Do states have a right to independence, or does the Union have a right to preserve itself against their will?
The question is invalid unless its premises include absolute elimination of: shady, back-room deals; unaccountable news media; and the influence of secret societies.
Is that your contention regarding what happened in 1860 in the Southern states? I would think that if the people of those states did not agree with the decision to have independence, they would have made their feelings known on the subject.
It looks to me like they did make their feelings known when they signed up to fight the invaders, like the Ukrainians are doing now.