As with Georgia, or Moldova, or the constantly unsettling situation in Armenia and Azerbaijan, if there is an invasion of Ukraine, I do not expect it to lead to an occupation of all of the country.
This is because west Ukraine would never accept any sort of Russian rule, - they see themselves as "western", and look to - and identify with - a different history, language, religion and political culture than the eastern part of the country, just as some of the eastern regions look to (and identify with) Russia culturally, theologically, linguistically, socially, politically and historically.
Yes, we can (and should) blame Stalin for this, given how he shifted the borders of what was then Ukraine several hundreds of miles (kms) further west in 1945, creating a fundamentally divided country, one with an enormous - and unbridgeable - cultural and political cleavage running right through the middle of it, - though this division and cultural conflict didn't really matter until the collapse of the USSR, - while, Khrushchev's decision to cede Crimea to Ukraine, (albeit while Ukraine was still within the USSR), also served to give a hostage to fortune (a hostage which was duly claimed by fortune in the form of a resurgent Russia).
However, - and this is insufficiently appreciated in the west - it is my opinion (and I have held this view for over a decade) that for cultural-socio-historical-theological reasons, - and it doesn't matter that they may not be logical, or rational, or that most people are secular, issues of identity still move and motivate them - I believe that Russia will go to the wire on Ukraine in a way that it will never do for any place else in the post Soviet space. This - Ukraine - matters viscerally to Russia in a way that nowhere else does.
Thus, this is not just about politics, or pure power (although, naturally, it also includes both of these); nor is it solely about restoring a core and key Slavic component of the old Soviet "Near Abroad" to Russian (neo-imperial) control.
Rather, above all, it is about laying permanent claim to what - and where - is viewed as the locus of Russian identity, to the cradle of Russian culture and civilisation.
To Russians, the cradle of their culture and civilisation comes from Ukraine (the old Kievan Rus), their sense of national identity (and that of the Orthodox Church) derive their legitimacy from this link with Kievan Rus, and thus, we cannot assume that rational considerations will be the only elements in play, here, if, and when, an invasion does take place.
For what it is worth, I cannot ever - though "ever" is a long time - see a single, unitary Ukraine emerging from this, though many in the 'west' will find this difficult to accept, wedded as they are to concepts of current boundaries and borders.
Rather, I would expect to see the emergence of two Ukraines, an eastern version and something that might be called west Ukraine; I would imagine that the existence of a "west" Ukraine will be contingent on (a reluctant) recognition of the Russian right to (extensive) influence - a resurrection of the old 19th century idea of respective "spheres of influence" and "great power" (not super power) politics - on and in and over - the 'eastern' part of Ukraine.
Whether it will be possible to construct or design some form of political architecture that will accommodate both entities (countries) under one (even nebulous) political umbrella (a confederation? a federal solution?) remains to be seen, but is something that I, personally, would doubt could come into being, not given the deep (and deepening) divisions that currently exist between both Ukraines and their respective backers.
