top of page

How the African Slave Trade Changed the Congo


Normally, in the United States, we study slavery for what it did to America.


1. Slavery created vast racial inequality in the United States. That inequality has declined between the years of the Ante-Bellum South and the twenty first century. However, in our country, there are still racial gaps in most indicators of human well-being that persist to the present day.

Equally important,

2. Slavery was a key foundation of American economic growth. The profits from growing cotton and selling it to Britain were reinvested in factories in the Northeast. The prosperity of Boston and Philadelphia were a byproduct of the economic success of the Southern slave economy.

*  *  * 

Probably most of the readers of this website knew both of those things already.

But what exactly did slavery do to Africa itself? We as Americans take interest in slavery from the moment a captured African slave gets on the boat for the Middle Passage. But slavery also had extremely destructive effects on the continent the slaves left behind.

The raw material for this essay comes from Robert Harms’ history of the Congo before it was colonized. (River of Wealth, River of Sorrow: the Central Zaire Basin in the Era of the Slave and Ivory Trade 1500-1891. 1981. Yale). However, Harm’s account of the Congo experience has striking parallels to other settings, such as the slave economy of Southeast Asia, the changes in global trade that came about with the discovery of Spanish silver and to the mercantile system that prevailed in Europe and the European colonies in the seventeenth and eighteenth century.

1. Slavery became a giant stimulant to trade within Africa itself. Human beings were an extraordinarily valuable cargo. There were few traditional African products that could command the price of captive human beings. There had a little bit of trading in gold and silver elsewhere in Africa. (The Congo has gold now, but none had been discovered in precolonial periods.) Ivory would become valuable later, but that was a product that catered to European and Asian tastes. There had always been slavery in the Congo and an inter-regional trade in slaves had always existed. However, the high prices that the Europeans were willing to pay were a gigantic stimulus to the business. The amount of slave raiding dramatically increased – and traditional trade routes that might have just been inland only were extended to the Atlantic Coast.

Making slave voyages viable facilitated trade in all sorts of other products. As long as boats were running up and down the Zaire River selling slaves to the Europeans, the boats could be filled with other products as well. These could be sold to the Europeans or to villages the slavers passed along the way. The return trips could also be profitable as well. Goods the Europeans provided as payment for the slaves could be sold in passing villages as well. Slaves could even be purchased on the return trip to sell to wealthy Africans in the inland areas.

It goes without saying that greater trade produced greater production, extending the wealth beyond the personal fortunes of the traders themselves.

This is part of a larger pattern of European economic activity in one part of the world leading to cognate development in other distant seemingly unrelated regions. The conquest of the New World led to Conquistadores mining vast amounts of Mexican and Peruvian silver. The silver made its way to Spain. From Spain, the silver went to Genoese banks where the Hapsburgs did their banking. The Genoese traded with the Ottomans and the Egyptians. The Egyptians traded with Ethiopians. Ethiopians traded with Indians. Indians traded with Javanese. Javanese traded with Chinese. Chinese traded with Japanese. In this way, New World Silver made its way all the way from Peru to Japan. It enriched every nation through which it passed.

Slavery was more unsavory and more brutal. But it had the same long distance effects on prosperity. It also had the same long distance effects on human misery.

2. Slavery massively increased warfare and social fragmentation among the Congolese. Slave raiding had always existed in the Congo. But the greater profitability associated with the rise of the European market vastly increased the volume of slave raiding that was occurring. This immediately put anyone who was “capturable” in great danger.

Who was capturable?

Individual people travelling away from home were automatically potential victims. This includes both random travelers and traders doing business on the river. The safest strategy was to stay home surrounded by people one knew who could provide protection. Leaving home alone was genuinely dangerous. Travelers had to have a big enough escort to discourage attackers. Slave traders and ivory traders travelled in large convoys of large boats, with the boats containing both cargo and guards. Because large scale trade required large scale military escorts, this raised the cost of any kind of long-distance dealing. This confined participation in the slave trade to wealthiest and most powerful chieftains. An indirect effect of both taking slaves and the escort requirements for moving slaves was to significantly increase the social inequality among the Congolese. The very rich got dramatically richer. The very poor and just-a-little-bit poor got enslaved.

