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Lincoln fought to save the union, not to free the slaves. Is it ok to kill so many to save the union? Probably not.
Does a country have a right to exist, despite the will of its people? No.
Can it constrain the right to secede?
The Declaration of Independence was an act of secession. And isn't it self-evident that when a government "becomes destructive of those ends", the people may alter or abolish it,
or try to.