Given the unstable situation in Ukraine, I can well understand Putin's desire to secure Russia's bases and interests in the Crimea. It seems unlikely that Ukraine can continue to exist in its current form given the strength of feeling on opposing sides. It might not be such a bad outcome for the country to spilt with a portion becoming a region of Russia and the remainder becoming a pro-western state. I can't imagine that the West would benefit in any way from intervening to prevent an area with a Russian majority being freed from Russian "protection". Not say that the West will not use the situation to its "advantage" to isolate Russia further.
Don't get me wrong, Putin is a nasty piece of work and I hope he gets him comeuppance but in this occasion he's not doing anything any other leader wouldn't do in the same situation...and I think he'll have more success than, say, a couple of western powers trying to secure Suez!!