I'm not disagreeing with you about Stalin's ambitions. What I'm pointing out - hypothetically - is that had Roosevelt not died and Churchill been replaced, the conversations about the final dispositions of the Eastern European nations might have gone very differently. No guarantees.

Churchill's comments about the entire matter are in his six-volume work covering the entirety of WW II. The annexation of Poland happened when Russia delayed its entrance into Warsaw. The city's defenders, seeing the advancing Russians, rose up, expecting the Russians to continue to advance and reach the city in the next day. Instead, the Russians inexplicably halted - despite pleas from Churchill - allowing the remaining German forces to quell the rebellion. Half the city died and the pro-democratic leaders who weren't killed in the fighting were actually executed by the Russians. That left only pro-Communists in the capital of Poland. Churchill - and a following Roosevelt - insisted on democratic elections, but after the massacre, they knew that Poland would go Communist. Russia installed a vassal state. Similarly, Stalin worked to install Tito in Czechoslovakia at the expense of other pro-Western groups. Russia was only forestalled in Greece because Churchill sent troops in to control Athens and stop the rioting of the pro-Communist forces. The Russian rampage through Eastern Germany was brutal: nearly anyone found by the Red Army was executed and their homes pillaged by the Russian invaders.

All this aside, however, Churchill had been pressing for a full disposition of the nations of Europe to return back to their original borders once the Allies landed in Italy. Yes, at Yalta Roosevelt failed to support Churchill in pressing for a return of Polish borders. (We can always criticize in hindsight.) Churchill was prescient in the matter, Roosevelt was more uncertain and so delayed a final arbitration until later on. The problem was that Roosevelt died before that final arbitration could be held and the new British Prime Minister was Labour Party - the same pacifist who had engineered the failed treaties and agreements between WW I and WW II. The new British leadership was only concerned about pulling British forces back to England and worrying about Indian independence (a leftover issue from before the War) than trying to maintain stable democracies in the face of communism, and so Stalin was never confronted.

Truman was advised by none other than General Patton that they shouldn't allow the Russians to maintain their control over everything East of Berlin. Patton urged a direct military confrontation with the Red Army if they wouldn't go back to their homeland. Eisenhower was more diplomatic, but he, too, had grave concerns about allowing the Russians to stay. Truman caved to pressures back home and concentrated on the necessary victory over the Japanese, but in so doing, he delayed until Russia had strengthened their grip and solidified their presence - executing anyone native who dared speak against them.

You are welcome to criticize Roosevelt for his missteps (I'm certainly not a policy supporter), but to me Truman had a chance and let it slip through his fingers not once, but twice. One resulted in a brutal communist regime and the executions of millions over the next 40+ years. The other resulted in another brutal communist regime, the execution of tens of millions, and a war that cost millions more and remains unresolved to this day. I speculate that the relationships forged through three years of war - especially when backed by the military might - would have been enough to sway Stalin. We will never know, however.