I found this sentence in a Reuters article by Andrew Osborne about Putin, NATO, and the two former Soviet republics of Georgia and Ukraine significant:
Under NATO rules, countries with territorial conflicts cannot join NATO.
Let’s consider a hypothetical example. Let’s say you were a country that very much did not want countries on its borders to join NATO. What would you do?
That is precisely the reason Russia is supporting what seems to be an enduring conflict in the Donbass.