‘In wartime’, Churchill told Stalin at Teheran, ‘truth is so precious that she should always be attended by a bodyguard of lies.’ Discussing the preparations for the forthcoming cross-Channel invasion, the two warlords were agreed on the importance of plans to deceive the Germans about allied intentions. The key plan, the work of a secret circle of British officers who wrote the script, was FORTITUDE, an entirely fictitious operation accompanied by all the appropriate theatrical props: bogus wireless traffic, dummy tanks, landing craft and so on. A British Fourth Army in Scotland was to invade Norway. Another phantom force, the United States First Army Group supposedly under the command of General George Patton, was stationed in the south-east of England, apparently poised for an assault on the Pas de Calais. At the core of the conspiracy was a truly fantastic web of misinformation supplied to the Germans by Juan Pujol, a Spanish double agent originally known as BOVRIL but happily, and felicitously in view of the starring role he played, renamed GARBO.

It worked. Though practising deception themselves, and always aware that it would be used against them, the German High Command were led to believe that the Normandy landings were a feint intended to draw German forces away from the main allied objective, the Pas de Calais. By the time they