> The new United Nations organization set up at Yalta should have been able to intervene, but the voting arrangements agreed there allowed the Soviet Union, as one of the five permanent members of the Security Council possessing a veto, to prevent action.
The Soviet Union had 6.4 million men along the Eastern Front [0], composed of battle hardened veterans led by brilliant generals such as Zhukov, with many millions more back in Russia that could be called into service. In 1945 there was no military force in the world that could have dislodged the Soviet Union from Eastern Europe.
In addition, while the US was developing nukes, nobody really could be sure how effective they would be. Thus, Yalta was very pragmatic in not wanting to get into a futile struggle with Stalin over Eastern Europe as well as enlist his aid in what was anticipated to be a very bloody invasion of the Japanese homeland.
0. https://en.wikipedia.org/wiki/Eastern_Front_(World_War_II)#F...