LAURENCE REES: And so to the Yalta conference. Was Yalta a betrayal of the ideals for which the Western Allies thought they were fighting the war?

ANDREW ROBERTS: It’s a betrayal of the ideals because we went to war for the integrity of Poland and the independence of Poland. In fact, what we wanted in April 1939 was a trip wire for Hitler, something that triggered a war. It could have been Poland, it could have been anywhere else frankly, the key thing was to get us into a war with Germany before Hitler took anymore of Europe. One could argue forever about whether or not it would have been better to have done it earlier at Munich but certainly by the time Poland was invaded there was no alternative.

But what we didn’t ever promise to the Poles was that we were going to be able to land an army on the other side of Europe and fight. We didn’t ever promise them that we were going to attack Germany from the west, and we were in no position to do so either. So in that sense it was a betrayal of the ideals, but I’m not sure if it was really a betrayal of the country itself because there was simply nothing that could be done short of using a nuclear bomb, or threatening to, which was obviously impossible against our great and glorious comrade that had lost 20 million people fighting the Nazis.

LAURENCE REES: Why then pretend at Yalta that we actually believe Stalin is going to allow free elections in Poland and elsewhere, when there is zero evidence from elsewhere that Stalin was likely to do this?

ANDREW ROBERTS: Because he promised he would, this is the thing. There was an enormous amount of wishful thinking involved, but at the same time you have to remember that Poland and the independence of Poland was not at that stage the No.1 issue. It wasn’t even the thing that they spent more time talking about than anything else. The key thing at Yalta was to try to get Stalin to stump up with the promise to go to war with Japan 3 months after the end of the war in Europe and also to try and get the Soviet Union into the United Nations organisation that was going to be set up after the war as well. After that you came up with free elections in Eastern Europe, and so it wasn’t even the prime desiderata of the British and Americans. I think, as well as wishful thinking, there was a sense that the kinder and nicer you were to Stalin the more likely it would be that he was going to going to come into a world organisation that was inclusive, and that he wasn’t going to sabre rattle and pursue imperialist pre-war style policies. This sounds ridiculous coming from somebody, Winston Churchill, who had, after all, denounced appeasements, which were precisely the same thing effectively only a decade earlier. But I think Churchill was an appeaser to the USSR up to and including Yalta.

LAURENCE REES: Still, Churchill at Yalta does speak passionately about how he feels about Poland - saying ‘we drew the sword for Poland’ and so on - but at Yalta Roosevelt gives every impression of not being really bothered about Eastern Europe.

ANDREW ROBERTS: Which is very interesting considering that Roosevelt actually had Polish voters in the Democratic Party who he could have played to. Nonetheless, as his letters to Stalin show, he was just as bad as Churchill when it came to the wishful thinking, and he’s also a very ill man it’s worthwhile remembering. He’s a dying man by the time of Yalta and possibly knows it, so he’s not on the kind of form that he was earlier on in the war. But, yes, there is the central problem that the allied powers are militarily incapable of doing anything else and are starting to look to the Pacific War, and wanting above all to try to continue to get on well with the Russians, and let Poland slip beneath the Soviet yolk.

LAURENCE REES: And even at the Tehran Conference, when Roosevelt isn’t sick, before the meeting about Poland's borders comes up, he says to Stalin, in effect: 'well I can’t support you, Marshal Stalin, officially in the meeting, but I can unofficially let you know that I’m not bothered about Poland'.

ANDREW ROBERTS: He reacts again because he’s got an election coming up in the following months in 1944. He refuses to rubber stamp Churchill’s percentages deal, which Churchill did tell him about as soon as he’d done it, thinking that it was a good deal. Actually, it wasn’t such a bad deal apart from if you happened to be a Pole, but it protected Greece and British and Western interests in the southern Mediterranean. But when one looks at the history of Poland, and the way in which Polish geography simply works against it on every level for two centuries prior to the outbreak of the Second World War, it is difficult to work out how we could have genuinely protected Polish independence and integrity between Tehran and Potsdam. What pressures could we have brought on Stalin once the Wehrmacht had been smashed in 1944?