You’ve been indoctrinated by the history books or text books written after the 50’s.
Go hunt up a school textbook or a history book written in the late 19th or early 20th Century and you’ll come away better educated.
Until then, take our word for it. The War of Northern Aggression was not about slavery.
Re-read this article a bit more closely.
EXCELLENT paper from Mises dot org.
http://mises.org/journals/jls/18_4/18_4_3.pdf
You all might want to read it or bookmark it for later.