Stalin (left) and Nazi Foreign Minister Joachim von Ribbentrop shake hands in 1939
The existence of these documents has allowed a true 'behind-the-scenes' history of the West's dealings with Stalin to be attempted. All of which means, I hope, that my findings contain much that is new.
I have been lucky that the collapse of the Eastern Bloc has permitted this work. It was certainly something I could never have predicted would happen when I was taught the history of World War Two at school back in the early 1970s.
Then, my history teacher got round the moral and political complexities of the Soviet Union's participation in the war by the simple expedient of largely ignoring it. At the time, in the depths of the Cold War, that was how most people dealt with the awkward legacy of the West's relationship with Stalin.
(It is important to note that the 'Soviet Union' is the accurate term to describe the country at the time of World War Two. However, many people used the word 'Russian' when they meant 'Soviet', and indeed many still do so today. Stalin would often describe the country he ruled as 'Russia', and Churchill, Roosevelt and the Nazis did the same. Not to use the terms 'Soviet' and 'Soviet Union' is to diminish the massive contribution to the war made by citizens from all 16 Soviet republics.)
The focus was on the heroism of the Western Allies - on Dunkirk, the Battle of Britain and D-Day. None of which, of course, must be forgotten. But it is not the whole story.
Before the fall of Communism, the role of the Soviet Union in the World War Two was, to a large extent, denied a proper place in our culture because it was easier than facing up to a variety of unpalatable truths.
Did we, for example, really contribute to the terrible fate that in 1945 befell Poland, the very country we went to war to protect? Especially when we were taught that this was a war about confronting tyranny?
And if, as we should, we do start asking ourselves these difficult questions, then we also have to pose some of the most uncomfortable of all.
Was anyone in the West to blame in any way for what happened at the end of the war? What about the great heroes of British and American history, Winston Churchill and Franklin Roosevelt?
Scene from 'Behind Closed Doors' featuring (from left) Stalin, Roosevelt and Churchill
Paradoxically, the best way to attempt an answer to all this is by focusing on someone else entirely - Joseph Stalin. It is Stalin who dominates the work. And a real insight into the Soviet leader's attitude to the war is gained by examining his behaviour immediately before his alliance with the West.
This period, of the Nazi-Soviet pact between 1939 and 1941, has been largely ignored in the popular consciousness. It was certainly ignored in the post-war
I remember asking one Russian after the fall of the Berlin Wall: 'How was the Nazi-Soviet pact taught when you were in school during the Soviet era? Wasn't it a tricky piece of history to explain away?'
He smiled in response. 'Oh, no,' he said, 'not tricky at all. You see, I didn't learn there had ever been a Nazi-Soviet pact until after 1990 and the collapse of the Soviet Union.'
Stalin's relationship with the Nazis is a vital insight into the kind of person he was, because, at least in the early days of the relationship, he got on perfectly well with them.
The Soviet Communists and the German Nazis had a lot in common - not ideologically, of course, but in practical terms. Each of them respected the importance of raw power.
And each of them despised the values that a man like Franklin Roosevelt held most dear, such as freedom of speech and the rule of law. As a consequence, we see Stalin at his most relaxed carving up Europe with Joachim von Ribbentrop, the Nazi foreign minister.
The Soviet leader was never to attain such a moment of mutual interest and understanding at any point in his relationship with Churchill and Roosevelt.
The aggressive listener
Both Churchill and Roosevelt had gigantic egos and both of them liked to dominate the room. And both of them liked the sound of their own voices. Stalin wasn't like that at all. He was a watcher - an aggressive listener.
It was no accident that it took two highly intelligent functionaries on the British side - Sir Alexander Cadogan, permanent under-secretary at the Foreign Office, and Lord Alanbrooke, chief of the Imperial General Staff - to spot Stalin's gifts most accurately.
They saw him not as a politician playing to the crowd and awash with his own rhetoric, but more like a bureaucrat - a practical man who got things done.
As Cadogan confided in his diary at Yalta: 'I must say I think Uncle Joe [Stalin] much the most impressive of the three men. He is very quiet and restrained... The president flapped about and the PM boomed, but Joe just sat taking it all in and being rather amused. When he did chip in, he never used a superfluous word and spoke very much to the point.' [David Dilks (ed.), 'The Diaries of Sir Alexander Cadogan OM 1938-1945', Cassell, 1971, pp. 708-9, entry for 11 February 1945.]
Field Marshal Lord Alanbrooke 'formed a very high idea of his [Stalin's] ability, force of character and shrewdness'. [Alex Danchev and Daniel Todman (ed.), Field Marshal Lord Alanbrooke, 'War Diaries 1939-1945', Phoenix, 2002, p. 483, entry for 28 November 1943.]