Small villages were also at risk of being raided. The rise of the slave trade meant a significant increase in warfare; strong villages raided weaker villages, with the losers being sold into slavery. The Congo had always been somewhat fragmented, as forested areas outside of empires tend to be. However, the increase in warfare and the increase in random raids on travelling individuals would have created massive mutual distrust, and would have divided an already divided territory into smaller mutually hostile units. Constant war is not good for economic growth. Men are kept busy fighting rather than improving their farms and their enterprises. Losers see everything destroyed.

Note that the Congo experience is not that different from the experience of Ancient Greece. The Ancient Greek city-states all kept slaves. Large cities such as Athens and Sparta went on raiding expeditions to capture slaves. Constant warfare led to major confrontations involving the largest and wealthiest cities. Both Troy and Athens were destroyed by poorer, less sophisticated cities.

There were parallel processes in Southeast Asia as well. The region around the Straits of Malacca (which comprise modern Sumatra, Singapore, Malaysia and Thailand) were long characterized by pirates who were slavers. Malacca Staits residents could not sleep within a quarter mile of the coast because it would be too easy to be seized by pirates and sold into slavery. Sleeping a quarter mile in provided an opportunity for one’s animals to warn of a possible incursion allowing residents to flee into the jungle.

Slavery lowered rates of economic development in ancient Southeast Asia. It also increased military fragmentation. Local warlords would ally with pirates (or the warlords would in fact be pirates), Raiding and warfare were common. Malaysia and Thailand did not significantly develop until the “end” of the slave era. Forced human trafficking of both sexual and manual labor continue in the region to the present day. The level however is far lower than what existed in the era of the pirates.

3) Slavery produced serious depopulation among the Congolese. An obvious cause for this was the forcible transshipment of slaves to the African Islands and the New World. However, population decline had more causes than mere forced outmigration, as serious as that was.

The slave trade introduced new Western diseases to the Congo – notably smallpox. Smallpox originated in China but spread to Europe. As Europe colonized the rest of the world, they brought smallpox with them, generally with devastating effects on local populations. The Congo was no exception. Europeans did not penetrate the interior until the end of the nineteenth century. However, Congolese slave traders contracted smallpox from Europeans they met on the coast. On their return trips, the traders were disease vectors, infecting every river village at which they stopped on the way.

The slave trade also introduced guns into the Congo. Just as warfare was becoming more frequent, because of the slave trade, it was also becoming more lethal. The same increase in mortality that was occurring in Western wars because of the invention of the modern rifle was paralleled when modern rifles were sold to non-Western populations.

Harms also notes that in Slave Era Congo, fertility was very low. Given the absence of historical records, it is unclear if Congo fertility was always low, or if the slave trade caused it to go lower. I suspect that fertility declined – even if this is nothing other than conjecture. My reasoning is that in preindustrial populations, ideal family size is a function of the ability to use children for economic activities. Children are meant to help on the farm or in whatever business the family engages. Constant raiding meant that there was no guarantee that people who had children would be able to keep their children. Much of the economic incentive to have children was reduced. Obviously, the emotional trauma of losing children would have contributed to lower interest in childbearing as well.

A further consideration is that slavery would have broken up couples as husbands and wives were individually seized on slave raids. Harms argues that many women were taken as slaves to be wives to rich men, and that the slave women would have withheld sex out of antipathy to their husbands. I would take issue with Harms on this one point. I suspect the slave women did not get a vote on whether they had sex with their masters. The other considerations however, suggest that fertility declined and that this decline was substantial.

4. The Congo developed a system of neo-mercantilism with individual villages fighting to retain exclusive trading rights to hinterland areas. Trade in the Congo moved on rivers. This gave any village with the ability to militarily block off a river the ability to maintain exclusive access to any villages upstream of the blockade. Establishing chokepoints became a common strategy for warlords to obtain military control of a region. The main Zaire River itself was too wide to effectively control from riverside villages. Wealthy villages could control the side of the shore where the village was located; however, boats seeking free passage could simply keep their distance from the shore in question – or as was not uncommon, pay off the village where they wanted to stop. In contrast, smaller rivers were narrow enough to be choked off with a blockade. These settings maintained exclusive upstream trading rights for the blockading village, although here as was the case with wider rivers, passage could be negotiated with the payment of sufficient tribute.

