>Russia did invade Poland...
Yep, the and at the end of WWII in Europe they forced Poland to 'move' to to the West, but seizing eastern parts of then-Poland for their own, and taking eastern Germany and making it Poland. If you're wondering why you can't find Prussia on a modern map, that's why.
Also, I it's not at all clear that Truman had the political capital to invade Russia. Europe was in ruins. Quite a few GIs had died or were injured badly. People wanted to be *done* with the war. If you mean post-Hiroshima, I'm just as glad we didn't have a wider nuclear war - and we just didn't have the troops to follow up with occupation. The USSR is huuuge. There's a Rand report on how many troops per population (on a mobile, so can't find it) but worth a look - on and that' s for a *cooperative* population.