The discovery doctrine, or doctrine of discovery, is a disputed interpretation of international law during the Age of Discovery, introduced into United States municipal law by the US Supreme Court Justice John Marshall in Johnson v. M'Intosh (1823). In Marshall's formulation of the doctrine, discovery of territory previously unknown to Europeans gave the discovering nation title to that territory against all other European nations, and this title could be perfected by possession. A number of legal scholars have criticized Marshall's interpretation of the relevant international law. In recent decades, advocates for Indigenous rights have campaigned against the doctrine. Discovery in international law The means by which a state can acquire territory in international law are conquest, cession by agreement, occupation of land which belongs to no state (terra nullius), and prescription through the continuous exercise of sovereignty. Discovery of a territory creates a mere inchoate title which must be completed within a reasonable period by effective occupation of that territory. Robert J. Miller states that by 1493, "The idea that the Doctrine [of discovery] granted European monarchs ownership rights in newly discovered lands and sovereign and commercial rights over Indigenous peoples due to first discovery by European Christians was now established international law, at least to Europeans." Kent McNeil, however, states, "it is not apparent that such a rule was ever part of the European law of nations." History Miller and others trace the doctrine of discovery back to papal bulls which authorized various European powers to conquer the lands of non-Christians. In 1452, Pope Nicholas V issued the bull , which authorized King Afonso of Portugal to conquer "Saracens, pagans and other enemies of Christ", and "reduce their persons to perpetual servitude" while also taking their land and goods "to convert them to you, and your use, and your successors the Kings of Portugal." In 1455, Pope Nicholas V issued , which extended Portugal's authority to conquer the lands of infidels and pagans for "the salvation of all" in order to "pardon ... their souls". The bull also granted Portugal a specific right to conquest in West Africa and to trade with Saracens and infidels in designated areas. In 1493, following a dispute between Portugal and Spain over the discovery of non-Christian lands in the Americas, Pope Alexander VI issued which drew a north-south line 100 leagues West of the Cape Verde Islands and gave the Spanish Crown exclusive