What orange said.
Similar situations have happened before, the most notable and recent example being Georgia, which erupted in a brief war over ethnic Russians in South Ossetia. Essentially the story goes South Ossetia was a largely unrecognised political entity within Georgia, ethnically and culturally Russian. Georgia tries to put an end to it as a political entity, Russia invades Georgia to protect the Russian population.
It does bear mentioning of course, that in that example Georgia was less than discriminate when it came to collateral damage, and was the primary aggressor by any reasonable metric.
Edit: Regarding the Crimea, Russia is very, very keen to hold onto it as a strategic base whatever happens, it's both strategically and culturally significant to them. There was a minor diplomatic crisis when Ukraine last tried to raise the rent on the Sevastopol naval base.
Which is why when Obama says Ukraine isn't a piece on a chessboard, one can't help but be sceptical.