The party lasted into the small hours, and when the Germans finally took their leave Stalin was, according to Hoffmann, “well and truly lit up”! The Soviet leader clearly understood the incongruity – almost the comic nature – of this pact with his former enemy.

“Let’s drink to the new anti-Cominternist,” he said at one point, “Stalin!” But his last words to Ribbentrop were spoken with apparent sincerity: “I assure that the Soviet Union takes this pact very seriously. I guarantee on my word of honour that the Soviet Union will not betray its new partner.”

Back at the Berghof, the atmosphere grew ever more anxious in the hours before news of the signing of the pact came through. Herbert Doring watched that evening as Hitler and his guests stared at the dramatic sky over the high mountain peaks.

“The entire sky was turmoil,” he remembered. “It was blood-red, green, sulphur grey, black as night, a jagged yellow. Everyone was looking horrified – it was intimidating….Everyone was watching. Without good nerves one could easily have become frightened.”

Doring observed one of Hitler’s guests, a Hungarian woman, remark: “My Fuhrer, this augurs nothing good. It means, blood, blood, blood and again blood.” ‘Hitler was totally shocked,’ said Doring. ‘He was almost shaking. He said, “If it has to be, then let it be now.” He was agitated, completely crazed. His hair was wild. His gaze was locked on the distance.’ Then, when the good news that the pact had been signed finally arrived, Hitler ‘said goodbye’, went upstairs and the evening was over.’

Behind Closed Doors, Laurence Rees