When Portugal began to expand outside of Europe to Africa and America (Brazil), the Portuguese learned that these regions were excellent areas for growing sugar cane. Selling sugar was very profitable. So when they took over uninhabited islands, first in the Cape Verde area and then on São Tomé (near the coast of Nigeria), they began growing sugar. Sugar was hard work, and they made use of slaves they acquired in Africa to be the labor force to grow it.

In 1512, as sugar production was growing in São Tomé, King Manuel of Portugal and Afonso of Kongo made an agreement to share a monopoly of the slave trade. The Portuguese crown would buy all slaves for the island and would purchase them only in Kongo. The crown would then set up their own estates. In the process, the many Portuguese settlers on the island who had developed the industry were pushed out of its profits. This included both buying slaves in Africa and selling sugar in Europe.

To get around the crown’s restrictions, the settlers began to explore other places to purchase slaves. One of these areas was south of Kongo along the Kwanza River. Kongo claimed the area as part of its domains, but they had very little control there. A new kingdom, Ndongo, was developing. Both Kongo and Portugal called Ndongo Angola, from the title of the king (e Ngola). The rulers of Ndongo were challenging Kongo’s authority and were aided by merchants from São Tomé.

Kongo was unable to contain this collaboration. In 1526, Afonso wrote several letters to João III complaining that Portugal could not control the merchants of São Tomé. He threatened to stop the slave trade if João could not enforce his authority. He contended that in the fighting in the area, Kongolese subjects and even nobles had been enslaved. Ultimately, the rulers agreed to establish a committee. This committee determined which slaves had been taken under the terms of the agreement and which were not.

Historians who study Afonso’s letters disagree on exactly what Afonso was complaining about. Some claim that Portuguese merchants were capturing people within the Kingdom of Kongo’s main territories. Others understand it to be referring to the struggle over the trade from Angola. If Portugal were actually capturing people in Kongo’s core, clearly that would indicate major weaknesses in Kongo. However, if it were in Angola, it would be more peripheral to Kongo.

In the early times most of the slaves sold by Kongo were from outside the country. However, in the 1670s, Kongo began a very long civil war that was never fully resolved. Powerful contenders for the throne seized provinces. Local bandits and raiders began building bases in the country, raiding the area around them and selling their captives as slaves. From the 1700s to the 1850s, people from the Kingdom of Kongo itself were captured and sold in large numbers overseas.