"And states should be allowed to legalise (sp) slavery. Because that worked so well the last time."
Sigh... Why not look at the history of the world as a whole. What nation ever fought itself over the issue of slavery? To the tune of over 600k dead?
Many of our abolitionist founders believed that slavery was a dead end doomed to end in the near future but elected to allow the south to maintain it for the interest of the union. It was essentially the cotton gin's invention which allowed slavery to remain economically viable far longer than they expected.
Without that economic leep after the 1790s, attitudes would have changed in the south as much they had in the north regarding slavery and it would have ended -- WITHOUT bloodshed. You've effectively made my point -- when a powerful central government makes a uniform law across a heterogeneous population, someone's rights are going to feel violated and they could very well go the route of the US in 1776 or the South in 1862.
I'm not saying that slavery or the south was in any way honorable. That war needed to be fought and is an exception brought on be a dramatic and swift economic change..