'Overthrow' was an Allied deception plan designed to persuade the Germans that the Allies were planning to launch amphibious landings against Calais and Boulogne in German-occupied north-eastern France (August/October 1942).
Designed to deter the Germans from reinforcing their forces on the Eastern Front or in the Mediterranean in the period immediately before the launch of 'Torch', the operation was initially designated 'Passover' and then 'Steppingstone', and was related to a number of other deception undertakings.
Designed and undertaken in parallel with 'Solo I', which was designed to keep large numbers of German troops in Norway, 'Overthrow' was based on the 'story' that the Allies were preparing to launch an operation to seize a beach-head in the Pas-de-Calais late in October or early in November 1942. This deception operation was supported by extensive and carefully exposed physical activities on and behind the south coast of England, including the construction of additional 'hards' (paved stretches of beach) for the loading of landing craft, the use of coastal shipping to simulate the gathering of invasion craft during the first 10 days of October, an exercise by the British 3rd Division, the strengthening of anti-aircraft defences, and increased aerial reconnaissance over the relevant areas of the French coast, and in addition to these the British Overseas Airways Corporation was ordered to relocate the base for its flying boat service to Lisbon from Poole on the Dorset coast of England to Shannon in the western part of Ireland, large numbers of French dictionaries were bought, enquiries were made in Lisbon for engineers familiar with the coal mining areas of northern France, double agents reported the establishment of a new command organisation in Kent and a mass of unusual activity in the south of England, veiled suggestions that the concentrations for 'Solo I' involved simultaneous attacks on Norway and France, and references to the 'Jubilee' raid of 19 August on Dieppe as a dress rehearsal.
The climax of the deception effort was to have been a real combined exercise in the English Channel just before the implementation of 'Torch', but the shipping and men for this 'Cavendish' could not be assembled before November, when bad weather was expected to render such an operation implausible, so it was canceled.
'Solo I' and 'Overthrow' played straight into the preconceptions of the German leadership, and most especially those of Adolf Hitler, who was preoccupied with Norway as a 'zone of destiny', and of Grossadmiral Erich Raeder who, on 26 August, emphasised the need to retain strong German naval forces in Norway as a result of the constant threat of an Allied invasion.
So far as 'Overthrow' was concerned, as early as the first days of August, the weekly situation report of Generalfeldmarschall Gerd von Rundstedt, the Oberbefehlshaber 'West', came to the conclusion that the U-boat campaign would force the Allies to move against northern France to take the bases from which the boats operated, and only one week later the concentration of small craft in the harbours along the south coast of English seemed to confirmed his expectations. After 'Jubilee', von Rundstedt thought that this operation had been only 'feeling the way' and that Brittany would soon be attacked, and one week later still the German commander in France referred to 'Jubilee' as a dress rehearsal, and that there was still the very real possibility of renewed major landings based on the experience gained in that operation. On 14 September, von Rundstedt reported that Allies activities confirmed his opinion that further operations were imminent.
On 5 October, Hitler decided, on the basis of reports from agents, that there was a threat to the French coast of the English Channel, and that this coast be put on alert and its defences strengthened. On 9 October, Hitler same to the conclusion that the most likely target for an Allied assault was Cherbourg. On 12 October, von Rundstedt reported that he had been informed of the exercise by the British 3rd Division, and decided that the British were more occupied than before in the implementation of amphibious landing exercises in divisional strength, and one week later was still convinced that the British were trained and ready for amphibious landing operations on a comparatively large scale. On 31 October and 1 November, the Luftwaffe carried out heavy raids on Canterbury, possibly in an effort to disrupt the new command in Kent reported by the double agents. On 2 November, von Rundstedt considered a major raid still likely in November, informing his subordinate army commanders that there was to be no relaxation of preparations during the winter months. As late as 16 November, eight days after the 'Torch' landing in French North-West Africa, von Rundstedt was still certain of an assault on France, a certainty which remained with him until the middle of December, when the worsening winter weather clearly made amphibious undertakings impossible.