The Maxwell Land-Grant Case was a landmark decision by the United States Supreme Court in 1887. The case involved a dispute between the United States and the heirs of a Mexican land grant. The grant had been made by the Mexican government in 1841 to Lucien Maxwell, a prominent American trader. The grant was for a large tract of land in what is now New Mexico. The dispute centered on the question of whether the United States had acquired title to the land through the Treaty of Guadalupe Hidalgo, which ended the Mexican-American War in 1848. The Supreme Court held that the United States had acquired title to the land, and that the heirs of Maxwell had no claim to it. The Court reasoned that the Treaty of Guadalupe Hidalgo had extinguished any rights that the Mexican government had to the land, and that the United States had acquired title to it. The decision in the Maxwell Land-Grant Case was significant because it established the principle that the United States could acquire title to land through treaties with foreign governments. This principle has been applied in numerous cases since then, and has been a cornerstone of American foreign policy.

The Maxwell Land-Grant Case was a dispute between the United States and New Mexico over land grants made by the Mexican government prior to its cession of territory to the U.S. in 1848. The Supreme Court held that, under international law, title to lands granted by one sovereign nation cannot be recognized as valid when they are within territories subsequently acquired by another sovereign nation without express recognition or confirmation from the latter. In his dissenting opinion, Justice Field argued that it would be unjust for an individual who had purchased land in good faith from a foreign government not to have their rights respected simply because of subsequent changes in sovereignty. He noted that such individuals should not suffer losses due to events beyond their control and proposed instead that Congress pass legislation confirming titles already existing at the time of acquisition so long as those titles were based on valid grants issued before cession occurred.