When the colonial powers arrived in West Africa, it was an unmitigated disaster. Three centuries of slave trading would follow, colonial borders were imposed, and western style governments instituted, superimposed over hundreds of years of tribal culture and customs. British policy was to "make the world England".
Since then, African societies have largely been under siege, first with colonial dominance, and since with the artificial borders and modern weapons. Europe has certainly done Africa few favors, and West Africa in particular.
Because of the slave trade, the balance of power shifted in this region. The areas on the coast that took slaves and sold them to Europeans became richer. Meanwhile, the nations from which the slaves were taken became much weaker politically as well as economically as they lost huge numbers of people in the prime of life.
It is impossible to separate early European intervention into African affairs without discussing the slave trade, as this was the catalyst for that intervention. The slave trade became quite lucrative not only for Europeans but for African tribesman as well. Many times, tribesmen waged war on other tribes for no other purpose but to find captives to be sold into slavery. A devastating social effect was a substantial decline in the male population, as males were the most desirable for the trade.
Because of contact with Europeans, a number of Africans, particularly in the old kingdom of Kongo became Christian. Christianity appealed to the kings of Kongo as it supported monarchical rule; and Jesuit missionaries drew strong parallels between Christian saints and the spirits of traditional African religions. The capital of Kongo, Mbasa, called San Salvador by the Portuguese had so many churches in the sixteenth century that it was known as "Kongo of the Bell."
The Portuguese tended to interfere in local political matters, which did not endear them to local chieftains. On one occasion, they defeated a Kongolese army and decapitated the King. Thereafter, seeking further trade opportunities, they intervened in Ndongo, which they renamed Angola, from the title of the local king, ngola.