I don't believe you have the full context and reading Wikipedia on the subject will yield a lot of surprises.
USSR got the following bits and pieces after WWII:
* Kaliningrad, added to RSFSR
* West Ukraine, added to Ukrainian SSR
* Some ex-Polish lands added to Belorussian SSR (Poland get even more of better, ex-German lands though)
* Baltic states
* Tuva.
Of these, perhaps three Baltic republics were not happy about Afghanistan (and everything else), but they were quite small.
The rest of stuff, such as UkrSSR, BelSSR, Caucasus and Central Asian SSRs, were carved out of RSFSR (Russia) in the 1920's by the USSR, after being part of Russian Empire for a century or more. Most of them did not exist as a separate state previously, especially in the assigned borders. Some briefly existed, often as anarchist or German puppet states, between 1917 and 1921.
USSR did indeed gain control of various Eastern European countries such as Poland or Romania, but they were not part of USSR so they did not have to fight in Afghanistan.
The rest of stuff about nations and statehood opportunity is frankly bullshit. These ASSRs were also carved by USSR out of Russia, and almost universally have 40% to 80% of Russian population within them - the only separatist opportunity they have is to be a particularly poor reissue of Bosnia. People are not stupid and they understand that.