This is one argument that will probably go on forever. But for what it’s worth, Confederate general John S. Mosby said it was all about slavery.
“I’ve always understood that we went to war on account of the thing we quarreled with the north about. I’ve never heard of any other cause of quarrel than slavery.”
https://en.wikiquote.org/wiki/John_S._Mosby#Letter_(1894)
Side note: Mosby was probably right, at least for the wealthy Southern political leadership. But for the dirt-poor Reb serving as an infantry private, I don’t think so. He was fighting against the North for the same reason his great-grandfather fought against England.
To my knowledge, Mosby never rose above the rank of Colonel.
Fine fighter. We needed more like him.
Why did Lincoln send an invasion force into the South?