Way back when, long before the US Civil War of 1860, slavery was confined to the Southern colonies which would form the confederacy for the Civil War. The North was into small farms owned and operated by free men. The big slave worked plantations were a Southern phenomenon. And the two systems did not interact much.
Starting back in the early 1700’s a feeling began to grow in the North that slavery was unfair, cruel, and close to a mortal sin. Abolition it was called later. For the Revolution, this feeling was submerged, all hands North and South, understood that to beat the Redcoats we Americans needed unity in the face of the enemy and we could not afford the political strains that abolition would bring, at least not until we had won the Revolution.
After the Revolution abolition grew apace. A lot of protestant preachers spoke in favor. Uncle Tom’s Cabin by Harriet Beecher Stowe became a best seller, selling nearly as many copies as the Bible. The Republican Party was created to push abolition. When the Republicans elected Abraham Lincoln in 1860, the Southern states seceded from the Union. When the South fired upon Fort Sumter the Civil War was on. If the south had just let Major Andersen’s Fort Sumter garrison continue manning their guns, the North might well have let the South secede in peace. But after the South fired on the Union flag, the North sprang to arms, It took 600,000 casualties, more that all we suffered in all the other wars we ever fought to win the Civil War and free all the slaves.
We didn't fight the Civil War for white supremacy. We fought it for freedom and liberty.