The U.S. Civil War fought by Lincoln is badly mis-named.  Rightly considered it was a war to prevent independence of the South from the united States (not a mistake in capitalization).  The South seceded.

Just like the Donbass today fights to fend off a Ukrainian army from eradication of ethnic Russians.  Just like the colonists did against King George III.

Argue the legality of secession all you want, ultimately people have the sovereign right to determine the course of their own destiny. Ultimately, they can simply just say, "No."