How did warfare and existing slavery in africa contribute to the rise of american slavery

The conflict and existence of slavery in Africa, particularly on the west coast, facilitated the importation of slaves to New World. Rather than having to invade Africa and attempt to take people by force, European slave traders were able to, "legally" but no less horrendously, negotiate the purchase of already-enslaved peoples from other Africans. This made repeat-purchases much easier, and helped to increase the scale of the transportation of slaves across the Atlantic. 

This is not to say that Europeans never took African slaves by force, but a great number were purchased as slaves from African slave-owners or people hostile to the enslaved groups.