One of the assumptions in much Ukraine commentary is that a full scale Russian invasion of the country would be followed at some point by what would, over time, be a potentially ruinous insurgency. That seems probable. And I suspect it’s the key reason — much more than even the most extreme sanctions — why Russia has so far held back from such a move. Even apart from the bad history of Great Powers getting bogged down in occupations of hostile territory, the military reforms under President Putin, which have revived Russia’s conventional military power, have prioritized rapid strike capabilities over holding territory. Such a scenario would be a catastrophe for Ukraine but it would likely be pretty bad for Russia too.
But this article in Foreign Policy shows how easily this scenario could bleed into a confrontation between Russia and NATO, even though neither side would have any interest in it doing so.