Rough seas in the English Channel could swamp the landing craft, to say nothing of their effect on the soldiers cramped on board. Low cloud and bad visibility presented another great threat, since the landings depended on the ability of the Allied air forces and navies to knock out German coastal batteries and defensive positions. General embarkation for the first wave of 130,000 troops was under way and due to be completed in two days’ time.

Stagg was plagued by a lack of agreement among the different British and American meteorological departments. They all received the same reports from the weather stations but their analysis of the data simply did not match up. Unable to admit this, he had to tell Major General Harold R. Bull, Eisenhower’s assistant chief of staff, that ‘the situation is complex and difficult’.

‘For heaven’s sake, Stagg,’ Bull exploded. ‘Get it sorted out by tomorrow morning before you come to the Supreme Commander’s conference. General Eisenhower is a very worried man.’ Stagg returned to his Nissen hut to pore over the charts and consult the other departments yet again.

Eisenhower had other reasons for ‘pre-D-Day jitters’. Although outwardly relaxed, with his famous open smile for everyone whatever their rank, he was smoking up to four packs of Camel cigarettes a day. He would light a cigarette, leave it smouldering in an ashtray, jump up, walk around and light another. His nerves were not helped by constant pots of coffee.

Postponing the invasion carried many risks. The 175,000 soldiers in the first two waves risked losing their fighting edge if cooped up in rough weather on their ships and landing craft. The battleships and convoys about to head down British coasts towards the Channel could not be turned round more than once without needing to refuel. And the chances of German reconnaissance aircraft sighting them would increase enormously.