Obviously, such internal customs barriers and monopoly privileges reduced overall levels of inter-regional trade. However, economic development could occur within these constraints.

Seventeenth and eighteenth century Europe was characterized by an elaborate patchwork of mercantile monopolies. Countries maintained exclusive rights to trade at specific ports, or in the colonies, at specific islands. Victory in warfare in Early Modern Europe gave the winner more trading ports; losing meant a loss of same. The wars of the Congo look fairly similar. Europe grew in the seventeenth and eighteenth century. It grew even more when such mercantilist restrictions were relaxed and free trade became more prevalent in the Continent.

How was long distance slave trading maintained in the face of all of these local river-blocks? As was the case in Europe, alliances and diplomacy bought access to trade centers one did not control personally. Both European and Congolese diplomacy were complex. Shifting alliances led to new wars and new conquests.

 *  *  *

Overall, the effects of slavery on the Congo were highly negative. To be sure, the slave trade was a business like any other business. Wealthy Congolese got rich from the slave trade, just as wealthy Portuguese, wealthy Britons and wealthy Southerners got rich from the slave trade. There would have been a further benefit from the expansion of other forms of trade not associated with slavery in general. However, the slave trade led to a dramatic increase in war and a dramatic depopulation of the Congo. Constant raiding and warfare would have undercut investment and any kind of long term economic perspective. The labor force and entrepreneurial pool was gutted by military casualties, the deportation of slaves, the introduction of savage and lethal diseases, and the general decline in childbearing and family formation. Fighting over local trading privileges also gutted the possibility of political unity. This would have significantly reduced the ability of the Congolese to resist the incursions of the French and Belgians at the end of the nineteenth century. The Belgian occupation in the early twentieth century led to the most brutal labor exploitation system that would be imposed on any colony by any European country.

The Congo was ravaged by internal violence, divided, and depopulated. None of this laid any foundation for prosperity in later periods.

For More Information 

On the extraordinary long distance trade links associated with European expansion, see Immanuel Wallerstein’s four volume tome Modern World System. (University of California Press, various dates). On Manila as a critical New World – Japan entrepot, see Birgit Tremml-Warner’s Spain, China and Japan in Manila, 1571-1644. (University of Amsterdam, 2015). For an edited collection of essays on the European-Asian trade nexus – and the equal role of Asian traders in that nexus, see James Tracy’s Rise of Merchant Empires: Long Distance Trade in the Early Modern World 1350-1750. (Cambridge, 1990)

On the complex relationship between slavery and warfare in Ancient Greece, see Peter  (Hunt’s Slaves, Warfare and Ideology in the Greek Historians (Cambridge, 1998) Hunt makes the additional point that the Greek city-states used slaves as soldiers, making slave raids an independent source of military might. The record of the Congo on this point is unclear, and may have varied based on local conditions.

On slave raiding in Southeast Asia, the most useful citations are books on other subjects that treat piracy as a secondary theme. See Colin Mackay’s History of Phuket and the Surrounding Region. (White Lotus, 2013) (The rise of the tin industry weakened the raiding economy) and Wilfred Blythe’s Impact of Chinese Secret Societies in Malaysia.(Oxford, 1969). (The secret societies stopped the raiding by fighting the pirates and winning.)

On the role of plagues and disease in world history, see William McNeill’s excellent Plagues and People (Anchor, 1976). On the lethal effects of the invention of rifles, see Brett Gibbons Destroying Angel: Rifle-Musket as the First Modern Infantry Weapon. (Self-published, 2018). Gibbons discusses the 1850’s. Every decade after the 50’s saw improvements in rifles and further increases in killing power. For a more general discussion of the effect of rifle improvement in European war, see Geoffrey Wawro’s Warfare and Society in Europe 1792-1914. (Routledge, 2000), notably Chapter 6.

Any demography textbook would contain a discussion of fertility in preindustrial societies. Gregg Carter’s Population and Society (pick any edition you like), is a perfectly reasonable first book to read.

On mercantilism and international battles for economic hegemony, Immanuel Wallerstein’s Modern World System(previously referenced) is especially informative. See in particular, Volume II.

bottom of page