No slavery - no civil war.
Not every Southerner supported Slavery and not every northerner supported it, but the few radicals of the north and south who brought on the war did so over the issue of slavery.
The best explanation I can give for the whole slavery vs. economics argument for the start of the WBTS is that both sides are right. It was largely an economic and political war, but slavery was tightly wrapped up into the Southern plantation economy. Thinking that the War started because of some moral component regarding slavery is a simplification...not many other people than the ardent abolitionists really thought there was that much wrong with it. Even Lincoln didn't turn the War into an anti-slavery crusade until it was politically expedient to do so in 1863.
The agrarian economy down here was based (too much) off of slavery. Threatening the expansion of slavery into the new territories threatened the power of the elite down here, and the economic issues are what pushed things over the edge. And once Fort Sumter happened, and the war really started, then patriotism toward their native states became the primary motivation for the common men of the South to fight.
Maybe that's a simplification (I'm sure it is, I'm no historian) but I've always believed that the economics vs. slavery argument is kind of specious, because you can't completely separate the two.
}:-)4