Are you kidding me? NATO? Wtf? What is NATO going to do? Go into World War 3 because of Crimea? C'mon. Be realistic. World War 3 wouldn't be profitable for anyone, no one would make money of it, and therefore it will not happen. Russian would have lost it, but if only 10% of Russian missiles would hit USA, only cockroaches would survive.
My best guess is that they will shout a lot, 'strongly condemn' and similar bullshit, and that's gonna be it. The only thing I don't know is how ready will Ukranians be to defend Crimea. One thing is certain, if they engage in full scale war with Russkies and lose, beside Crimea they will lose eastern part of the country. And I don't see how they can win, best thing they can hope for is a stalemate. After which there would be some bullshit negotiations, where everyone will agree that they should respect right of Crimeans to self-determination. After which it would also come to the same end - Crimea would separate from Ukraine.
However, if I was Ukrainian, I would also be ready to defend my country.