Foster v. Neilson

Foster (plaintiff) claimed Louisiana land based on a grant from a Spanish governor, but Neilson (defendant), the land's possessor, argued the grant was invalid because the territory had already passed to France and then the United States before the grant was made; the political branches of the U.S. government had already taken the position that the territory belonged to the U.S. at the relevant time, a determination the Court felt bound to follow. Foster separately argued that Article 8 of the 1819 Treaty of Amity between Spain and the U.S., which stated that Spanish land grants made before January 24, 1818 "shall be ratified and confirmed" to their possessors, itself validated his grant without any further action needed.