Really? Then again, why was it not done? What stopped the US from launching attacks on the USSR?
There were, I guess, many reasons. One of the main one was that US/UK were not paranoic about the USSR destruction. Surprised?
Yes, I do not clame that the US wanted to destroy USSR. Not the same way Hitler wanted. US is pragmatic, not paranoic.
And how was the US military supposed to “occupy” an ‘A-bombed-into-submission’ Soviet Union with only about 400,000-500,000 soldiers in its (all-volunteer) army.
Please see my aswer to the previous paragraph. But, if it went so far to the start of war, USA not necessary needed to occupy the whole country. Strategical points would be enough. But depending on the goal it might be unnecessary altogether. One may just bomb the his way through (like during the Battle of Britain or strategical bombing) untill the enemy does what ever you want them to do. Because nukes are so cheap, as you say and therefore tempting. 
The same happened in 1807 during the second battle of Copenhagen (I currently live in this city) when British bombarded and burned the city (http://en.wikipedia.org/wiki/Battle_of_Copenhagen_(1807)). Do you know why they did it? Do you think that Denmark and UK were at war? Ha-ha-ha. Nope!
The Brits were afraid that Napoleon capture Denmark and get hold of the Danish fleet. So the Brits “suggested” that Danes should pass they ships to Britain. And when danes rejected they bombarded the city with the Congreve Rockets which caused fires. 2000 dead, 30% houses burned. The fleet was taken after Danes capitulated. They actually could not defend Copenhagen because the whole army was on the other side of the country defending against… french. And do not forget that the was also the first battle of Cepenhagen in 1801 with similar British role. Here you are, a nice little preventive war.
But this is just history lesson… or wait… we are in a histocal forum! That is great! 
I mean, could it be that the US feared an eventual Soviet thrust into Western Europe where Red Army conventional forces were vastly superior?
Right. OF course there was fear in the west. Why? That is debatable. There is no straight black-white answer there, I think.
By the way, what period are you reffering to?
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