I like it to call it the War Between the States, and not the Civil War. If you look up Civil War in a dictionary, then you will find out that it’s a struggle for control over a government. The South did not want anything to do with that government, and separated to form their own government.
I always wondered how Constitutional the War Between the States was? I did not think their is a clause that says you cannot leave unless 2/3rds of the states agree. Never heard that before.
I think most fair minded individuals believe that it was legal to succeed. This topic is covered at length in "The Real Lincoln" as well as books by the Kennedy brothers like "The South Was Right". I believe that even some strong supporters of Lincoln agree that his indivisible union was not the original design.
But that's pretty far off topic.
Or, maybe not, sadly.