The Arab leaders made deals with the African chiefs to enslave tribal mmembers for tribute or other rewards.
BTW, slavery is still practiced in only muslim nations today with Africa's Sudan enslaving many Black Christians in the name of allah.
Even more ironic is that Christianity was adopted by blacks long before it spread to Western Europe. The Ethiopian Orthodox Church was founded bare years after the Crucifixion. And the Ethiopians were never slave traders. So you have a religion with long roots in Africa, and instead of adopting it, black militants pursue a religion that has traditionally served to enslave blacks.