The horrors of Europeans and then we Americans invading, conquering, enslaving, and killing millions of indigenous people across the Americas can be directly traced to an ages-old philosophy referred to benignly as "The Doctrine of Discovery."
     The principles (or lack thereof) of the theory are probably as old as humanity, and certainy go back to the Roman Empire of two thousand years ago. More advanced civilizations encountering less advanced ones seem to inevitably and instinctively use brutal practices including warfare, dispossession, and enslavement to assimilate (conquer) the indigenous peoples they meet. "Might is right" exemplifies the doctrine, justifying exploitation of resources and/or taking control of indigenous-claimed land.
     This was certainly true in the European invasions of the Americas starting in 1492, as detailed in Esteban: the African Slave Who Explored America, as well as Manifest Destiny in the United States, the colonization of Africa, and the Christian/Islamic wars by both sides for centuries in the rest of the world. 
    In its essence, the doctrine maintains that any civilization which considers itself more advanced socially or religiously — and most importantly more militarily powerful — than the recently "discovered" country has the right to do whatever is necessary to claim the land and claim the "discovered" land's natural resources and wealth.
     Though in practice for centuries, the doctrine under different names was put into words by Pope Nicholas V  in his 1452 and 1455 papal bulls and later documents declaring that Christian monarchies had a right to conquer and enslave Muslims and other non-Christian people and seize their lands. It became part of commonly accepted international law by the U.S. Supreme Court in 1832.