In January 1941 Winston Churchill entertained Harry Hopkins, the American presidential adviser, to dinner at Chequers. During the course of the evening, the prime minister launched into one of his flowing monologues about the nobility of Britain’s war aims, emphasising the need to uphold liberty and democracy. ‘We seek only the right of man to be free,’ said Churchill, before asking Hopkins what Roosevelt would think of his statement. Hopkins replied that the president would not ‘give a damn for all that ... You see, we’re only interested in seeing that that Goddam sonofabitch Hitler gets licked.’

At the time of that exchange, it seemed unlikely that Hitler would get ‘licked’ any time soon. Indeed, the ruthless German war machine appeared to be almost invincible, having brought much of Europe under Nazi rule. The British might have defended themselves heroically against the Luftwaffe in 1940, but without a massive expansion of its army