Don't you think Putin is just kicking back and enjoying the shitshow? I think he's trying to get the old band back together — he came of age, politically, when the USSR was disbanded. That dissolution didn't happen because Russia wanted it to; it was basically forced upon them. And Ukraine was the last domino to fall, so I'm not surprised it's his first target.
I'm no expert in politics or history, but my perception from here is that he has almost no downside to the position he's taken. He can unclench his fist without losing face, or he can keep on clenching without anyone able to do much against him. EU sanctions mean nothing. We're not gonna nuke him. He's a psychopath, IMHO.
ETA: Mr Disconnected rightfully pointed out that Ukraine is not in fact Putin's first target; he used the "protection" excuse in Georgia already.