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.
The Discovery Doctrine is a concept of public international law expounded by the United States Supreme Court in a series of decisions, intially in Johnson v. M’Intosh in 1823. The doctrine was Chief Justice John Marshall’s explanation of the way in which colonial powers laid claim to newly discovered lands during the Age of Discovery. Under it, title to newly discovered lands lay with the government whose subjects discovered new territory. The doctrine has been primarily used to support decisions invalidating or ignoring aboriginal possession of land in favor of colonial or post-colonial governments.
John Marshall, who is most credited with describing the doctrine, did not voice wholehearted support of the doctrine even while using it to justify judicial decisions. He pointed to the doctrine as simple fact, looking at the possession-takings which had been supported by it as things which had occurred and had to be recognized. The supposedly inferior character of native cultures was a reason for the doctrine having been used, but whether or not that was justified was not relevant for Marshall.
This Doctrine governs United States Indian Law today and has been cited as recently as 2005 in the decision City Of Sherrill V. Oneida Indian Nation Of N.Y.