Secrecy had always been the greatest concern. Much of the southern coast was covered with elongated military camps known as ‘sausages’, where the invasion troops were supposedly sealed off from contact with the outside world. A number of soldiers had, however, been slipping out under the barbed wire for a last drink at the pub or to see sweethearts and wives. The possibilities of leaks at all levels were innumerable. An American air force general had been sent home in disgrace after indicating the date of Operation Overlord at a cocktail party in Claridge’s. Now a fear arose that the absence from Fleet Street of British journalists called forward to accompany the invasion force might be noticed.

Everyone in Britain knew that D-Day was imminent, and so did the Germans, but the enemy had to be prevented from knowing where and exactly when. Censorship had been imposed on the communications of foreign diplomats from 17 April, and movement in and out of the country strictly controlled. Fortunately, the British security service had captured all German agents in Britain. Most of them had been ‘turned’ to send back misleading information to their controllers. This ‘Double Cross’ system, supervised by the XX Committee, was designed to produce a great deal of confusing ‘noise’ as a key part of Plan Fortitude. Fortitude was the most ambitious deception in the history of warfare, a project even greater than the maskirovka then being prepared by the Red Army to conceal the true target of Operation Bagration, Stalin’s summer offensive to encircle and smash the Wehrmacht’s Army Group Centre in Belorussia.

Plan Fortitude had several aspects. Fortitude North, with fake formations in Scotland based on a ‘Fourth British Army’, pretended to prepare an attack on Norway to keep German divisions there. Fortitude South, the main effort, set out to convince the Germans that any landings in Normandy were a large-scale diversion to draw German reserves away from the Pas-de-Calais.