Theology of New World Conquest, Part I

Authority to conquer the New World by European colonial powers was granted by the highest levels of the Christian world. Pope Nicolas V granted Spain and Portugal the right to conquer and enslave the Amerindians (savages who practiced human sacrifice):
“We grant you [Kings of Spain and Portugal] by these present documents, with our Apostolic Authority, full and free permission to invade, search out, capture, and subjugate the Saracens and pagans and any other unbelievers and enemies of Christ wherever they may be, as well as their kingdoms, duchies, counties, principalities, and other property [...] and to reduce their persons into perpetual slavery.” (Dum Diversas. Nicolas V. June 15, 1452) 
God told the Israelites to genocide the pagan Canaanites who practiced similar, probably less severe, atrocities. The Europeans were generous, our ancestors introduced these depraved nations to Christ.

SOURCE: Hayes, Diana. 1998. "Reflections on Slavery." in Curran, Charles E. Change in Official Catholic Moral Teaching.