The Civil War has been coming up with some regularity in the news, and it occurs to me that I've never offered here my view of it, which is in some respects different than the standard view. I base this off of reading my way through my library's collection of Civil War books as a child, and a few years thinking about constitutions, civil war, and the rule of law during my academic career.
In a nutshell: the Civil War was 'about' constitutional issues, where slavery is the driving force behind those issues. Rather than deflecting or diminishing slavery as a cause, it amplifies the extent to which the South acted in bad faith. That is to say: the southern states seceded because they wanted to establish independent political control over their own territory and did not want to/in actuality recognize the authority of the United States federal government to do so. The reason they wished to have this control was to maintain slavery. But, taken as independent arguments, constitutional reasons alone served as a spectacular violation of the Rule of Law: the southern states were happy to leverage strong federal control when it suited their purposes, but once it no longer appeared that they would be able to dictate national policy, they picked up their ball and left rather than engage in the work of politics; they were so convinced that any defeat would have been so unjust that the prospect of facing one was intolerable. (I am of the belief that Lincoln would have been considerably more moderate on slavery had the South not radicalized the issue; the constitutional arguments may have been pretexts to the South, but they were nearly the whole of the point for Lincoln.) That is to say, the South was wrong on slavery, conveniently and deeply hypocritical on the Rule of Law, and--most offensive to a certain type of southern sympathizer--guilty of bad form.
One should probably also note that this bad faith tendency--"I might lose, and if I do, I will impugn the legitimacy of the people who disagree with me. In fact, I will pre-emptively impugn their legitimacy in the hope this will increase the chances of my side winning."--has unfortunately become one of the default modes of handling political and social disagreement.