[gozur88]

I'm not sure you realize just how ubiquitous the institution of slavery was prior to the 19th century. Those "populations being enslaved" themselves had slaves when they were strong enough to take them.

Slaves made empires (true empires) possible because captive people are a net positive for the conquerors. You could conquer your neighbors and... sell them to pay for the war, which isn't the case today. Outside Syria, I guess. When the Roman Empire was at its peak citizens could pay their taxes with only two days of work per year.