I am not taking about modern tax slavery, bank slavery or marriage slavery. I am talking about true slavery where people are worked to death and traded as livestock. If we do not want to see a world wide return of slavery, we most consider possible hypothetical scenarios where it could return.
First lets look at all the ways that people were traditionally turned into slaves. In the Ancient world if one city/state went to war with another, the loser would be enslaved. The logic behind this was that the loser was damned by the Gods. The thinking at the time was that such people damned by the Gods should be put to death (men, women and children). An exception was made for those to be used as slaves. In the Ancient World Slaves were considered dead people. Hence they had no rights as live human beings.