Yalta was not a success, it was a series of choices between evils. FDR did not know what the bomb woud do, nobody did for sure. And keep in mind that we had to drop two of them to seal the deal.
It is real easy to second guess people after everything has worked itself out. From the start of the war, US policy people figured we needed the Russians to do most of the heavy lifting to avoid losing millions ourselves. So we slow played Europe while the commies bled the Germans on the eastern front and didn't invade France until halfway through 1944. The wait was worth it; we lost a lot less men and the Nazis were a lot more exhausted than in '43.
We slow played Japan as well. We passed on Iwo to go after the Philipines and then set up for the invasion. We did not know how effective the bomb would be or whether it would even work when used in war situations, as opposed to out in the desert in New Mexico.
FDR conceded nothing at Yalta that Stalin wasn't going to be taking anyway. The only way to get him out was a war and the greatest generation was already tired of the ones we were in and would not have supported another. And we didn't have a whole lot of atomic bombs. If we went to war with STalin over Poland and Hungary, et al, the reds would have been all over Europe before we had enough atomic bombs to fight back.
Yalta was a series of bad but necessary decisions.
Wars seldom turn out the way you expect, which is why they are best avoided if possible.
As for the socialism bit, Obama is not doing anything that every democratic president since Truman has not given a shot. So if he is a socialist, then so were Truman, Kennedy, Johnson, Carter, Clinton. And the American people have ratified with their votes, repeatedly, the steps first taken by FDR in terms of a social safety net.