In 1720 CE, during the peak era of the European trans-atlantic slave trade, Agaja, king of Dahomey raided and sacked all slave trading forts that were set up on the coastal areas of his kingdom by Europeans. King Agaja burnt everything down and blocked off supply routes to the hinterlands. Slavery was in conflict with Dahomey's development.
Europeans were bitter. They tried to sponsor Africans they had made slave dealers, against King Agaja. "They failed to unseat him or to crush the Dahomey kingdom. But in turn, king Agaja failed to persuade the Europeans to introduce a new line of trade or other economic activities such as local plantations agriculture. Being eager to acquire firearms, Agaja agreed to resume the slave trade in 1730 CE." - (Walter Rodney, How Europe Underdeveloped Africa).
During the time of King Wegbaja in the 17th century CE, European often preferred to obtain a shipload of captured and enslaved men and women from one spot. Dahomey was in constant conflict with the Oyo kingdom of the Yoruba people, which was in present-day Western Nigeria. And this meant that there was always a teeming number of Africans captured on both sides and given to Europeans in exchange for firearms and junks from Europe (This is the reason why millions of Yoruba speaking people are found today in Brazil, Trinidad and Tobago etc. In modern day Argentina, the population of these Africans were wiped out in the 19th century CE, that is why there is no black Argentine today but there are black Brazilians).
Sometimes one horse was exchanged for 15 captives from these battles. In other times, horses were gotten from the Hausa traders from the Savannah of present-day northern Nigeria, in exchange for enslaved persons, who were sold to merchants in Kano, who in turn sold these persons to the Arab traders along the trans-saharan trade routes.
Europeans often miscalculated how long it would take Dahomey to gather a shipload of captured men and women, hence the chained, enslaved persons often exhausted the food supplies on the coast. Thousands died of starvation in this way. King Wegbaja thought out a scheme that often estranged descendants of enslaved men and women today. He began sacrificing these men and women to a deity he conceived. He convinced his subjects that he was in that way, doing the captives a favour, instead of allowing the chained men and women to die of starvation. These were their enemies... Besides, he told his subjects that the sacrifices will ensure the perpetual power of their kingdom over rivals like the Oyo kingdom to their east(which the Dahomey people hated) and the Ashanti kingdom to the west.
This was the root of mass human sacrifice that is often attributed to the Dahomey kingdom and Voodoo.
As research indicates, there were enslaved men and women(mostly those from Ketu, a town of the Oyo kingdom, near Abeokuta) who would express thankful relief when European slave ships arrived; 'okpe oo!' was the shouts in the Yoruba language, amidst cries as the sacrifices were halted on sighting slave ships. But it most be noted too that, it was the Yoruba kingdom of Oyo that first made Dahomey a tributary state of their Kingdom and demanded bond servants as tribute to the Oyo crown. Dahomey broke free eventually and declared war on Oyo kingdom, which was eclipsed by the the conquest of both by the British and the French.
Obviously, Europeans saw these bloodied scenes on some occasions, giving rise to popular conclusions that Europeans saved Africans from mass sacrifices and that it was the responsibility of the African chiefs... that "Europeans merely turned up to buy the captives." This was captured in the book 'Sins of our fathers' where the author wrote that it was many Europeans who persuaded him to state clearly that the trans-atlantic slave trade was the responsibility of the African chiefs. In response to this, Historian, Walter Rodney stated; "...as though, without the European demands, there would have been captives sitting on the beaches in their million" waiting to be chained into the slave ships.
Image: King Agaja and his guards of Female warriors that were known for their fierceness and courage. They were called the "Mino" (meaning, 'our mothers' in the Fon language).
The History of Africa magazine