The only way Stalin's ambitions could have been contained is if the Allies had refused to send aid to him after the Germans invaded the Soviet Union in 1941.But that would have resulted in a German conquest and would have made the task of defeating Hitler enormously harder if not downright impossible.By the time of Yalta in February 1945,Stalin's forces were in control of all the Eastern European capitals and were poised to take Berlin (Roosevelt agreed to let him get there first).At that stage,the only way to free Eastern Europe from his grip was to go to war with him.A few months later,the Americans would no doubt have had the option of threatening him with the Atom Bomb but would have had to be prepared to make good on that threat and with the war with Japan still unresolved and public opinion at home looking for an end to hostilities,would dropping an A-bomb on a major ally against Hitler while preparing to do the same to the Japanese (against whom the Russians were also seen as a potential ally) REALLY have been a realistic option?

: Had Truman been at earlier conferences, rather
: than the sick FDR, he may have been more
: forceful with Stalin ... in the end of course,
: several decades later, the soviet system
: collapsed -- thankfully ... but it could have
: been prevented.

Truman may have wanted to be more forceful with Stalin,but he would have been faced with exactly the same dilemma facing Roosevelt for exactly the same reasons I have given above.It was not a question of willingly handing over Eastern Europe to Stalin but being forced to swallow the de-facto reality of Russia's occupation of the region.It was the bitter price for accepting Russia as an equal ally and partner in the struggle against Hitler and Japan.Of course it was unfortunate that it was the people of Eastern Europe who had to pay that price!