The astonishing thing about the grand alliance that defeated Hitler was not that two Western democracies made a pact with one of the vilest tyrannies the world has ever known. It was that Churchill, Roosevelt and then Truman fell prey to their own propaganda that Stalin's was an essentially benign regime and allowed themselves to be manipulated by the Soviet dictator into a morally indefensible and politically self-destructive position.
For 50 years after 1945, the received image of the "good war", in which the evil Nazis were overcome by a morally superior coalition, obscured this unpleasant reality. As Laurence Rees points out, the story of this aspect of the war could not be told before the collapse of the USSR, since witnesses dared not speak and documents were inaccessible. He could have added a third reason: the denial indulged in by educated classes in the West where Communism and Russia were concerned.
Rees begins his narrative with the Ribbentrop-Molotov pact which, he points out, was the opening move of the Second World War. Without that pact, Hitler could not have risked an invasion of Poland. He also makes it clear that Stalin entered into it with the same motives as Hitler - the destruction of the Polish state, the absorption of its territory and the liquidation of its people. It follows that the war was not started by Germany alone, but by a Russo-German alliance.
Nobody was prepared to face up to this at the time. While socialist fellow-travellers in the West stood on their heads to explain and justify Stalin's action, the British and French governments did everything to avoid treating the Russian invasion of Poland as they did the German one. This introduced an element of dishonesty into the Allies' conduct of the war that would never go away.