The North opposed expanding slavery, while the South went to war to do so.
Yes, the North objected to slavery and the South wanted it continued. But both held their respective positions exclusively out of a concern for wealth; who got it and where. Just like all other wars, that war was about wealth, power and territory. This is not to be misread as some sort of white-washing of slavery. I'm merely pointing out the true motivation for the war.
And even this is overly simplistic. For instance, in writing about a book he recommends, "Slaves Without Masters: The Free Negro in the Antebellum South" by Ira Berlin, Henry Louis Gates, Jr. writes:
Here’s where the monolith falls apart, however. As critical as Berlin’s findings about the North and South was his revelation that the South really consisted of “two Souths”: an Upper and a Lower, distinguished, among other things, by their histories, geographies and outlooks.
The Upper South (think Delaware, Maryland, Virginia, North Carolina and later Kentucky, Missouri, Tennessee and D.C.) had been marked by its earlier history of manumission following the Revolution; it also had a more negative outlook about slavery’s future as a result of its increasingly inhospitable soil (for more on this, see Amazing Fact, “What Was the Second Middle Passage?”).
The Lower South (think Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, South, Carolina and Texas), by contrast, had never embraced manumission fever, and because there was still so much money to be made off the cotton trade (see Amazing Fact, “Why Was Cotton King?”), it never wavered in its commitment to the slave economy.
http://www.pbs.org/wnet/african-americans-many-rivers-to-cross/history/free-blacks-lived-in-the-north-right/
I've never read Berlin's book, but upon reading Gates, I'm inclined to pick it up.