A friend and I are in a friendly debate with each other about various things about WII, one of which was about the atomic bombings. We were debating if the bombings were justified. He says they were because an invasion would have cost many more lives than the bombings did. I said the bombings were unnecessary and the war was already won, and that the U.S. dropped the bombs for more post war political reasons, such as the Japanese surrendering to the USSR instead of the US and fears of a soviet invasion of Western Europe that the bombs would intimidate the soviets from considering, given that the Soviets had the most powerful land army on Europe that the West would have had a lot of difficulty stopping if the soviets decided to advance into Western Europe.
We don't really want answers on any of those previous questions, as one facet of my argument particularly caught our interest. The question is if there were any serious fears in US/UK leadership about soviet invasion in the first place and if so how serious were they. When I brought it up my friend effectively said that sounded like a load of bull, and that there were no fears or any hostilities in relations between the West and USSR in the end and immediate aftermath of the war.
This question was asked earlier here however the only u deleted comments don't really answer my question. http://www.reddit.com/r/AskHistorians/comments/2rstw1/was_the_soviet_union_ever_seriously_planning_to/
It is hard to generalize, but in general I think one can say that the US policymakers did not think that the USSR was planning to wage a truly aggressive military expansion in Europe anytime soon, but that this was contingent in part on US forces being present there as a deterrent. In other words, if you took the US out of Europe completely, then all bets would be off. If the US was there, then what you would get is a slower, more subtle form of expansion, like manipulating elections and things of that nature. (They recognized even before WWII was over that countries which the USSR was occupying were becoming essentially vassal states.) This was the essence of Kennan's "long telegram," and became the basis of the policy of "containment."
This does not mean that they didn't think that war might not flare up — they did. They already knew from their experiences with the Soviets in Berlin that close proximity with the untrusting could lead to misunderstandings that could, in principle, spiral out of control. They did not trust Stalin in the least. They also knew that the Soviet Red Army was numerically superior to their own forces and that the European countries were still in a hole because of the war. So they saw it as a dangerous situation.
The US was not in any position to use nuclear weapons against the USSR with any rapidity or speed until the late 1940s. In 1948, during the Berlin blockade, Truman authorized atomic-bomb-capable B-29s to be sent to the UK, with the idea that he could have actual bombs sent later if necessary. In 1950, he authorized the non-nuclear components of several bombs to be sent to the UK as well, but the pits remained in the US. It was not until the Eisenhower administration that fully-capable nukes were stationed in Europe (Truman did authorize the DOD to have some full nuclear assemblies in the Pacific in 1951, however). (More here.)
As for the Soviets themselves — they found World War II unbelievably depleting in terms of resources, manpower, and citizens. They were not eager to start another war in the slightest. They also, however, had learned a harsh lesson about the importance of having buffer zones around them, and wanted to secure the aforementioned satellites as reliable Soviet allies. They did not trust the United States, who they saw as inherently unfriendly to them for ideological reasons, and Truman as particularly anti-Soviet. Stalin did worry that while the US had a monopoly on the atomic bomb, the USSR was potentially more vulnerable than it might otherwise be, but he also recognized that the US was in no mood to start a new war either. Hence a willingness to try some very aggressive ploys (like the Berlin blockade), to see what the response would be.
But anyway — as to whether there were fears and hostilities, there were plenty. The "honeymoon" of being joint victors in World War II was already over, at the highest levels, by the time of the Potsdam Conference, but by 1947 or so it was clear that the USSR and USA were not going to be great pals. This is not to overstate the inevitability of this (in late 1945 through 1946, there were still many who hoped that some kind of partnership could be established), but the US top policymakers had already made up their mind on the trustworthiness of Stalin, and he had made him his mind on how much he wanted to trust them.