Slavery was a secondary, important issue of the Civil War, but centralization of government was the primary issue.
“My paramount object in this struggle is to save the
Union, and is not either to save or to destroy slavery. If
I could save the Union without freeing any slave I would
do it, and if I could save it by freeing all the slaves
I would do it; and if I could save it by freeing some and leaving
others alone I would also do that.” — Abraham Lincoln, August 22, 1862, letter to newspaper editor, Horace Greeley