The civil war was not fought over slavery. The union chose to free the slaves because it hurt the south economically more than it hurt the north economically.
Mississippi’s is even more explicit https://avalon.law.yale.edu/19th_century/csa_missec.asp - “Our position is thoroughly identified with slavery — the greatest material interest of the world”
The civil war was fought because slave states wanted to force slavery on other parts of our country, outside their own borders.