Astronomiya has raised some pretty compelling arguments/evidence that slavery was paramount to (though of course not solely) the cause of conflict; I'm surprised nobody has yet addressed his post.
Because it's too long.
I'd offer another interpretation for you: John Brown was the proximate cause of the Civil War.
Not his actual actions, but the way he was heroized in the North in general and by the Republican Party in particular for his plan to incite a racial war and have most people of power and property in the South murdered in their beds. Lincoln himself expressed admiration for Brown a couple times. When a man is elected who openly supported murdering you in your bed, bad things will happen one way or another.
Now, to say that slavery is a root cause is also true, but you're all viewing it as a moral issue rather than an economic one. The Civil War was inevitable because of the marriage of two completely different economic systems with no overlapping interests. The Emancipation Proclamation is the proof of this, by annihilating slavery
in the states where it was the pillar of all economic activity (excepting Maryland, but only because Lincoln had arrested the legislature before they could secede and there was suspension of habeus corpus and the like). Karaj's point about it is thus turned on its head.