Annnnddddd there it is... The Tariff Argument I was afraid 67 was making.
If it was about Import Taxes, why didn't the Civil War begin under any Whig Presidents exactly? And why secede before Tariffs were raised? Why not fight that in the Senate rather then by force of arms when they can win that fight?
It's worth differentiating between immediate causes and ultimate causes. The ultimate cause of the Civil War was that there were (at least) two, increasingly mutually exclusive social systems growing up within the antebellum Union: One based on slavery, one on state capitalism. The Northern states whose elites thrived on state capitalism had interests that had diverged from those of the Southern states whose elites thrived on chattel slavery sometime at the end of the 18th century, so, at the very least, there had to be a split.
The immediate cause of the Civil War happening in 1860 was twofold:
1. The Slave Power had unequivocally lost control of the Federal government, permanently. No more slave states were likely to enter the Union, and Lincoln was elected without a
single electoral vote from below the Mason-Dixon line. The rapidly tilting population balance guaranteed Northern states control of the House and the lack of new slave stats guaranteed them control of the Senate, going into the long run.
2. A manifestation of this was the incoming Morrill Tariff (which was far larger than any previous tariff except the tariff of abominations itself) which, taking the previous point into account point,
the export-import dependent slave lords had absolutely no way to effective oppose.
So, it's not incorrect to say the Morrill Tariff caused the Civil War. But the Morrill Tariff was important because of the divergent social systems of the two major halves of the country. Saying the Morrill Tariff caused the Civil War doesn't suppose that slavery didn't, it
requires that slavery did. The North needed a large, wealthy captive market to sell manufactured goods to, so it couldn't let the South go, and the South knew it would never be able to effectively oppose Northern policies, and getting rid of slavery and adopting their own version of Northern state capitalism was essentially out of the question, so we got a war of secession instead.
This need to simplify things into easy to understand mono-causal frameworks is, IMO, another manifestation of the need to see history as a morality play. I'm more or less convinced that a South that left the Union peacefully would not descend into a dictatorial 3rd world hell hole, and I feel like the opposition has utterly failed to argue otherwise. I'm also convinced that slavery was just about the most evil thing to ever exist on this continent; in fact, I blame slavery for almost every social problem we suffer from today. The two positions are, from my view, not mutually exclusive.