Naturally all this took some explaining to populations schooled to hate their neighbours. Moscow's propagandists in particular had their work cut out justifying the astonishing volte-face to the Communist faithful beyond its boundaries. It was too much for the British Communist Party chief Harry Pollitt to stomach, but the great majority of the Central Committee eagerly accepted Moscow's declaration that the "division into fascist and democratic states has now lost its former sense" and that the USSR had the right to defend the revolution in whatever way it saw fit.

The government reaction was cool, measured and pragmatic. The professional view of the diplomats was that the arrangement could not last and it was essential that Moscow was kept in play.

On the ground, though, everything started off fine. Agreements were honoured and both armies paraded and even socialised together as they trampled over the inhabitants of their new conquests. Beneath the surface differences there was much shared ideological DNA. This was indeed a pact between two devilish creeds. Both Nazism and Communism had rejected a notion that until then had sustained civilisation, that of the universality of mankind. Both sides thus set about murdering and enslaving with the same merciless efficiency. For the Germans the racial enemy was the primary target; for the Russians, the class enemy, a category that, as well as Trotskyists and conservatives, included Esperantists, philatelists and those with "white hands" that had never done manual labour.

Each had gone into the bargain imagining they were hoodwinking the other. "I have them!" exclaimed Hitler on hearing that the Russians had signed. The pact brought him peace in the rear while he set about his Polish and Western conquests, as well as access to Soviet raw materials to feed the war machine and circumvent the threat of blockade.

To Stalin it looked like a sweet deal. Despite their overtures, France and Britain could give him nothing but promises, whereas the Nazis were offering military technology, a free hand to expand his domains, and a stay of execution that might extend indefinitely. If things went really well the Germans and the Allies might bleed each other white, leaving the USSR to dominate Europe.

Moorhouse makes it clear that the pact meant more to Stalin than it did to Hitler, for whom ideology would always eventually trump expediency. Despite overwhelming evidence that Germany was planning to renege, Stalin almost to the end refused to accept that the game was over, literally shooting the messengers who brought the bad news.

As the ultimate victors, the Russians never had to undergo the national self-examination and acts of contrition forced on the Germans for their wartime behaviour. There is no sign of them doing so now. Instead in all likelihood they will use next year's anniversary to reinforce the message that it was their sacrifices that ultimately defeated Hitler. While that may be true, it is good that we have Moorhouse's book to remind us of the bigger and darker story.