Er, Corey? Virtually all ordinances of secession stated slavery was the prime cause. And Alexander Stephens, the Confederate VP, stated in his Cornerstone Speech that the war was all about slavery and white supremacy:
...But not to be tedious in enumerating the numerous changes for the better, allow me to allude to one other though last, not least. The new constitution has put at rest, forever, all the agitating questions relating to our peculiar institution African slavery as it exists amongst us the proper status of the negro in our form of civilization. This was the immediate cause of the late rupture and present revolution. Jefferson in his forecast, had anticipated this, as the “rock upon which the old Union would split.” He was right. What was conjecture with him, is now a realized fact.
...Our new government is founded upon exactly the opposite idea; its foundations are laid, its corner- stone rests, upon the great truth that the negro is not equal to the white man; that slavery subordination to the superior race is his natural and normal condition. This, our new government, is the first, in the history of the world, based upon this great physical, philosophical, and moral truth.
Stephens then goes to claim that the North didn't welcome secession because it needed the taxation derived from slave labour, and ends the speech by basically asking, "Why can't the north just respect our right to secede and let us get on with enforcing our white supremacy over our slaves?"
So Stewart is a little bit correct. States rights was partially the cause, but only because the Confederacy wanted to keep negro slavery, and impose their white supremacy over their slaves. The prime cause was negro slavery. And how do we know this? Because the British decided to use slavery as a wedge to divide the United States: first, Andrew Jackson was used, wittingly or unwittingly, to destroy a major source of finance to stpo the south from developing their economy away from slavery; and second, Giuseppe Mazzini was sent to exploit slavery as a wedge issue, which he did through Young America.