To prevent Ukraine from joining NATO. Prospective NATO members can’t have ongoing border disputes.

And hey, if you can’t join NATO, might as well become a vassal to Moscow. You know — that’s a beautiful country you have there and it’d be a shame if something happened to it.

Also, shores up their claim to Crimea. And steals a lot of ocean-front property from Ukraine, along with all the Black Sea oil and gas rights that come with it. And Russia loves warm water ports, especially on the Black Sea.