What is the Doctrine of Discovery?

The Doctrine of Discovery is the collective term for three papal bulls (decrees) written between 1452 and 1495 by Popes Nicholas V and Alexander VI. The Doctrine of Discovery is a set of legal principles that gave authority to Europeans to explore the world and seize anything they found not already claimed by a Christian: land, resources, and people. A translated Doctrine of Discovery excerpt:
“... invade, search out, capture, vanquish, and subdue all Saracens and pagans whatsoever, and other enemies of Christ wheresoever placed, and the kingdoms, dukedoms, counties, principalities, dominions, possessions, and all movable and immovable goods whatsoever held and possessed by them, and to reduce their persons to perpetual slavery, and to apply and appropriate to himself and his successors the kingdoms, dukedoms, counties, principalities, dominions, possessions, and goods, and to convert them to his and their use and profit.”