Our son actually learned that in school. It was also about states rights vs a big federal government. They added slavery to get more people interested and fighting.
States’ rights yes but specifically the right of new states entering the union to permit slavery. The North sought to limit or even prevent the expansion of slavery into the territories, which would lead to congress being dominated by the South. The South hoped to gain leverage and forge policies that favored their largely agrarian economy. The issue of slavery was not an add-on. It was key, although not so much as a moral issue as an economic one. The abolitionist movement of course fueled the sentiments for emancipation on moral grounds, but they didn’t cause the war.
State’s rights were respected insofar as the states were allowed to secede (even military bases in those states were abandoned). Only when those states (as their own union) attacked what was unquestionably USA 🇺🇸 military territory (which literally made the land out of ocean) did a fierce defense commence to retain that land, completed by conquest of the attacking country.
(Yes, violence would have happened anyway under some other pretext - but those cases didn’t happen, this one did,)