Well, in fairness, all he really needs to do (and appears intent on doing once his Blitzkrieg attempt failed utterly) is occupy all the coastal territory and land-lock the rest of Ukraine, forcing Ukraine to pay through the nose to ship their export goods--and receive imports, obviously. It's a potential strangle hold financially.
But that's now. If Ukraine accedes in any way to Russia on this occasion, it will only be a matter of time before the rest of the country goes the way of Crimea and the Donbas. To put it another way, do Russians stop at half a glass of vodka?
Edit: I didn't realize until after I replied how far behind I am ITT.