It is not commonly appreciated that the first target of slavery in West Africa were the Moors (the Portuguese and the Spanish referred to all Muslims regardless of racial differences as Moors). A description of the first raids has come down to us through the writings of the Portuguese writer Azurara. In 1441, a certain young Portuguese captain Golcalves sailed along the coast of southern Morocco and Mauritania gathering ivory, animal hide and sea lion oil for sale in Lisbon. In a chance encounter, he met up with a Muslim couple, wounded the man with a javelin and took them both aboard ship as slaves. At that time the jurisdiction over the Portuguese colony of Tangier was with Prince Henry, an enthusiastic supporter of a naval thrust along the Atlantic seacoast to outflank the Maghrib. The couple was presented to Henry. Sensing an opportunity to capture more slaves, he authorized an ambitious raid the same year under a seasoned and experienced captain Tristao who was familiar with the Atlantic coast of West Africa.
Captains Golcalves and Tristao netted more than a dozen Muslims and enslaved them. Elated, Henry wrote to Pope Eugene IV who gave a decree that capturing the Moors as slaves was a part of the Crusade and whoever sailed south in this pursuit would receive ablution of his sins (1442). This was the origin of the slave trade, which began with Portuguese piracy on the Moroccan coast in 1441. The process was systematized in 1444 when the Portuguese Lagos Company was chartered under the patronage of Prince Henry.
An extract from Prof. Dr. Nazeer Ahmed, PhD on the Trans-Atlantic Slave trade and how it relates to Muslim people and Muslim history in Africa.
ETA: The Portuguese explorer named in this post is Antão Gonçalves, the first recorded European to buy and bring enslaved Africans to Europe.dynamicafrica)