the region with European manufactured goods, which competed with domestic industry (especially textiles). When we add these costs to the unspeakable horror experienced by scores of African people on the Middle Passage and on the plantations of the New World, we can see that the slave trade was truly an absolute catastrophe. Ultimately, the trade ruined many of the monarchs who had actually benefited from it, as European companies constructed large slaving fortresses along the coast to control the trade directly.

The slave trade weakened most states of Africa, though it did help some of them.  Let us see why this is the case.

When Europeans came to Africa to take slaves and bring them to the Americas, they did not simply come ashore and start raiding to capture as many slaves as they could.  This would have been difficult for them to do with the small numbers of Europeans who could come on any given ship.  Instead, the Europeans worked with Africans to get slaves.  The African states on the coasts typically raided inland, took slaves, and sold them to the Europeans.

Of course, this helped the coastal kingdoms.  They became rich from the proceeds of their sales.  However, it was not at all helpful to the inland states.  This is because it depopulated them and, in particular, because it took many of their most important people.   When slavers came raiding, they of course took the people in the prime of life who could fetch the best prices.  This meant that the best workers and warriors were being taken.  This clearly hurt the economies of the inland states.  As huge numbers of people were taken away, the social and political fabric naturally suffered.  It is hard to maintain a society or a political structure when large numbers of people are being forcibly removed from that society and political structure.

Thus, the inland states of Africa were badly harmed because they lost so much of their population, particularly those who were at a stage in life when they were most important to the economy, the society, and the political system.

It is generally estimated that around twelve million African slaves crossed the Atlantic during the height of the African slave trade. This massive number excludes the millions who were sold into slavery within Africa and those who were sold in Asia. Because the Atlantic slave trade (as part of the triangular trade with Europe and the Americas) saw the highest volume of African slaves, West Africa saw considerable negative effects.

The volume of people sold into slavery impacted African societies by stunting both diplomatic and technological development. The financial payoff was so substantial that millions of able-bodied men and women were hunted down and removed from their communities to be sold. African communities survived without some of their most capable contributors for hundreds of years. As a result, they fell behind as other countries around the world saw agricultural and industrial revolutions. The slave trade also kept rival communities at odds as they worked against each other to profit from the slave industry (most slaves were prisoners of war).

Ultimately, from a long-term perspective, the lack of development and stability over the centuries made it easier for European nations to colonize the vast majority of Africa, hampering African cultural growth and freedoms.

The slave trade had a variety of effects on the economy of West Africa. Starting in the early sixteenth century, between 12 million to 15 million West Africans were sold into bondage over the next four hundred years. The differences in the economic effects varied mostly by what role different groups of West Africans played in the process of human trafficking.

The vast majority of West African slaves were sold to Europeans by other Africans. Many coastal tribes grew exceedingly wealthy through the slave trade. Most slaves were captured by these tribes during wars. Some wars were fought exclusively for the purpose of acquiring prisoners to sell in the slave ports. As coastal tribes grew wealthier from the sale of Africans from the interior, they were able to finance even larger armies and slave-capturing forces.

African rulers and merchants in these areas found it necessary to safeguard their ability to capture and sell Africans from the interior. They often did not allow Europeans to venture beyond the port towns to capture their own slaves. In fact, this was convenient for the Europeans as well, who found it more profitable to supply other Africans with the weapons and means to capture potential slaves than to do it themselves.

The slave trade introduced many European wares to West Africa. As West African rulers grew wealthier through the direct trade of slaves or through taxing slave merchants who passed through their kingdoms, they became increasingly fond of European imports to Africa, such as cloth, jewelry, and weapons.

While coastal areas of West Africa prospered greatly through the slave trade, it devastated the economy of the interior. Although all people were targets, slave catchers tended to focus on kidnapping young men who could fetch a higher price in the slave ports. This practice, especially after a large raid, could wipe out the entire work force of a tribe at once. This led many communities to starve or disintegrate entirely as individuals sought shelter in the wilderness or sought to join other communities. As a result, the slave trade led to a huge decrease of the local population.

Overall, the slave trade had a negative impact on the economy of West Africa. Individual kingdoms may have benefitted financially from the trade, but overall, it was a huge loss for the region. Since human trafficking involved disruptive conflicts, the destuction of communities, and the removal of a large amount of the workforce, regional trade routes and partnerships were very unstable. A successful economy needs peace and stability to thrive. The chaos brought about by the slave trade greatly held back any economic enterprise not directly related to human trafficking. The impacts of the slave trade were so far-reaching that many economic historians believe that the region has yet to recover from it.

