There is a significant slave population in the United States today. We call them prisoners. The difference of a (pre-modern) slave from a (modern) prisoner, is that prisoners are not much good to anyone at all; including themselves. Some readers might be inclined to equivocate and draw lines between prisoners and slaves to separate them. The reality is that a prisoners are a subset of slaves. All the descriptors of slavery apply to prisoners.
Because I’m not insane, I’m in favor of prisons and jails. Making slaves of criminals (for a time)–i.e., to put them under the legal authority of another more competent, temperate, and law-abiding person to control them–is a superior choice over the alternatives to either execute all criminals or allow them to wander around. It would be superior still to control them to good purposes.
There are also other kinds of slaves, in other institutions, and with varying degrees of personal freedom and worth: juvenile delinquents, the elderly in nursing homes and hospices, those under power of attorney, military personnel, and the mentally ill.
Just as with my contemplation on what fatherhood is (especially from the Biblical perspective): Once I recognized with horror that I was actually in favor of slavery …all grounds for liberalism within my mind were harrowed. An add-on effect is that I understood slavery in the Bible better. It was no longer an unconsidered evil tolerated by benighted desert nomads.
 Under certain circumstances and for certain people and only for certain periods of time.