Tribes would make the tribes into slaves after a battle or just on a raid. They enslaved anyone they wanted to.
One thing that made the Five Civilized Tribes civilized was the fact that they owned slaves. The Cherokee I know for a fact, held black slaves because when emancipation was ordered they offered freedom or tribal citizenship many chose the tribe. I think the tribe recently tried to remove the black Cherokees but were thwarted in their effort.
What about the Aztecs? Being their war slaves had particular, let’s say even heart-felt, consequences.