From Wikipedia

Congo

Part Three

 By the 16th century, the manikongo held authority from the Atlantic in the west to the Kwango River in the east. Each territory was assigned a mani-mpembe (provincial governor) by the manikongo. In 1506, Afonso I (1506–1542), a Christian, took over the throne. Slave trading increased with Afonso's wars of conquest. About 1568-1569, the Jaga invaded Kongo, laying waste to the kingdom and forcing the manikongo into exile. In 1574, Manikongo Álvaro I was reinstated with the help of Portuguese mercenaries. During the latter part of the 1660s, the Portuguese tried to gain control of Kongo. Manikongo António I (1661–1665), with a Kongolese army of 5,000, was destroyed by an army of Afro-Portuguese at the Battle of Mbwila. The empire dissolved into petty polities, fighting among each other for war captives to sell into slavery.

Kongo gained captives from the Kingdom of Ndongo in wars of conquest. Ndongo was ruled by the ngola. Ndongo would also engage in slave trading with the Portuguese, with São Tomé being a transit point to Brazil. The kingdom was not as welcoming as Kongo; it viewed the Portuguese with great suspicion and as an enemy. The Portuguese in the latter part of the 16th century tried to gain control of Ndongo but were defeated by the Mbundu. Ndongo experienced depopulation from slave raiding. The leaders established another state at Matamba, affiliated with Queen Nzinga, who put up a strong resistance to the Portuguese until coming to terms with them. The Portuguese settled along the coast as trade dealers, not venturing on conquest of the interior. Slavery wreaked havoc in the interior, with states initiating wars of conquest for captives. The Imbangala formed the slave-raiding state of Kasanje, a major source of slaves during the 17th and 18th centuries