If Yalta did not happen - the Soviets would still have occupied what they occupied but there would have been no legitimacy to their holdings. I don't think it would have changed a thing in the long run but it would not have soiled "our" hands.
I repeat again because I bet most Americans don't remember or know this fact that except for Poland all the Eastern European nations that fell under Soviet occupation were allies with the Nazis.
No American was going to die to save former Nazi allied nations from the (what I am sure most Americans considered the rightful) wrath of the Soviets.
Well, and in addition, most Americans I would say, perceived the Soviets as present allies, not future adversaries. I believe Churchill cringed at what was coming but FDR posed as the supreme strategist who had "Uncle Joe" well in hand.