Bletchley Park, the highly secret complex about fifty miles north-west of London which decoded enemy signals, adopted a new watch system for Overlord from 22 May. Its experts were ready to decrypt anything important the moment it came in. Thanks to these ‘Ultra’ intercepts, they were also able to check on the success of Fortitude disinformation provided by the main ‘Double Cross’ agents, Pujol, Dusko Popov (‘Tricycle’) and Roman Garby-Czerniawski. On 22 April, Bletchley had decoded a German signal which identified the ‘Fourth Army’, with its headquarters near Edinburgh and two component corps at Stirling and Dundee. Other messages showed that the Germans believed that the Lowland Division was being equipped for an attack on Norway.

Ultra decrypts revealed in May that the Germans had carried out an anti-invasion exercise, based on the assumption that the landings would take place between Ostend and Boulogne. Finally, on 2 June, Bletchley felt able to report: ‘Latest evidence suggests enemy appreciates all Allied preparations completed. Expects initial landing Normandy or Brittany followed by main effort in Pas-de-Calais.’ It looked as if the Germans really had swallowed Plan Fortitude.

Early on 2 June, Eisenhower moved into a trailer hidden in the park at Southwick under camouflage nets. He dubbed it ‘my circus wagon’, and when not in conference or visiting troops, he would try to relax by reading westerns on his bunk and smoking.

At 10.00 hours that Friday, in the library in Southwick House, Stagg gave Eisenhower and the other assembled commanders-in-chief the latest weather assessment. Because of the continuing disagreement among his colleagues, particularly the over-optimistic American meteorologists at SHAEF, he had to remain Delphic in his pronouncements. Stagg knew that by the evening conference he must produce a firm opinion on the deterioration of the weather over the weekend. The decision to proceed or to postpone had to be made very soon.