In my humble opinion...
The obvious moral repugnance of enslaving another human based on ethnicity (or any other factor, at that) is forced to balance against the equally obvious self-serving corruption of Northern business interests and Federal collectivist (i.e.: tyrranical) tendencies when we look back at the American Civil War (or the War Between the States, or the War of Northern Aggression, or the War Between Union Goons and Dixie Thugs, if you will). Hindsight is 20/20 of course, but if I had been President and could persuade a sizable majority of my colleagues in Washington to agree, I would have let the South secede... good riddance to backward blowhards. Then the United States of America would suspend all trade with the Confederate States of Dixie or whatever they would call themselves, and declare that any slave who made it into the North was automatically free and any white (or other) person caught entering the North with the purpose of kidnapping and returning said slave would be jailed. A loud declaration to the rest of the world that the "peculiar institution" is abhorrent and contrary to the principles of freewill and natural law, constantly updated with intense economic and idealogical pressure, would put a fairly decisive crimp in the viability of such antiquated racist institutions and do irreparable damage to the emerging nation... perhaps enough to change the minds of the average Southerner, if not the slave-owning elite. I'm guessing this would go on for about 20 years - maybe less - before the South would be begging to rejoin the United States, rescinding slavery as a matter of course to do so.
I could be wrong, of course. And I'm sure there are those who take exception with my characterizations of intention, especially our friends in the "New South". Confederate flag-waving Freepers... FLAME ON!
WELL, Whatever would all those Susie Sunshine perfect Northern ancestors who kept slaves have done with THEIR slaves? Perhaps the Southerners could have given their slaves a horse to ride North and offered the thousands of Northern sweatshop and factory workers good healthy outside work planting and picking cotton in the great outdoors instead of being locked up in Mr. Yankee Bigshot's widget factory with the painted over windows with pittance pay.