Reddit mentions of Clash of Extremes: The Economic Origins of the Civil War

Interesting note: most african slaves from the West African slave ships actually went to Brazil with some of the least actually leaving to America. Also Slavery wasn't fully banned in Brazil until 1888.

Also because this involves slavery and economics, I would really recommend reading Clash of Extremes by Marc Egnal. It goes deep into how the split between the north and the south was a clash of economic interests that were taken up by extremists (spoilers: it was all the Great Lakes region's fault). (I'm a massive history nerd, so I felt the need to post all this interesting word-blah)

That being said I don't think Commargh is a proper comparison to the Pre-Civil War United States as Commargh is a city-state where the US is a country and also the US hadn't fallen (yet?) like the Eldar. Also the US never directly got its slaves from US citizens raiding Africa instead the US received slaves mainly through warlords in Africa willing to sell their fellow Africans for guns and money.

Instead I believe a more apt comparison would be the slave capturing piracy of the mediterranean by Barbary Pirates. These pirates operated out of city states along the North African coast and enslaved many European sailors (including British ones) with some estimates reaching the millions. This theory probably makes more since as very few Warhammer historical references are taken from American history and many more taken from European history (ie: Holy Roman Empire Style church, Imperium of Man = British Empire anyone notice Colonial Governor = Planetary Governor (with a little 19th century European totalitarianism for flavor), Tau caste names based on Aristotle's theory about all matter, all the other ones that I can't think of off the top of my head).