Everybody in the west is expecting a hollywood invasion, a declare of war with tank brigades blowing through the board and conquering southeastern Ukraine in a week. Putin is much smarter than that. What we are seeing now is a slow motion invasion. The question is, what is Putin's end game. Does he just want to establish firm control over the Donbass region before coming to the negotiating table, or will we eventually see Russian paratroopers in Kiev?
I believe, at this point it only depends on the strength of resistance. So far, at least, Ukrainians have been able to inflict what appears substantial casualties on the Russian troops - sufficiently serious that it has become pretty much necessary to semi-acknowledge that Russian soldiers are there and that they are dying (at this point it is no longer possible to deny - too many funerals going on near major Russian military bases). Still, this appears to be not at all enough to stop Putin - the assault accelerates. The longer-term goal, of course, is Kiev and beyond - the question is how big the damage has to be for it to be redefined.
Of course, given what we know of Ukrainian mood and history, once the army gets demolished (which may well happen), a major partisan war will start. And that will, probably, go on for decades.
I have to think trying to take all of Ukraine now would blow back on Russia in a major way. In that case, they would be responsible for the financial mess Ukraine is in, they would bear the cost of the partisan war, and they would have expended a massive amount of men, money, and material to defeat a Ukrainian army which looks to be half-way competent.
The smarter route, IMO, would be to take and hold Donbass and demand its independence at the negotiating table, along with economic concessions. Maybe Ukraine takes the deal, maybe they leave the Donbass as a frozen conflict zone. Wait a few years and the IMF will start applying the screws to Ukraine; they'll want their money back no matter what. The Ukrainian people will snarl, maybe a government will fall, and Putin will step in with another offer. Maybe they take it this time, maybe they reject it and another Separatist republic pops up, situated between Crimea and Donbass.
