“Reports of Spanish mistreatment of the New World natives prompted a severe crisis of conscience among significant sectors of the Spanish population in the sixteenth century, not least among philosophers and theologians.” – Thomas E. Woods, Jr., How the Catholic Church Built Western Civilization [p. 136]
This was a new thing in world history. Before the rise of Catholic nation states, nobody much cared how conquerors treated the conquered. Might made right, as they say. It was standard practice for the mighty invaders to loot and pillage the natives, to slaughter those who resisted, to burn their property, and to enslave the survivors.
And, while the slaughter of New World natives was a heinous evil, the fact is that the foundations of international law were laid in the sixteenth century because Spanish Catholics were appalled about it.
“Laws governing the interaction of states had remained vague throughout the years, and had never been articulated in any clear way. The circumstances arising from the discovery of the New World gave impetus to the study and delineating of those laws … when theologians applied themselves to a serious reckoning with these issues. … Here again does the Catholic Church give birth to a distinctly Western idea.” — – Thomas E. Woods, Jr., How the Catholic Church Built Western Civilization [p. 136-137]