The effects of the Atlantic slave trade on West African societies were complex and characterized by change over time. At first, the slave trade was actually controlled by West African kings, who offered enslaved people instead of the gold that many early European voyagers sought to trade for. Over time, as the kingdoms of Europe established colonies overseas, the increased demand for labor that accompanied this transformation altered the trade. The slave trade enriched the kings of such empires as the Kongo, but it did so at a massive cost. It led to endemic warfare in the region, as Europeans sought captives to enslave. It later led to major social stresses, as village life was often disrupted by slave gangs who kidnapped young people for sale. Over time, European powers sought to conduct the trade on their terms, building large fortresses that served as the centers of slave trade and the ports of departure for millions of unfortunate people. One very significant way that the trade affected Africans was by causing a major gender imbalance in African towns and villages. Since slavers favored male captives, fewer were left behind. This led to polygamous marriage practices, one significant way in which traditional African lifeways were disrupted. The slave trade also deprived West African kingdoms of healthy young laborers, which not only ripped families and villages apart, but also inhibited the development of the region's economy. 

Between the fifteenth and nineteenth centuries, an estimated 12 million Africans were enslaved and transported across the Atlantic Ocean. Socially, this process had a dramatic impact on Africa's population because it removed a huge number of men and women. By 1850, for example, the population of Africa was 25 million people but historians believe that it would have been double this number, had slavery not taken place. 

For those left behind, slavery had important economic consequences too. Because so many able-bodied men and women were transported abroad, Africa did not have enough workers and entrepreneurs to bring about an agrarian revolution and, in the longer-term, to industrialise. A number of Africans turned instead to working in the lucrative slave-trade and this created important political consequences because traders found it harder to enslave people during peace time. Slave traders, therefore, created political rifts and conflicts to ensure a steady supply of prisoners of war who could be sold into slavery. In Ghana, for example, the rise of slavery coincided with the introduction of gunpowder and explosives to the country. 

The answer to this question covers such a vast expanse of time, territory, and peoples, that in this particular forum I will only be able to give you the broad strokes—however, I will also gladly provide you with some references for further research.

In the first place, the slave trade caused the involuntary export of anywhere from 8 million to 20 million human beings, in a period of time stretching from the 1400s, which were the earliest days of the Atlantic slave trade, all the way up to the mid-nineteenth century—and this range of numbers represents a conservative estimate. To get an idea of the scope of this number of human lives, the population of the United States as a whole at the outset of the American Civil War was in the neighborhood of 31 million people. To say that the removal of tens of millions of men, women, and children would have a deleterious effect upon the social development is a gross understatement. In many sub-Saharan African nations, it is hypothesized that the extraction of such huge numbers of people had such a deleterious effect upon the economy of those nations, that they never recovered from it; it is speculated that because of this, the course of the development of those nations was irrevocably transformed.

Secondly, the social development of African as a continent of nations, the social development within those nations, and the perception of African and persons of African descent, was indelibly changed by the slave trade. The precise effects are a matter of some debate, as are the precise numbers of human beings taken into bondage in Africa. However, Walter Rodney, who was a prominent Guyanese historian and prolific commentator on the effects of the African slave trade, stated that the population of Africa stagnated for centuries following the inception of the slave trade, while the populations of Europe and the Americas continued to increase.

Politically, it has been speculated that the slave trade caused factionalism within ethnic groups and thus irrevocably changed the structure of power in many African nations. Historical consequences of war between factions, such as enslaving one’s enemies, now became profitable as these enemies could then be sold to European slavers. This may have further destabilized the political structure in many areas, because it may have provided an impetus for conflict. Another tragic consequence is the perception of Africa and African peoples by outside nations and peoples. Africa, a continent vast in territory and ancient in history, with rich cultures and civilizations, artistic and technological history, became a source of human slaves who were seen by outsiders and by slave traders and those societies dependent on slave labor, such as the American South, as being not even human. Laws codified this racism: the Three-Fifths Compromise of 1787 during the United States Constitutional Convention was designed to level the number of representatives, and thus, the amount of influence, that Southern states had. For the purposes of determining numbers of representatives, Southern delegates preferred that their huge numbers of slaves be counted as population, which would have provided the South with more representatives. However, slaves were property and not even thought to be human, and therefore had no voting rights. The compromise was that only three-fifths of the slaves could be counted for population figuring purposes and thus for the distribution of representatives of the Southern states. Events such as this led to a tragic result: once enshrined in law, the fight for equality for black Americans, and the ripple effects of institutionalized racism, with all of its attendant human brutality and misery, is suffered in the United States to this day.

