of the Donbas separatists and bolster its national security in the Black Sea and along its
western borders. It believed it could do this quickly and with minimal carnage/ damage
......it was wrong. "
Yes I agree the Russians indeed made a foolish assumption: one made due to a belief that they were dealing with rational actors in the West who would see sense and consequently the whole thing as you say would be wrapped up quickly and without too much damage.
That "mistake" though has been a valuable lesson. What they found is that what they were and are dealing with in Ukraine and the greater West is far from rationality: it met instead contempt, narcissism, delusion and a psychopathology that cares nothing for human life.
Western Perfidy has been confirmed and the loss of trust has been enormous. Consequently the Western desire that Russia would ever be a mere comprador state as under Yeltsin has become even further removed from any potential reality.