Putin is betting Europe won't unify.
Besides NATO, which Ukraine is not a part of, the European Union is the only force capable of checking Russian aggression. The chances of that are pretty slim, and Putin knows it. The Russian military may not rival its American counterpart, but it's still a potent force to be reckoned with.
I still firmly believe that if Russia sticks to the Crimea, nothing much will happen. Even if the EU did respond, they'd likely be fired on by the citizens they were trying to protect. Putin also knows that if Europe really steps in militarily, all the natural gas supplies that power a good portion of the EU electrical grid might suddenly be turned off. I mean if he really wanted to, Putin could cripple the European energy grid nearly overnight. That might hurt Russia temporarily economically, but no doubt they can find new buyers in the East, I'm sure.
Like I said before, I'm no fan of Putin, but I think he thought this one out pretty well. It's really a win-win for him. Europe can't do much without crippling their energy grid. The United States is certainly capable of doing so (we've been dismembering Russian military equipment with ease for years), but it'd be both politically and economically disastrous for us since not only are we a bit tired of war, but Europe is also one of our largest trading partners. Russia also has an immense nuclear arsenal. That pretty much lets them do whatever they want. I wonder if that's what Putin and Obama talked about?
After this fizzles out, I [loosely] predict this will happen:
-The EU will reconsider energy independence.
-Likely military re-hardening of NATO border states (which US companies will certainly benefit from).
-This one is more far fetched, but it might shake the confidence in NATO for their inability to act (even though it's not a NATO matter, it is a European one).
Let's hope no one dies in this. I hate war.