When Europeans settlers came to the Americans, Native Americans were treated very poorly. They often became slaves, subjected to harsh treatments that could cause death. Many of them also got infected by diseases for which they did not have antibodies, leading to millions of deaths.
These conditions made some Europeans uncomfortable, particularly missionaries. They argued for better treatment of the Native Americas, specially Spanish missionary Bartolomé de las Casas. For this reason, the Spanish crown forbade Native American slavery. However, they were still oppressed by other forms of work that were very much like slavery.
