The situation of Portugal in coastal Africa from ~1430-1700 did not resemble Spanish and Portuguese conquest of the Americas from 1500-1700.
The Spanish were able to conquer the peoples of the Caribbean, Mesoamerica and the Inka empire (and the Portuguese able to conquer Brazil) because of 3 major factors:
The effective use of alliances and local political rivalries to gain influence. (i.e. Cortez making an alliance with Tlaxcalans against Aztecs)
Demographic collapse of indigenous societies in Americas because of novel diseases combined with violence. (i.e. a smallpox outbreak during the siege of Tenochtitlan. Deaths of Taino people because of enslavement and disease. Quechua worked to death in mines in Andes.)
Military advantage of having access to cavalry, firearms and cannon, steel weaponry and armor. All of which the peoples of the Americas lacked.
The Portuguese could not rely on these factors in their encounter with African societies. Where the Americas did not have the capacity to smelt and forge iron, African societies did use iron tools and weapons before contact with Europeans.
In the West African savanna, horse cavalry was already a well-known method of warfare, and cavalry did make its way to the Oyo empire in what is now southwestern Nigeria (i.e. Yoruba territory).
Alaafin Oyo c. 1910
On the other hand, there were areas where horses were novel, like Congo, Angola, Namibia, Zimbabwe.
And the Portuguese did possess the advantage of firearms/artillery that were unknown to the African societies they encountered.
However, the Portuguese didn’t hold much military technological advantage in conflicts with African polities. We see this when the Portuguese attempt to conquer up the Zambezi valley in order to gain control over the gold mines of the Monomotapa kingdom (successor of Great Zimbabwe). Monomotapa was able to resist this invasion and force a Portuguese expeditionary force of 1500 men to retreat.
Ditto, when the Portuguese first tried to establish a colony at Luanda in 1579, they faced powerful opposition from the kingdom of Ndongo which controlled the area around Luanda. The Portuguese experienced military defeat by Ndongo armies, and it was only the intervention of Kongolese allies that saved the situation for Portugal.
In a later war against Queen Njinga of Ndongo and Matamba in the 1640s, the Portuguese again experienced significant setbacks. Queen Njinga made diplomatic overtures to the Dutch, who were incoming commercial rivals to the Portuguese. The Ndongo-Dutch alliance managed to capture Luanda and hold it from 1641-1648 (when the Portuguese/Kongo alliance recaptured it).
Another major difference was that unlike the Americas, African societies did not suffer major demographic collapse in the wake of contact. Partly, this was because diseases like smallpox, cholera, etc. were already part of the African disease environment. Partly, diseases like Sleeping Sickness, Malaria, Dengue fever were novel and debilitating to Europeans.
However, many historians of early-colonial Americas have stressed that demographic decline happened because of disease and violence/genocide together. Populations which are already stressed by food scarcity, or violence are more susceptible to disease. As I already said, the Portuguese saw repeated military defeat (along with some military victories) in battles with Kongo, Ndongo, Monomotapa. There never really arose a situation for conquest and disease to combine to reduce Kongo or Mutapa populations by 90% as seen in the Americas.
Which brings us to alliances and exploiting political rivalries. To be honest, the Portuguese did try to employ this tactic. I already mentioned the Portuguese alliance with Kongo against Dutch-Ndongo alliance. Furthermore, the Portuguese would sometimes intervene in succession disputes and civil wars in Kongo, backing whichever claimant seemed most favorable to Portuguese interests.
In fact, the initiative to try and found the colony of Luanda/Angola in 1579 happened in the context of a succession dispute in Kongo. The Portuguese decided to take advantage of Kongolese being preoccupied with that conflict to expand their power in Luanda, which Kongo considered part of its sphere of influence (and Ndongo also felt was part of their sphere of influence).
But, on the coast of West Africa, the Portuguese mostly constrained themselves to leasing territory on the coast from local rulers and constructing forts at Elmina, Axim and Ouidah. These forts were really only toe-holds on the coast, and were very dependent on the good will of local rulers, who could effectively cut off trade and supplies of food and water to these forts if conflict arose.
