Crimea and Eastern Ukraine are full of ethnic Russians. What happens to the rest of Ukraine is the tricky part. Russia can't afford to lose her to the EU and Ukraine can't afford to lose gas and oil from Russia.
Catch-22.

Poor Ukraine, always caught in the middle of empires. Rarely a moment to be just Ukrainians. It's the breadbasket of Europe, producing much more food than they can consume. Sometimes assets are your biggest liability.