Doctrine of Discovery
Papal Bulls of the 15th century gave Christian explorers the right to claim lands they "discovered" and lay claim to those lands for their Christian Monarchs. Any land that was not inhabited by Christians was available to be "discovered", claimed, and exploited. If the "pagan" inhabitants could be converted, they might be spared. If not, they could be enslaved or killed.
This concept of a public international law was used to determine the decision of the Supreme Court case, Johnson v. McIntosh (1832)
View the video links to learn more about the Doctrine of Discovery
Steve Newcomb at the Indigenous Peoples Forum on the Doctrine of Discovery, Arizona click for video
Doctrine of Discovery video