in the antebellum United Strates, most people, especially in the South, considered themselves to be first and foremost citizens of their states and only secondarily, Americans.
The non-slave-owning southerners fought because their primary allegiance was to their home states rather than to the Union.
Just finished reading, "Born Fighting," by James Webb, it was very enlightening on the topic of why the South fought. It is well worth the read for those wishing to understand this issue.
You are correct on your answer. The book's main emphasis is on the Scots/Irish and how they are the invisible ethnic group that made the South and Midwest what it is.
Anyway...a good read.
Except North Carolina. Many in this state believed in the Union and even provided soldiers to the Union when they had advanced far enough into Dixie.
I think that the South had their own propaganda machine working over time to recruit soldiers. Many had a romantic notions of chivalry. Almost all of them didn't understand the killing power of industrialized warfare (which happened here before the trench warfare in the European theatre of WW 1).
That's like saying that our soldiers in WWII fought because they wanted to protect their buddies. It's a true statement, but it doesn't answer the question as to cause.
Think counter-factually: If all of the states had been slave states, or none of the states were slave states, would the Civil War still have happened? Obviously not.
The fact that US leadership said it was about preserving the Union doesn't change the fact that the southern states left because they knew slave-owning was being slowly squeezed out. They didn't want to stay on that path. And that's why they left, not because of tarriffs or cultural issues or whatever. The war could have easily been avoided if the Northern states would have agreed to a constitutional ammendment protecting slavery. But the North was not about to even consider any such thing. Hence the war. It was between the South, which insisted on slavery in perpetuity, and the North, which insisted that slavery be phased out.