If you are suggesting that the allies could have repelled the Soviet occupation of Eastern Europe without starting an all-out war with the Soviet Union, I think the idea is absurd. The Motivation for Soviet expansion into eastern Europe was to have a buffer between Russian and perceived hostile powers to the west and any interference would be interpreted by the Soviets as a provocation of war.
Well, apparently we agree: it was an act of cowardice on the part of Churchill and Roosevelt. I'd say there's nothing more to discuss but, of course, there's always more to discuss...

Britain entered the war when its its immediate allies were attacked by its historical enemy and it was obvious they were next. The US also entered the war in Europe without any attack by Germany.
I didn't mean to imply that the US was attacked by Germany. I meant the attack by Japan. The US declared war on Germany on December 11th 1941.
And it's something in this point that I mean to discuss further. You say that Britain entered the war when its immediate allies were attacked. I take it you count Poland among them. If Britain entered the war on account of its allies being attacked, why leave them out in the cold 5 years later? Obviously, allegiances had nothing to do with it. Neither Britain nor the US gave a damn about their allies. If you read a bit about Polish military contributions to World War II (here, I'll make it easy: follow
this link), you will find out that, for instance, Polish troops took considerable part in what "Britain" achieved in, say, the Battle of Britain (where the all-Polish Squadron 303 had the highest number of kills of any Allied squadron). Now, isn't that a bit disingenuous? First, "use" your ally in battle, then don't give a fiddler's fart what happens to him...?
Another point, and definitely not a minor one, is that the costs of that decision were certainly far greater than Roosevelt and Churchill were pretending (before themselves?) they would be. We are still living in the aftermath of WWII - we wouldn't be so excited discussing it otherwise! And we could long have ceased to be in that situation. Economically and politically that decision had dreadful consequences. I can't imagine them ever being that bad had the "Allies" not backed out in Tehran and Yalta.
I find it a bit silly to deny the fact that what Churchill and Roosevelt did during those conferences was wrong, and contrary to popular opinion - NOT in their best interest.
Besides, leaving what happened after the war aside, what about the Warsaw Uprising? Do you seriously think there was a threat of war with Stalin if the Allies had assisted the insurgents? I don't think so. The fact is they were acting like sissies in negotiations with him because, apparently, he was an intimidating monster. But what sort of politics are those? Making decisions on account of fear?
There's a pretty good wikipedia article on this subject too:
Western betrayal.
Now the interesting turn this discussion could take is if someone accused Poland of ruining it. No, not in the 1940s. Not even in the 1930s but during the signing of the
Peace of Riga. Had Poles acted the way they ought to have acted, like true victors - Europe would have been completely different afterwards. As it happened, Russia came out of that war (which it lost, for crying out loud!) practically unharmed, its strength remained basically unchanged...

Not being a professional historian I can't really back this claim properly but I suspect it's not completely irrational.