In the Indian Ocean, the Portuguese enjoyed a short period (circa 1500-1525) where they were a new interloper, able to impose their power on Swahili city states, trade with India, and plunder muslim shipping.
After the Ottomans conquered Mamluke Egypt in 1517, they became a growing naval threat to Portuguese power in the Indian Ocean. This rivalry caused the Portuguese to construct Fort Jesus in Mombasa to protect their naval supply lines on the Swahili coast.
Fort Jesus, Kenya’s Portuguese Fortress.
But this competition and Portuguese disruption of existing trade patterns meant general decline in Swahili trade with Persian Gulf and India and shrank Portuguese influence to Fort Jesus, Sofala, etc.
Just as the Portuguese had Dutch and English competition in the Atlantic, they quickly faced Ottoman and later Busaidi Omani competition in the Indian ocean.
So, basically, the situation that happened from 1850-1900 where European powers enjoyed a pronounced military advantage and quickly conquered and partitioned the continent of Africa was not possible from 1450-1700. The profound divergence in commercial/miltary/technological power that enabled the partition of Africa arose as a consequence of the wealth flows from New World colonies to Western Europe and the subsequent commercial and industrial revolution.
Also, Albert Kwadwo Adu Boahen has made the point that one of many factors that encouraged European imperialism in the 19th/20th century was metropolitan population growth and malthusian world-view. The British, French, German and Italian political leadership saw growing populations in the 1800s which they feared would lead to “overpopulation”. There were also large flows of immigrants from Germany, Ireland, and Italy to the North and South American republics.
The solution, according to Boahen, was the creation of settler colonies. Italy could hold out Libya or Eritrea as territories where “surplus population” of the Italian countryside could be resettled, but remain as Italian nationals in the colonies.
There was not this worldview in 16th and 17th century Portugal. There was not the dilemma of “too much population growth”. There was already favorable land in Brazil that could be taken from indigenous South Americans, for settler colonialism. There just weren’t these pressures of land-hunger, and the disease environment of tropical Africa made settlement unattractive in the 1500s and 1600s. It was only with medical advances in the 1800s in managing tropical disease that it became plausible for the large-scale resettlement of Europeans in tropical Africa became considered healthy.
Warfare in Atlantic Africa; 1500-1800 by John K. Thornton discusses Portuguese wars against Ndongo and Kongo in the 1500s and 1600s. Thornton stresses that Portuguese weaponry and military formations did not grant much/any advantage against Kongo or Mbundu military traditions.
Empires of the Weak by J. C. Sharman offers a revisionist account of the “military revolution” in Europe. The thesis of his book is that Europeans enjoyed naval power and control over seaborne trade. African and Asian rulers were content with land-based power and power over people, and so did not consider Europeans a threat to be driven away. Which allowed the development of European hegemony in the 1700s and 1800s.
Wars of Imperial Conquest in Africa- 1830-1914 by Bruce Vandervort. He devotes the first two chapters to describing the military/commercial/technological situation at the outset circa 1830 and how the industrial revolution and revolution in weaponry really changed the ability of Europeans to conquer African states.
African Perspectives on Colonialism devotes the first (or second?) chapter to the period of colonial conquest circa 1850-1900, and emphasizes the degree to which African soldiers were involved and the rationales of African leaders to accommodate or resist European hegemony.
Toby Green has written several books about this early period from 1400-1700, including: Rise of the Trans-Atlantic Slave Trade- 1300-1589, A fistful of Shells and Brokers of Change; Atlantic cultures and commerce in pre-colonial Western Africa which all help explain what Portuguese commercial interests were in Atlantic Africa.
African Kings and Black Slaves by Herman Bennet delves into Iberian worldview in the 1400s and 1500s, and argues forcefully that the Catholic church strongly pushed a view that African kingdoms should be seen as equals of a European kingdom. And argues that European ideas of social/technical inferiority arose in 17th and 18th centuries, obscuring earlier more egalitarian worldviews.
Machines as Measure of Men by Michael Adas treads on some of the same ground as Bennet. Adas argues that initially, Europeans viewed Christianity as the defining difference separating them from others. He argues it was only in the 1600s on that Europeans considered themselves more technologically and commercially (and societally) advanced.
Credit: Josue Dennis Chance from Quora