To argue that slavery, which was recognized from the time of the Articles of Confederation and the later formulation of the Constitution as something that would eventually split the country wide open, was unimportant to secession and civil war is either obtuse or ignorant.
It was certainly important to my great grandfather who lived in a slave state that did not secede (Kentucky) and fought in a Union regiment against neighbors and kinfolk. We've been voting Republican for around 140 plus years as a result. So please don't try to lecture me about the Civil War. I've got history handed down in my family from people who fought it.