LAURENCE REES: And then Stalin sends Molotov, in November 1940, to Berlin. So what is Stalin's thinking here?

SIMON SEBAG-MONTEFIORE: Well, he gave clear instructions, and the instructions were that Finland had a big influence, France had a big influence and you’ve also got the Balkans, and Stalin regarded the Balkans as traditional Russian territory. All those states had really been created under a Russian umbrella in the 19th Century, the Orthodox pan-Slavism and all that stuff. Stalin still thought in that old-fashioned way about the Straits, so Bulgaria, Romania should have been Soviet Russian satellites - Yugoslavia even. And of course the fact that Germany was gradually encroaching on those territories very aggressively really made Stalin realise that this policy was actually falling apart, and Germany really was not ever going to be a partner in Europe.

So he sends Molotov with very strict instructions to really toughly test Hitler and come back and find out what Hitler’s real intentions were and whether they were serious - even honour among thieves, a sort of alliance of bandits, if you like. So Molotov is actually a very good person to go there to do this; he’d never had any experience with diplomacy until 1939 when he became Foreign Minister so he was the perfect person to be very good at it and he was probably one of the few people that would be totally unbamboozled by Hitler. And his performance was astonishingly good, if you think they wanted to tease out Hitler, and Hitler’s display of intolerance and frustration and his attempt to just buy them off by saying, oh, well, you know, you can invade India or fight them, you can have the Far East, you can have the shore of Arabia and the Persian Gulf, that really opened their eyes and they realised that the whole relationship was in serious trouble.

LAURENCE REES: So despite the Molotov talks clearly not having gone well, and despite receiving worrying intelligence reports, Stalin doesn’t actually think Hitler will invade, does he?

SIMON SEBAG-MONTEFIORE: Somewhere around the winter of 1940, after Molotov’s visit to Hitler, something strange and very personal comes into all this, and that is that Stalin suffers from that thing which Saddam Hussein was later to suffer from. Tyrant blindness you could call it, or a sort of mental cul-de-sac, where he becomes the victim of his own tyranny. And having killed so many people so recently you’d have thought that after that no one would dare disagree with him, but again and again you have these reports reaching him - these kind of quite brave apparatchiks' reports - and all of them must have been thinking, well, maybe this is real, maybe the Nazis are planning an invasion. And somewhere in this colossal gamble that was the Ribbentrop-Molotov pact Stalin is forced into this mental cul-de-sac where he feels he can no longer turn back from it without losing immense credibility.

And, after all, he had a very sensitive sense of his own prestige and what the Bolshevik hierarchy would take. Even though it was terrorized, there were ways that he could be overthrown. He could be arrested, for example, or just removed. So I think he suddenly began to feel that he just simply could not turn, this had to be right, he had to stick with the Hitler pact because to turn would undermine all his prestige and credibility as a ruler, after having made this incredible decision to turn all Bolshevik tradition on its head and make an alliance with the Nazis on his own personal say so, trusting a German right-wing dictator more than the Western democracies. He suddenly found himself unable to turn, and therefore stuck on this kind of narrowing cul-de-sac which he simply couldn’t risk turning back from, and he believed that his own will, which had achieved such colossal things, impossible things: industrialisation, collectivisation against vast odds, he simply believed that it could be self-fulfilling.

And so you enter, by the time you get to April, May, June [1941] a period where he really is under immense pressure and behaving quite irrationally and quite strangely and sort of trying to fight against impending doom and impending reality. But up to then the whole thing made sense. Even up to the very end of 1940 it wasn’t as crazy as we now see it. But in the beginning of ’41 with these intelligence reports coming in it really does become increasingly insane that he sticks to this policy, and it’s only explainable as a kind of personal thing. But there’s also the fact that, as he said to someone afterwards, I just simply expected Hitler to behave in a way that I would behave, and Hitler was the ultimate gambler on a colossal scale. Hitler was the sleepwalker who was willing to risk everything on these colossal bloody enterprises. But Stalin never thought like that, for him it was always a long game thinking in centuries, it was totally different. His essential mistake was to think that Hitler was like him, and that Hitler would never launch an invasion of Russia while he still had the huge, powerful British Empire undefeated at his back. And so those are the two things that I think essentially led to this insane decision to ignore the threat of Barbarossa despite all the intelligence warnings.