Maybe USSR and Warsaw Pact dissolved because there was no prospect to wage war in Europe unless someone wanted nuclear war since Europe was pretty much divided into NATO on one side and Warsaw Pact on the other.
What I want to say is that maybe Russian - Soviet generals got frustrated that they can not wage war, send tanks, make huge military movements and so on, so they decided to allow Warsaw Pact and USSR to be destroyed so they can again experience they adventures of their fathers and grandfathers in World War 2 in the future.
What do you think people ?
Perhaps the reason why Putin may not try to take over whole of Ukraine or to crush current regime completely is because in that case Russia again borders NATO and no future war is possible which could make Russian army extremely sad and depressed !
This could be worth exploring.