Putin's move seems to be strategically sound, as much as I disagree with where things are going.
1. His supporters & electorate back home probably wanted him to do something.
2. He wants to be seen as a strong & confident leader, reminiscent of Soviet days.
3. Russia does have a military base there after all.. Things need to be looked after.
4. This is seen as a national security issue by Putin and a lot of other Russians - something that is happening in their back yard and their ancestral stomping grounds. A place where a majority Russian population exists.
5. Who's going to stop Russia.. in Crimea? The military engagement appears to be bordering on the minimal. It's a bit shady, but if he gets called out on it by U.S. warships & stealth bombers, which probably won't even happen, he can just back down before they even arrive.
6. It's a minimal risk move, even though it does sour his relationship with the west a bit.
7. The Russian military has been modernized and beefed up a bit recently, let's give them something to do.
8. It puts Russia in a good starting negotiating position if it controls the things with boots on the ground.
In terms of a chess move, it almost seems to be the obvious one to make, from Putin's point of view anyway.
I just wonder what's going to happen once the dust settles a bit. Ukraine is obviously going to move closer towards EU membership. Crimea might become independent? Ideally it would remain a part of Ukraine with maybe even more autonomy, but what if that's not what the people who live there want?