Out here in CA you find a lot who will argue that it was illegal for the south to leave the Union. And then, in the next breath say that CA should leave it. Go figure.
I don't think it would so much be a matter of finding people who would give their lives, fortunes, and sacred honor if it became necessary. It would be a matter of, would the members of the US military be able to pull the trigger on their friends and family? (I can see that there would be some minor clashes, and some exchange of small arms, but I can't see an armored unit firing on a modern American city, nor can I see air support being called in on, oh, Fresno, Redding, or San Francisco) And, would there be the political will to force states to remain in the Union? Look at the hew and cry over just a relative handfull of deaths in Iraq. Can you imagine the outrage if another Sharpsburg were to happen?