Sorry for the long post, but I get a bit bent out of shape when I see people calling that a war against slavery Which is exactly what it was.
Read some of the great Civil War authors, such as Bruce Catton, or watch Ken Burns' PBS series.
You'll see that you missed a very fundamental point.
(You might also take a look at the 13th Amendment. It isn't about "leaving the Union")
I refuse to get involed with any more "War of Norther Aggression" nutjobs, but you might want to point out to him that history is written by the winners. To the victors go the spoils.