People should understand that from the Russian perspective Putin did not have much choice. Basically Russia lost the control over Ukraine, due to a inconsistent policy towards it. In a way Russian foreign policy is quite inefficient in maintaining economical or political control over the countries which are of interest to it, the way USA does it.
Regarding Ukraine it was clear with strange gas policy (basically there were some serious stealing through some middle-men companies, see Gunvor etc., instead of using gas prices as the way to control Ukrainian political landscape).
So when Russia basically fucked up with Maidan events Putin just couldn't let is slide and had to move, and move fast. Which proves him to be a great tactitian, but a poor strategist.
But in a way that was the smartest move he made during all his years in power. Basically he solved all inner political problems with it, and bolstered his international image of the bad mofo.