces were strong enough to carry out operations of such magnitude as 'Starkey', 'Tindall' and 'Wadham', which were in themselves deceptions, brought home to the UK for the first time the need for order of battle deceptions of the type created in North Africa and the Mediterranean by Colonel Dudley W. Clarke’s 'A' Force organisation.
Separate projects to suggest enlarged orders of battle were conducted for the US and British forces, carried out entirely through the double agents and known as 'Larkhall' and 'Dundas' respectively.
'Larkhall' was conceived as the means to inflate the progress of the 'Bolero' build-up of US forces in the UK, and began in May. A memorandum of 23 April from the London Controlling Section to the Joint Security Control organisation set out in some detail the kinds of help the Americans might provide in terms of 'special means' messages and the dissemination of false rumours in Latin America, with some suggestions as to detailed methods of implementation. The memorandum included as models examples of gossip concocted for dissemination by British diplomats in neutral countries. A fair sample is one for Switzerland in aid of 'Barclay' that was clearly designed to appeal to Nazi ears: 'I’m told that Israel Sieff has persuaded Ralph Grimthorpe into selling that lovely villa of his at Amalfi for £75,000. These Jews are amazingly shrewd. No doubt Sieff realizes that the end of the war is in sight already and he realizes that however hard we blitz the Italian cities we should never bomb a little place like Amalfi; whereas a villa anywhere on the South Coast of France is liable to suffer from serious war damage in the next few months.'
The Operations Division of the War Department’s general staff, which was the planning and operational command element for the US Army in Washington, DC, agreed on 7 May, and approved by Lieutenant General F. E. Strong, the head of the COSSAC (Chief-Of-Staff of the Supreme Allied Commander) planning team in the UK, on May 12.
At that time there were 107,000 US troops, most of them USAAF supply personnel, in the UK. The only combat force was Major General Leonard T. Gerow’s 29th Division, which was due to be joined in July by Major General Cortland Parker’s 5th Division from Iceland, and in September by Major General Maxwell D. Taylor’s 101st Airborne Division and Major General Leroy H. Watson’s 3rd Armored Division from the USA.
Double agents were used to 'tell' the Germans in May and June that the 3d Armored Division was already arriving, together with the fictitious 46th Division. When the 5th Division moved to the UK it was replaced in Iceland by the fictitious 55th Division, supposedly specially trained in mountain warfare, while the movement of the 101st Airborne Division to the UK was reported at the same time. The Germans were further given an overstated number for the USAAF personnel in the UK, and that these were mostly bomber crews.
One minor method employed to exaggerate US strength, at least in the eyes of stray observers, was by confusing British with US troops. British vehicles were marked with the white stars customarily used for US vehicles, and while all Allied vehicles were eventually marked in this manner, the fact had not yet been announce. US troops in the rear areas were dressed in British battle dress with US insignia, the 'story' suggesting that this was being tested against US battle dress.
'Larkhall' progressed smoothly, although the British had to be reminded that such activity should be co-ordinated by having requests for US action passed to the Joint Security Control through the European Theater of Operations US Army (ETOUSA) command. By August, when 'Larkhall' ended, the Germans had been 'informed' that there were 570,000 US troops in the UK, whereas the actual number was 330,000.
The Germans had arrived at a greatly exaggerated estimate of the Allied strength in the UK during 1943, though as usual it is hard to tell how much of that exaggeration resulted directly from the US 'Larkhall' and British 'Dundas' efforts. The Fremde Heere West army intelligence and assessment branch estimated in October that there were 43 Allied divisions available for a landing in Scandinavia or North-West Europe. At the beginning of November, Generaloberst Alfred Jodl, the chief of the operations staff of the Oberkommando der Wehrmacht, gave a comparable estimate to an audience of Nazi local administrators in Munich, adding that the Allies possessed a sea lift capability to deliver an assault force of 11 or 12 infantry and one or two armoured divisions, as well as an air lift capability for 20,000 airborne troops.