I am not going into it here, but there was great alchemy at play in Yalta, between 4FEB-11FEB, 1945.
The US and the world, will never recover from Yalta -
What really happened is mind numbing
https://www.conservapedia.com/index.php?title=Yalta_conference&redirect=no
*Alger Hiss was Soviet Agent
https://www.conservapedia.com/Alger_Hiss
I have read a great deal on Roosevelt and Yalta. Yes Hiss was an agent. Roosevelt was a dying man, but his mind was still clear enough. In addition he had Admiral Leahy with him , who was in fact, the unofficial acting President of the U.S., he had been doing his job and Roosevelts since the summer 1944. Few people knew this.
I suggest you read Nigel Hamilton’s three part books on Roosevelt as CIC. After a I read them I was more impressed by Roosevelt than I had been and Thank God he was running the war rather than anyone else.
We can be armchair QB’s all we want but I doubt there was little that could have been done to dissuade Stalin from what he wanted. Roosevelt was convinced he could charm him into doing what he wanted and that was a failure.
Admiral William Standley, who had been the Ambassador to Russia during the toughest part of the war tried to tell Roosevelt how handle Stalin and Molotov, but he would not listen. If Standley had been in charge there would have been a very different outcome, but Roosevelt did not even take Standley with him to Yalta.