I don't think Russia is going to invade continental Ukraine. But Putin wants to give Obama an off ramp and a way to save face over Crimea. Putin knows that he can't just take Crimea and not throw Obama a bone. So he is going to mass all these troops on the border and appear like he is planning to invade and then he'll draw them down and Obama can go around saying, see, our response worked, it deterred Russia from invading Ukraine. And Putin can look like a reasonable guy because he didn't do something crazy that he wasn't going to do anyways. A win win for everyone involved.
I agree, that's most likely. I don't think he's crazy enough to invade Ukraine proper.
In hindsight, Putin played this well, at least in the short term. He knows the West is not stupid enough to go to war over Crimea. He knows that Europe is too dependent on Russian gas and other trade to impose real sanctions (even though those sanctions would hurt Russia more than Europe, he guessed correctly that Europe was not willing to endure that economic pain just to get back over something like Crimea). He knows that it's all a bluff, and he called it.
And before someone says, "well, it's because the West is weak", no, it's because this was a game that Russia couldn't lose. What could a theoretically "strong" West do differently? The options would still the same: a costly war with a nuclear power or a sanctions regime that would be too painful to justify for something as minor as Crimea.
The pressing concern is with the Ukraine itself and Russia massing troops on its border.
I think the Obama admin should strongly consider supplying arms to the Ukraine. I don't understand why we can do so to shady militia groups in Arab countries but not the Ukraine. I don't think there is any possibility such arms could ever find their way to radical Islamic jihadists or that the Ukraine will use them against a neighbor (other than Russia if it attacks).
The difference is that the Mujahadeen were willing to fight and die for their cause. The Ukrainians? Um. You don't know how many of them will will turn around and join the Russians if invaded. And of the remaining who are loyal to Ukraine, you don't know how many will stand and fight instead of flee. Weapons aren't everything--that will to fight is almost (if not more) important. Give Ukraine weapons, and there's a good chance they'll just end up in Russian hands.
Stationing some token NATO troops that Russia must fire upon if they wish to invade would be a far more effective deterrent, methinks.