It seems like it should have been pretty clear that the Soviets were making puppet states out of almost all of Eastern Europe. So why did the US allow for this to happen, especially as it became more and more clear that the Soviets weren't going to be friendly to us? Did the US just not care about Eastern Europe and think the Soviets weren't going to be a threat? It seems like, as the country possessing the atom bomb, the US would be able to stand up to the Soviets and say that Poland needs free elections, or Germany needs to be reunited. So why did America allow the Soviets to gain control?
Well, in short the Americans couldn’t really stop them. Sure, the West has its naval supremacy, the bomb, and a booming economy in the USA. But the USSR has millions of battle hardened soldiers sat on central and Eastern Europe at the end of the war, so it was sort of a moot point.
So, whilst the West had the A-Bomb, it wouldn’t have been enough to win a conflict against the state that had just defeated Hitlers previously all-conquering Wehrmacht.
Furthermore, control of Central and Eastern Europe was at this point vital to Soviet national security; after all Russia had been invaded via the Polish corridor 4 times since the start of the 19th century; Stalin wasn’t about to give up control of that hard won, strategically important area. So even if the West did have the ability to remove Soviet control of C&E Europe at the end of the war, the price demanded by the USSR would have been prohibitively high.
Also, it wasn’t just the West conceding ground. The Soviets and the West game to a Gentleman’s Agreement at, I think, Yalta, where they carved up Europe into spheres of Influence. The soviets got the east and Central Europe. Austria would be released by both sides as a neutral zone. Greece & Norway would be part of the western sphere.
The soviets actually liberated the northern Norwegian territory of Finnmark from Nazi rule and returned it to Norway, so they were conceding ground too.
And the West got control of Western Germany with the incredibly valuable Ruhr & Saarland industrial regions, as well as the German North Sea coast, thus excluding the soviets from that region.
So, there was a give and take which was slightly more complex than the USA just letting the Soviets have Eastern Europe.