I have high admiration for Walter, and I don't entirely disagree with his argument here.  Much of the positioning about the validity of the Civil War has to do with slavery.  The case that it was technically about the right of secession is strong, but the reality of slavery as the basis for secession is just as strong.  As in the Declaration of Independence the consent of the governed is a key part of the argument. 
I have come to the opinion that when "the governed" are all subject to the same laws and treated equally it is a reasonable standard.  But if a selected segment of the governed (the blacks, in this case) is subject to different laws and rules then their consent is also required for that government to be legitimate.