Slavery is sometimes called “an inevitable evolvement”. I remember studying in school about the  “Trade Triangle” during colonial America. New Englanders would take rum to the West Coast of Africa where they would sell it and pick up slaves. They took the slaves on the “middle passage” to the West Indies, where they would trade them for molasses. Then the molasses were taken to New England and the process began all over again.
Needless to say, slavery existed as a legal right in the colonies before the American Revolution. but it gradually declined in the North. Unfortunately, because of the large land holdings in the South, slavery  continued there until the war and some states, Kentucky, Missouri, Maryland and Delaware still had slaves even then. Slavery wasn’t totally abolished until 1868.
According to an 1860 census, only 31% of families owned slaves and 75%of those owned less than 10 and often worked beside them in the fields. The Confederate Constitution, according to an article I read, banned the overseas slave trade, and permitted Confederate states to abolish slavery within their borders if they wanted to do so.
The CSA’s highest ranking generals, Robert E. Lee and Joseph E. Johnston were not slave holders and did not believe in slavery. By 1864 the Confederate States began to abandon slavery and there are some indications that even without a war, the “peculiar institution” as it was often called, would have ended on its own as it did in most other countries.