War isn't civil. That being said, a civil war is a war fought between two factions over the control of a single government. The altercation in this country was being fought between one side who wished to have nothing to do with the other government, indeed had formed it's own government. The other side was fighting to prevent this. There is nothing in the constitution that I am aware of that prohibited that secession. Only armed force and subsequent occupation by armed federal troops enforced the results of the conflict.