Just for historical context. When the southern states seceded, the north under Lincoln didn't go charging in. The south formed their own armies, a mistake on their part since each southern state wanted control of their own forces, so they had no central command. Once they quickly formed their own armies, a "southern" army attacked a union fort, Fort Sumter. The north didn't attack first.
The Republican Party was formed as an anti-slavery party, which the Democrat Party objected to. Lincoln had a Democrat, Andrew Johnson be his running mate in an effort to promote and maintain unity. A mistake, because after Lincoln's assassination, Johnson, returned the "good ole boys" in the south, back in power.
The war was definitely about slavery. It was the economics of slavery. The leaders in the south during the war, including senior officers, were slave holders. It didn't have to be a large number who were actual slave holders. There were blacks in the south who had slaves and they sided with the Confederacy. The low income whites who fought for the south, did so because they were told that the north (Yankees) were coming to take their land. Had they not seceded, they would have lost one major thing, the buying and selling of human beings as if they were livestock.
Slavery ended in Mexico in 1829 as they recognized its evil, England followed in 1838, France in 1858 and of course, the US in 1865. The last nation to end slavery (at least on paper) was Mauritania in 1981. Prior to those years, it was a worldwide scourge, and the white Europeans weren't the first to start it. It began millenniums ago. Even whites became slaves in Islamic nations. It was what it was.
Should we have separated from England? Yes. If we were still part of England, we would be a failing nation, as England is now.