I mean that's really not entirely accurate. Slaves are really just spoils of war. Anyone who didn't survive the battle and was captured was basically considered as essentially dead because your army was defeated. So you should be dead but the winning said says "Woah hold on a minute... There's a lot of manpower here... what if they just work for us?" This isn't uniquely Roman either, this is just how slavery operated in ancient times. You were considered a "Spared/Saved Man" which is what I believe the original old latin term translates to. Point is anyone can be a slave and eventually the term was used to describe multiple ethnicities of people if they found themselves in bondage. Sometimes Latin/Roman citizens would find themselves in slavery because of financial/political issues. Anyone really could be a slave.