[Sivad]

I suspect that every human who walks the earth is descended from both slaves and slave masters. Since white Europeans got in the slave business relatively recently in terms of history and because that enterprise has been well documented by white Europeans, the world gets to ignore the transgressions of others and pretend that only white people
have committed that detestable act.

[Zhang Fei]

http://en.wikipedia.org/wiki/Slavery_in_medieval_Europe

Slavery’s been around a long time. For individuals on the losing end of military campaigns, offensive or defensive, it was the alternative to summary execution.