If an internationally (as in: not Russian-run) supervised referendum showed that Crimea et al want to break away from Ukraine, then as far as I'm concerned they have a right to do so. And if a later referendum and agreement between this separate region of ex-Ukraine and Russia involve Crimea joining Russia, then power to Crimea.
The land, ultimately, belong to the people who live on it. Not to some geopolitical contraption that has no right in and of itself save that which it derives from its people.
In the meantime, until such a time as a referendum can be organised, any policing to avoid conflicts and protect any stray threatened minority should go to an actual third party, preferably a country that has little national stake in the conflict (Read: not you, Russia).