The Origins of the African Slave Trade

In 1807 Britain outlawed slavery. In 1820 the king of the African kingdom of Ashanti inquired why the Christians did not want to trade slaves with him anymore, since they worshipped the same god as the Muslims and the Muslims were continuing the trade like before.

The civil rights movement of the 1960's have left many people with the belief that the slave trade was exclusively a European/USA phenomenon and only evil white people were to blame for it. This is a simplicistic scenario that hardly reflects the facts.
Thousands of records of transactions are available on a CDROM prepared by Harvard University and several comprehensive books have been published recently on the origins of modern slavery (namely, Hugh Thomas' The Slave Trade and Robin Blackburn's The Making Of New World Slavery) that shed new light on centuries of slave trading.
What these records show is that the modern slave trade flourished in the early middle ages, as early as 869, especially between Muslim traders and western African kingdoms. For moralists, the most important aspect of that trade should be that Muslims were selling goods to the African kingdoms and the African kingdoms were paying with their own people. In most instances, no violence was necessary to obtain those slaves. Contrary to legends and novels and Hollywood movies, the white traders did not need to savagely kill entire tribes in order to exact their tribute in slaves. All they needed to do is bring goods that appealed to the kings of those tribes. The kings would gladly sell their own kins.
This explains why slavery became "black". Ancient slavery, e.g. under the Roman empire, would not discriminate: slaves were both white and black (so were Emperors and Popes). In the middle ages, all European countries outlawed slavery (of course, they retained countless "civilized" ways to enslave their citizens, but that's another story), whereas the African kingdoms happily continued in their trade. Therefore, only colored people could be slaves, and that is how the stereotype for African-American slavery was born. It was not based on an ancestral hatred of blacks by whites, but simply on the fact that blacks were the only ones selling slaves, and they were selling their own kins. (To be precise, Christians were also selling Muslim slaves captured in war, and Muslims were selling Christian slaves captured in war, but neither the Christians of Europe nor the Muslims of Africa and the Middle East were selling their own kins).
Then the Muslim trade of African slaves came to a stop when Arab domination was reduced by the Crusades. (Note: Arabs continued to capture and sell slaves, but only in the Mediterranean. In fact, Robert Davis estimates that 1.25 million European Christians were enslaved by the "barbary states" of northern Africa. The USA bombed Morocco, Algiers, Tunis and Tripoli in 1801 precisely to stop that Arab slave trade of Christians. The rate of mortality of those Christian slaves in the Islamic world was roughly the same as the mortality rate in the Atlantic slave trade of the same period.)
Christians took over in black Africa, though. The first ones were the Portuguese, who, applying an idea that originally developed in Italian seatrading cities, and often using Italian venture capital, started exploiting sub-Saharan slaves in the 1440s to support the economy of the sugar plantations (mainly for their own African colonies of Sao Tome and Madeira).
The Dutch were the first, apparently, to import black slaves into North America, but black slaves had already been employed all over the world, including South and Central America. We tend to focus on what happened in North America because the United States would eventually fight a war over slavery (and it's in the U.S. that large sectors of the population would start condemning slavery, contrary to the indifference that Muslims and most Europeans showed for it).
Even after Europeans began transporting black slaves to America, most trade was just that: "trade". In most instances, the Europeans did not need to use any force to get those slaves. The slaves were "sold" more or less legally by their (black) owners. Scholars estimate that about 12,000,000 Africans were sold by Africans to Europeans (most of them before 1776, when the USA wasn't yet born) and 17,000,000 were sold to Arabs. The legends of European mercenaries capturing free people in the jungle are mostly just that: legends. A few mercenaries certainly stormed peaceful tribes and committed terrible crimes, but that was not the rule. There was no need to risk their lives, so most of them didn't: they simply purchased people.
As an African-American scholar (Nathan Huggins) has written, the "identity" of black Africans is largely a white invention: sub-Saharan Africans never felt like they were one people, they felt (and still feel) that they belonged to different tribes. The distinctions of tribe were far stronger than the distinctions of race.
Everything else is true: millions of slaves died on ships and of diseases, millions of blacks worked for free to allow the Western economies to prosper, and the economic interests in slavery became so strong that the southern states of the United States opposed repealing it. But those millions of slaves were just one of the many instances of mass exploitation: the industrial revolution was exported to the USA by enterpreuners exploiting millions of poor immigrants from Europe. The fate of those immigrants was not much better than the fate of the slaves in the South. As a matter of fact, many slaves enjoyed far better living conditions in the southern plantations than European immigrants in the industrial cities (which were sometimes comparable to concentration camps). It is not a coincidence that slavery was abolished at a time when millions of European and Chinese immigrants provided the same kind of cheap labor.
It is also fair to say that, while everybody tolerated it, very few whites practiced slavery: in 1860 there were 385,000 USA citizens who owned slaves, or about 1.4% of the white population (there were 27 million whites in the USA). That percentage was zero in the states that did not allow slavery (only 8 million of the 27 million whites lived in states that allowed slavery). Incidentally, in 1830 about 25% of the free Negro slave masters in South Carolina owned 10 or more slaves: that is a much higher percentage (ten times more) than the number of white slave owners. Thus slave owners were a tiny minority (1.4%) and it was not only whites: it was just about anybody who could, including blacks themselves.
Moral opposition to slavery was widespread even before Lincoln, and throughout Europe. On the other hand, opposition to slavery was never particularly strong in Africa itself, where slavery is slowly being eradicated only in our time. One can suspect that slavery would have remained common in most African kingdoms until this day: what crushed slavery in Africa was that all those African kingdoms became colonies of western European countries that (for one reason or another) eventually decided to outlaw slavery. When, in the 1960s, those African colonies regained their independence, numerous cases of slavery resurfaced. And countless African dictators behaved in a way that makes a slave owner look like a saint. Given the evidence that this kind of slavery was practiced by some Africans before it was practiced by some Americans, that it was abolished by all whites and not by some Africans, and that some Africans resumed it the moment they could, why would one keep blaming the USA but never blame, say, Ghana or the Congo?
The more we study it, the less blame we have to put on the USA for the slave trade with black Africa: it was pioneered by the Arabs, its economic mechanism was invented by the Italians and the Portuguese, it was mostly run by western Europeans, and it was conducted with the full cooperation of many African kings. The USA fostered free criticism of the phenomenon: no such criticism was allowed in the Muslim and Christian nations that started trading goods for slaves, and no such criticism was allowed in the African nations that started selling their own people (and, even today, no such criticism is allowed within the Arab world).
Today it is politically correct to blame some European empires and the USA for slavery (forgetting that it was practiced by everybody since prehistoric times). But I rarely read the other side of the story: that the nations who were the first to develop a repulsion for slavery and eventually abolish slavery were precisely those countries (especially Britain and the USA). As Dinesh D'Souza wrote, "What is uniquely Western is not slavery but the movement to abolish slavery".

(To be completely fair, what was also unique about the western slave trade is the scale (the millions shipped to another continent in a relatively short period of time), and, of course, that it eventually became a racist affair, discriminating blacks, whereas previous slave trades had not discriminated based on the color of the skin. What is unique about the USA, in particular, is the treatment that blacks received AFTER emancipation, which is, after all, the real source of the whole controversy, because, otherwise, just about everybody on this planet could claim to be the descendant of an ancient slave).
(That does not mean that western slave traders were justified in what they did, but placing all the blame on them is a way to absolve all the others).
To this day, too many Africans, Arabs and Europeans believe that the African slave trade was an USA aberration, not their own invention.

By the time the slave trade was abolished in the West, there were many more slaves in Africa (black slaves of black owners) than in the Americas.