Jumping to the Civil War, the issue was a complex split between the North trying to preserve the Union and modify the Constitution as they saw it; the South trying to form another union and preserving (with modifications) the Constitution as they saw it. The Northern Union of States were forcing a zealous sense of law upon the Southern Union of States determined on a course of national Self-determination.
So the orignial question is too simplistic to associate to the Civil War with a simple answer because the primary issue of that war was went to the core of the rights of States. The question included the words "most important" which is very subjective and without a context to associate to whom. So the answer had to fall back unto using basic definitions to determine "importance" to preservation. I didn't give a valued weight to the importance, only the importance of the relativity to union and constitution.