Between the early sixteenth and the late nineteenth centuries, ships carried about 12.5 million African men, women and children across the Atlantic Ocean in chains. It was the largest forced migration by sea in human history, and it shaped the societies of the Americas as deeply as any treaty or revolution. To see how it began, we have to start before Columbus, on the coasts of Portugal and the islands of the eastern Atlantic.

Slavery before the Atlantic

Slavery was old and widespread. Ancient Rome, medieval Islamic states, Christian Europe and many African and Asian societies all held people in bondage. What was usually missing was a fixed link between slavery and one continent of origin. In the Mediterranean around 1400, an enslaved person might be a Russian, a Circassian from the Black Sea coast, a North African Muslim or a Greek captured at sea. The English word "slave" comes from "Slav", because so many captives once came from eastern Europe. Then two things changed. The Ottoman capture of Constantinople in 1453, followed by Ottoman control of the Black Sea ports, made Black Sea captives harder for western Europeans to obtain, and Portuguese ships were opening a new supply on the coast of West Africa.

Portugal on the African coast

Portuguese captains, backed by Prince Henry, sailed south along Africa in search of gold, a sea route to Asia and allies against Muslim powers. In the early 1440s they began seizing people on the coast of what is now Mauritania. In August 1444 about 235 African captives were landed at Lagos in southern Portugal and divided among new owners. The royal chronicler Gomes Eanes de Zurara described mothers clinging to children as families were split apart, and onlookers weeping, even as he praised the enterprise for bringing souls to Christianity. That uneasy mix of pity and justification would echo for four centuries.

The Portuguese soon learned that raiding was dangerous and that buying captives from African rulers was easier. Church authority gave cover. In the bull Romanus Pontifex of 1455, Pope Nicholas V authorized the king of Portugal to reduce the non-Christians he conquered to perpetual slavery.

The sugar islands

The decisive step came on islands. Sugar cane demanded enormous labor: cutting the cane fast, crushing it in mills, and boiling the juice day and night before it spoiled. Portuguese settlers planted it on Madeira, then on the Cape Verde islands and above all on São Tomé in the Gulf of Guinea, settled from the 1480s. On São Tomé, settlers joined large sugar estates to enslaved African labor in the pattern historians see as the model for plantations in the Americas. By the mid-1500s it was a leading sugar producer.

Across the ocean

Columbus reached the Caribbean in 1492. Spanish conquest, forced labor and, above all, epidemics of smallpox, measles and influenza devastated the Taíno people, whose numbers on Hispaniola collapsed within a few decades. The friar Bartolomé de las Casas denounced the cruelty done to Indigenous people. Early on he also suggested bringing Africans to do the work instead, but Las Casas later regretted proposing that Africans replace Indigenous laborers, writing that their enslavement was just as unjust. In 1518 the Spanish crown under Charles V began licensing shipments straight from Africa, and the first known voyages sailing directly from Africa to the Americas date from the mid-1520s.

Spain had no trading posts in Africa, so it relied on others and controlled them by contract. The asiento was a contract from the Spanish crown granting a merchant or company the right to supply enslaved Africans to Spanish America. It was valuable enough that Britain won it at the Treaty of Utrecht in 1713 and handed it to the South Sea Company.

From servitude to race

Over the sixteenth and seventeenth centuries, the link between Africa and slavery hardened into a racial order. Earlier enslavement often turned on religion or capture in war, and conversion sometimes opened a path out. As plantations spread, colonial lawmakers closed those paths, making slavery lifelong, inherited and tied to African descent. A Virginia law of 1662 declared that a child's status, slave or free, followed that of the mother. Children born to enslaved women and white fathers therefore stayed enslaved, and enslaved women's bodies became a source of their owners' wealth. Historians long debated whether racism caused slavery or slavery bred racism; most now argue that the two grew together, each feeding the other.

Worked example: one sugar estate on São Tomé

Picture an estate on São Tomé around 1550. Its owner is a Portuguese settler who came with a royal grant of land. The cane fields climb the hills; at the bottom sits a mill turned by water, and a boiling house where fires burn through the night. Perhaps two hundred enslaved Africans work here, most bought from traders on the mainland coast, some from the kingdom of Kongo to the south.

Ask the questions a historian would. Where did the money come from? Merchants in Lisbon and Antwerp advanced credit and bought the sugar. Where did the people come from? African rulers and traders sold them, and Portuguese officials taxed every sale. What did the system need to survive? A constant supply of new captives, because overwork and disease killed people faster than children were born. Each answer points across the ocean, to Brazil and the Caribbean, where the same estate would soon be rebuilt on a far larger scale.