As I said, the question of slavery and its effects is hundreds of years old, and affected millions of people of many very different cultures and nations in the continent of Africa. For more information, I encourage you to look further into the works of Walter Rodney, in particular, his book How Europe Underdeveloped Africa; and also into the delightful Ebony and Ivy: Race, Slavery, and the Troubled History of America’s Universities by Craig Steven Wilder, among many, many other books.

I also encourage you to explore the resources of the brilliant Charles H. Wright Museum of African American History in Detroit, Michigan, which can be found online at thewright.org.

I hope this helps.

In my view, the best source to consult on this is C.L.R. James's The Black Jacobins. Please pay particular attention to the first chapter, "The Property."

James uses the lectures of the late anthropologist and African scholar Emil Torday to assess the violence wrought on African societies by the Atlantic slave trade. He posits that, according to Torday, "in the sixteenth century, Central Africa was a territory of peace and happy civilisation [sic]" (James 7). Other historical sources, including John Hope Franklin's From Slavery to Freedom, place the beginning of the slave trade in the early fifteenth century. However, demand for slaves was not as high at this time due to the plentiful cheap labor offered by British indentured servants at this time.

James discredits historical revisionists who claim to have rescued Africans from tribal warfare by bringing them to the New World. He also counters those who cite systems of slavery which existed in West and Central Africa to excuse or diminish the inhumanity of the Atlantic slave trade: "It was on a peasantry in many respects superior to the serfs in large areas of Europe, that the slave-trade fell" (James 7). 

The strongest and healthiest men and women were sold to European traders. This was the likely cause of tribal life being "broken up," resulting in "millions of detribalised [sic] Africans [being] let loose upon each other" (James 7). The destruction of communities allowed for recklessness. Violence and ferocity survived due to weakened tribes and poor leadership. The horrors did not end there:

The unceasing destruction of crops led to cannibalism; the captive women became concubines and degraded the status of the wife. Tribes had to supply slaves or be sold as slaves themselves (7).

With that last sentence, James takes on a more sympathetic view of decisions made by tribal chiefs and kings. Recent historians, such as Henry Louis Gates, Jr., have been more critical. Arguably, the truth is somewhere in between. Certainly, there were those who were blinded by greed, who coveted the gold and guns that European traders brought in exchange for human chattel. However, as James argues, there were probably also those who feared the Europeans' navies and weaponry -- those who offered men, women, and children not out of cruel avarice but for survival.

The United States and Great Britain ended the African slave trade in 1807, though slaves continued to be traded throughout the British colonies and in the United States. Other European countries, such as Spain, continued to kidnap and import slaves to their colonies, though some shipments were derailed by British navies. 

Despite the end of the slave trade early in the nineteenth century, the exploitation of Africa -- this time, all over the continent -- would continue during the Age of Imperialism. By the 1870s, European nations, particularly Britain, began exploring Africa for mineral resources which would help fuel and supply their industries. The Berlin Conference of 1884-85, also known as the West Africa Conference or the Congo Conference, regulated colonization between European nations (i.e., allowed nations to divide Africa according to their own industrial needs), and helped lead to the emergence of Germany as a colonial power.

While it is certainly true that the African slave trade greatly diminished African societies -- some, according to James, as far south as Mozambique -- the exploitation of the continent did not end there.

Source: James, C.L.R. The Black Jacobins. New York: Vintage Books. 1989. Print.

One effect was felt directly in Africa—the European need for chattel slavery made the slave trade grow exponentially.  African tribes searched for more and more slaves and became dependent on European trade goods.  This would eventually impoverish Africa, making it easier to colonize in the nineteenth century.  While slavery had existed in Africa well before the time of Europeans, Europeans made slavery generational.  

African slavery also made large-scale agriculture in the Americas practical.  Native Americans were quick to run away from their European masters, and English indentured servants often died of mosquito-borne diseases.  The Europeans used Africans because they were not familiar with the territory and, therefore, were less of a flight risk.  Also, Africans were less likely to catch diseases like malaria.  African labor made the sugarcane and tobacco fields possible, thus making a small number of whites very wealthy and enriching the imperial powers who owned the colonies.  

Finally, the transatlantic slave trade enriched many port cities.  Before the Revolutionary War, New York and Boston were common ports for slave catchers to stop and sell their cargo.  This increased business around the cities, as the trade needed auctioneers to sell the slaves.  There was also a need for taverns and boardinghouses for people coming from out of town to buy slaves.  In the early days of America, the slave trade was a major part of the national economy, though some were complaining about the social issues that slavery raised even back then.