Spanish conquest of the Aztec Empire
The conquistadors rewarded people who had helped them defeat the Aztecs.  Many received an Encomienda, a village full of Aztecs who were forced to work for them.  This was not much different from what many serfs had done during the Aztec Empire.  However, workers were badly abused, and many died.  Because of this, a Spanish bishop named Bartolomé de las Casas suggested using African slaves to work in New Spain instead.  Later, when he saw how much worse African slaves were treated, las Casas changed his mind about this.