The war was not about slavery. Just ask Mr Lincoln. He said so.
It may not have been “about slavery”, but the Confederation absolutely ceded the moral high ground in a strategic blunder of epic proportions when they failed to recognize that liberation of their enslaved population and incorporating their numbers in military and economic activities would have greatly leveled the field of conflict AND denied the Union that “hill to plant a rally flag on”.
That commented upon, far greater long term damage resulted when the Democrats via Wilson countenanced racism and pitting us against each other when he resegregated the civil service. Personally, I still think Biden is still trailing him a bit in “largest POS to hold public office” (resegregation, bailing out CITI/JP Morgan w/WWI entry, signing off on Federal Reserve etc…)