Putin is an idiot. He started playing games with Ukraine and never saw the long game.
I disagree. I think he sees the long game and is even perhaps winning at it. He's pushing for all he can get and doing a decent job of it. He may have lost all of the Ukraine, but East Ukraine is still up for grabs and at this point, even if he had to fold, he'd walk away with Crimea. This is probably all about EU sanctions as the US won't do anything without the EU, and as long as the EU isn't going to put up some serious sanctions, they certainly aren't going to flex their military muscles. So he'll probably push up to the serious threat of sanctions and then ease off just enough to keep them from happening.