In particular, Alanbrooke was impressed that Stalin 'displayed an astounding knowledge of technical railway details'. [Alanbrooke, 'War Diaries', p. 608, entry for 15 October, 1944.]
No one would ever accuse Churchill or Roosevelt - those biggest of 'big picture' men - of having 'an astounding knowledge of technical railway details'.
And it was Alanbrooke who spotted early on what was to be the crux of the final problem between Stalin and Churchill: 'Stalin is a realist if ever there was one,' he wrote in his diary, 'facts only count with him...[Churchill] appealed to sentiments in Stalin which I do not think exist there.' [Alanbrooke, 'War Diaries', pp. 299-300, entry for 13 August 1942.]
As one historian has put it, the Western leaders at the end of the war were 'not dealing with a normal, everyday, run-of-the-mill, statesmanlike head of government. They confronted instead a psychologically disturbed, but fully functional and highly intelligent dictator who had projected his own personality not only onto those around him but onto an entire nation and had thereby with catastrophic results, remade it in his image.' [John Lewis Gaddis, 'Presidential Address: The Tragedy of the Cold War', p. 4, quoted in Amos Perlmuter, 'FDR and Stalin: A Not So Grand Alliance', University of Missouri Press, 1993, p. 17.]
One of the problems was that Stalin in person was very different from the image of Stalin the tyrant. Anthony Eden, one of the first Western politicians to spend time with Stalin in Moscow during the war, remarked on his return that he had tried hard to imagine the Soviet leader 'dripping with the blood of his opponents and rivals, but somehow the picture wouldn't fit'. [Quoted in Ben Pimlott (ed.), 'The Second World War Diaries of Hugh Dalton 1940-1945', Jonathan Cape, 1986, entry for 13 January, 1942, p. 348.]
But Roosevelt and Churchill were sophisticated politicians and it is wrong to suppose that they were simply duped by Stalin. No, something altogether more interesting - and more complicated - takes place in this history.
Roosevelt and Churchill wanted to win the war at the least possible cost to their own respective countries - in both human and financial terms.
Keeping Stalin 'on side', particularly during the years before D-Day when the Soviets believed they were fighting the war almost on their own, was a difficult business and required, as Roosevelt would have put it, 'careful handling'.
As a result, behind closed doors the Western leaders felt it necessary to make hard political compromises. One of them was to promote propaganda that painted a rosy picture of the Soviet leader.
Another was deliberately to suppress material that told the truth about both Stalin and the nature of the Soviet regime.
In the process the Western leaders might easily, for the sake of convenience, have felt they had to 'distort the normal and healthy operation' of their 'intellectual and moral judgements' as one senior British diplomat was memorably to put it during the war. [PRO FO 371/34577, O'Malley's report on Katyn, 24 May 1943.]
Facing the consequences
But I didn't want to only examine the mentality and beliefs of the elite. I felt from the first that it was also important to show in human terms the impact of the decisions taken by Stalin and the Western Allies behind closed doors.
And so in the course of my research I travelled across the former Soviet Union and Soviet-dominated eastern Europe and asked people who had lived through this testing time to tell their stories.
Uncovering this history was a strange and sometimes emotional experience. And - at least to me - it all seemed surprisingly fresh and relevant.
I felt this most strongly standing in the leafy square by the opera house in Lviv. This elegant city had started the 20th century in the Austro-Hungarian empire, become part of Poland after World War One, then part of the Soviet Union between 1939 and 1941, then part of the Nazi Empire until 1944, then part of the Soviet Union again, until finally in 1991 it became part of an independent Ukraine.
At various times in the last hundred years the city has been called Lemberg, Lvov, Lwów and Lviv.
There was not one group of citizens I met there who had not at one time or another suffered because of who they were. Catholic or Jew, Ukrainian, Russian or Pole, they had all faced persecution in the end.
It was the Nazis, of course, who operated the most infamous and murderous policy of persecution against the Jews of the city, but we are apt to forget that such was the change and turmoil in this part of central Europe that ultimately few non-Jews escaped suffering of one kind or another either.
I was fortunate to have a chance to meet these witnesses to history - all the more so since in the near future there will be no one left alive who personally experienced the war.
And after having spent so much time with these veterans from the former Soviet Union and Eastern Bloc I am left with an overwhelming sense of the importance of recovering their history as part of our own.
Our nations were all in the war together. And we owe it to them, and to ourselves, to face up to the consequences of that truth.