The war of Northern Agression was not about slavery, it was about Federalism. The south had already agreed to stop slavery, however...
The winner gets to teach in schools.
The South was right. A union that is not freely chosen by its State members is illegitimate. We have forgotten our founding principles.
When did they do that?
Have you ever bothered to read South Carolina's "Declaration of the Immediate Causes Which Induce and Justify the Secession of South Carolina from the Federal Union"?
It doesn't beat around the bush about why the slave States were leaving the Union.
It requires no infidelity to the notion of a Constitutionally limited Republic to acknowledge the slave states left, by their own admission, because of the hostility within the Union to slavery.
The other State legislatures declarations are also available, if you're curious what they have to say on the subject as well.