edit: for all you people downvoting me this is what i meant: https://www.theguardian.com/world/2017/aug/23/charlottesville-un-committee-warns-us-over-rise-of-racism
There is a rise in racism, the more we are discussing racial divide the worse it is getting.
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We have a rule against politics more recent than twenty years ago, so I'm not going to comment on the UN warning or present trends within the US. I'm also mostly going to discuss this from an American POV because I am one and that's what I study.
We should teach children about the slave trade for the same reason we teach them about democracy or industrialization. We usually consider those virtuous, or at least progress, and so celebrate them. The slave trade is a centuries-long atrocity in which the Anglosphere is deeply implicated and which is at least as important for understanding the modern world. During its height in the eighteenth century, British-flagged slavers dominated the transatlantic slave trade. They even supplied non-English colonies in Latin America. If you're an American (like me), Canadian, or UK citizen now, these are some of your national ancestors. The stealing of twelve million lives from Africa radically transformed three continents and had profound effects on a fourth. Slavery is what made colonization of the New World commercially viable, transforming scraggly settlements clinging to the edge of the Atlantic and forcibly depopulated islands into engines of a new world economy. Without the slave trade, there would be nothing like the lamentable history of slavery in the Western Hemisphere, to say nothing of its leviathan legacy that remains with us more than a century after emancipation.
I understand that coming from a guy flaired in the subject, that probably all sounds a touch self-serving. My history is the important one; I'm interested in it, so it must be. It's true that it's my history. I suspect that it's yours too. The demographics of Reddit are very white and male. (Like me.) Whiteness is one of those things that most of us who have it take for granted because it has been a concept, often the ultimate concept, organizing our societies for around four centuries. It's so pervasive and central that it doesn't require distinction. Unless stated otherwise, when the white Americans I study say "people" they mean "white people." White is the norm and deviation from it places one outside its circle of protection. Non-whites can be abused, raped, enslaved, and killed with impunity in the era I study. Few nineteenth century whites would think to pass a law excluding, say, Frenchmen from their state. They were overwhelmingly in favor of laws excluding black Americans from their states, passing those laws with tremendous majorities in the 1850s.
The nineteenth century is a ways, albeit not a long one, down the road from the slave trade. In the era I study most, the twenty years prior to the American Civil War, white Americans have a fully-developed theory of whiteness. That theory came from many sources going back centuries before. Fully accounting for it involves three continents. It's a massive task that I'm not competent to do a comprehensive job with. But I can talk about one of the biggest sources of whiteness: the transatlantic slave trade.
Whiteness is an in-group marker. If you have it, you are to some degree "one of us" from the POV of other people who apply the concept to themselves. In the Early Modern Atlantic, whiteness coexists with many other forms of identity. The two biggest are religion (Christianity of some kind) and nationality. Both of those identities are somewhat older than whiteness, but also formed from continuous, close contact with people and cultures defined out of them. Christians are not "pagans" or Muslims. Englishmen are not Frenchmen. Those distinctions hold a lot of meaning for people of the time, though not necessarily in the same ways they do today. When the other identities don't conflict, they aren't as pertinent, so one wouldn't expect Englishmen to regularly stress that Frenchmen aren't Christians (though given the era they might object vociferously to their being Catholic Christians). It's enough that they're French (or Spanish, etc) and not English, for the Englishmen.
At the height of the Atlantic slave trade, the English were the largest players. They owned the big ships (if not all of them) and ran the voyages (ditto). A ship would depart from Liverpool, Bristol, or London (mostly earlier on) usually with a captain, some officers, a crew, and a hold full of stuff to use to buy people. The crew would probably have no shortage of English seamen along, who may or may not be their of their own free will, but slaving crews often had a markedly international character. Sailors tend not to be too fussed about the details of piety at the time, so they're probably not going to get in brawls over the right way to pray. They're also going to be together for probably a year in quite close quarters, often in dangerous situations that require close cooperation for as much safety as they can muster. This is not an ideal time to pick fights over nationality.
This is also a time in which they will come into contact with people who don't look like them in a specific way that might not have mattered much in other situations or centuries: skin color. Specifically, they will come into contact with darker-skinned people from Africa in a variety of circumstances. Africans are trading partners, sometimes partners who speak English, but with whom they have a necessarily adversarial relationship. The slaver at the fort or who comes aboard on a boat to negotiate is a person a trader needs to be on the good side of to get better deals. Coastal African polities are strong enough that Europeans can't simply sail in and help themselves. They have to deal with local authorities on the West African coast.
Dealing with those polities gets the Europeans holds full of African slaves, who they will have to control (read: torture) for usually some months. Most African slavers don't have a ship's worth (as many as a few hundred) human beings that the European slavers want so there's a lot of time spent anchored offshore dealing with various factors, traders, and so forth until the captain has what he deems an acceptable cargo. Once his ship is fully "slaved", he would turn it for the West Indies (usually) and he and his unhappy human cargo would have a few weeks in transit to their first disembarkation point. It might be one of several as the captain tries to get rid of enslaved people passed over at the first few ports.
That's a long time for Europeans to be closeted with people who they know full well are hostile to them with the best of possible reasons. For all that time, they need to both maintain some kind of solidarity among the crew to preserve their own lives (revolts on slavers are fairly common and sometimes deadly) and keep enough of the enslaved alive for the voyage to turn a profit. Along the way, they will also help themselves to the enslaved in ways that would be unacceptable, or at least less acceptable, if done to a fellow crewman. Captains and officers might take several "wives" or "favorites" from among the enslaved women, switching to a new one when they've had their fill of raping her. In order to do something like that, you have to have some kind of concept that justifies it in your own mind. Misogyny can go a long ways, but slavers are conscious that the way they treat enslaved women is not the same way they treat women back home. Likewise they don't treat enslaved men quite the same.
Just being a member of the crew could do some of that work. You're stuck with these people for about a year, after all. But off the coast of Africa, slavers are rarely all to their lonesome. When they face difficulties, including with their crews, captains frequently get together. They also share information about the state of the trade and cooperate to navigate the shoreside politics and protect one another when their unwilling cargo revolts. That's a whole off-shore collaboration specifically to facilitate the slave trade. It crosses national and denominational boundaries. The thing holding the crews together and holding the slavers offshore together is whiteness, to the point where crewmen are referred to as "white" men even if some of them are from the same ethnicities as found down in the hold. The slave trade isn't the sole fount of whiteness, but it's one of the most important.
Which brings me to the question of whether discussing this causes more problems than it fixes. Let me be blunt: I don't believe that people learn racism from history taught well, which includes teaching this. They might well learn it from history taught poorly, by omitting it, as removing the source of inequities leaves people groping for an explanation. Without an understanding of how whiteness was created, the door is left wide open for students to assume it's just a natural phenomenon and some people (who aren't white) are fundamentally different and less deserving than others (who are). It's difficult and often intensely uncomfortable to talk about this stuff. We must ask questions about ourselves which have answers we may not like.
We are all embedded in this history, just as we are embedded in the world made by industrialization. It's not a matter just of individuals, now or in the past, being racist or not. The system set in place long ago, if modified since, keeps rolling along. It didn't end with the prohibition of slave imports to the US at the start of 1808. It didn't end with emancipation in 1865. It didn't end with the Voting Rights Act in 1965. There's a reason we call slavery one of America's original sins. It is entailed on us all and has shaped us, just as the "frontier" claimed by genocide against Native American nations did. You don't have to have been on that slave ship or held a whip on your own hands to be a beneficiary; the cotton in Abraham Lincoln's clothes was picked by slaves in the American South.