In 1526 the king of Kongo, a Christian, wrote to King João III of Portugal in alarm. Afonso I of Kongo complained that Portuguese traders were seizing his free subjects, even nobles and his own relatives, and asked Portugal to stop sending merchants. This level asks a different question: what did the trade do to Africa itself?

From gold to captives

Portuguese ships worked their way down the West African coast from the 1440s, and in 1482 they built Elmina castle on the Gold Coast, at first to buy gold. As sugar plantations spread to the island of São Tomé, then Brazil and the Caribbean, demand for enslaved labor grew and grew. The Atlantic slave trade peaked in the eighteenth century, when more people were shipped than in all earlier centuries combined. Captives came from Senegambia, the Gold Coast, the Bights of Benin and Biafra, and above all from Kongo and Angola: West Central Africa supplied the largest share of captives, close to half of the total.

How the trade worked

Europeans rarely captured people inland themselves. Malaria and yellow fever killed newcomers at terrible rates, and African states were strong enough to keep foreigners on their terms. Disease and the strength of African states kept most Europeans confined to coastal forts and ships. African rulers and merchants supplied captives taken in wars and raids, condemned in courts, seized for debt or simply kidnapped, and in return took goods. Textiles were the largest category of imports, many of them Indian cottons, followed by metal bars, cowrie shells used as money, alcohol, tobacco, and guns and gunpowder.

Slavery already existed in many African societies, in forms that varied widely. In some, enslaved people were absorbed into households over generations and could rise to positions of trust; in others they were harshly exploited. The Atlantic system was different in kind: in the Americas slavery became permanent, hereditary, tied to race and geared to mass plantation labor.

The debate over responsibility

Historians disagree about how to describe Africa's role. The Guyanese historian Walter Rodney, in a famous book of 1972, argued that the slave trade and later colonial rule drained Africa of people and wealth and blocked its development. The historian John Thornton, writing twenty years later, stressed a different point: Thornton argued that African rulers and merchants controlled the trade on their own terms, since Europeans lacked the power to force it. Critics of Thornton reply that control by elites does not mean their societies benefited. Both positions can be held at once: some Africans profited, while Africa as a whole lost.

Effects on African societies

Because planters wanted male field hands, roughly two men were shipped for every woman, leaving societies with skewed sex ratios and fewer young men to farm and defend villages. Population in West and Central Africa probably stagnated for long stretches while it grew elsewhere. Some states built their power on the trade, such as Dahomey, whose kings ran annual wars for captives, and Asante. Others broke apart. Many states were caught in a trap historians call the gun-slave cycle: they needed guns to defend themselves against slave raiders, and captives were the main currency for buying guns.

The effects may persist today. The economist Nathan Nunn found that the areas that lost the most people to the slave trades are poorer today, and later work with Leonard Wantchekon found lower levels of trust in neighbors and local government in the same regions.

Abolition and its paradox

Britain banned its slave trade in 1807 and the United States in 1808, and the Royal Navy patrolled the West African coast to stop slave ships. Yet the trade to Brazil continued until about 1850 and to Cuba into the 1860s. Europeans now promoted what they called legitimate commerce: palm oil for soap and machine grease, and peanuts. The result was a bitter paradox. Slavery within West Africa expanded in the nineteenth century, as enslaved people were set to produce palm oil and peanuts for export. European claims to be ending it would later become an excuse for conquest.

Worked example: Afonso's letters

Afonso, whose Kongo name was Mvemba a Nzinga, ruled from about 1509 to 1542. He had become a Catholic, read Portuguese, and sent his son Henrique to Lisbon, where Henrique was ordained and later made a bishop.

His 1526 letters show something more complicated than a victim pleading for rescue. Afonso did not ask to end slavery; Kongo held slaves of its own. He objected to the kidnapping of free Kongolese and to traders who undermined his authority by dealing directly with his subjects. He set up a royal board to inspect each captive before sale and confirm that the person had been lawfully enslaved. The measure shows a king trying to regulate a trade he could not stop. Within a century and a half, Portuguese-backed wars in Angola and a disastrous defeat of Kongo at Mbwila in 1665 had set off civil wars that fed the trade for generations. Afonso's story displays both African agency and its limits.