United States v. Fox is a United States Supreme Court case that was decided in 1876. The case involved a dispute between the United States and the Fox family over the ownership of a tract of land in the state of California. The Fox family had purchased the land from the Mexican government in 1845, but the United States claimed that the land had been acquired by the United States through the Treaty of Guadalupe Hidalgo in 1848. The Supreme Court held that the United States had acquired the land through the Treaty of Guadalupe Hidalgo and that the Fox family had no valid claim to the land. The Court reasoned that the Treaty of Guadalupe Hidalgo was a valid treaty and that the United States had acquired the land through the treaty. The Court also held that the Fox family had no valid claim to the land because they had purchased it from the Mexican government after the United States had acquired the land through the treaty. The decision in United States v. Fox established that the United States had acquired the land through the Treaty of Guadalupe Hidalgo and that the Fox family had no valid claim to the land. The decision also established that the United States had the right to acquire land through treaties and that private individuals could not purchase land from foreign governments after the United States had acquired the land through a treaty.

Justice Field delivered the dissenting opinion in United States v. Fox, arguing that the majority's decision was contrary to both precedent and common sense. He argued that a contract between two parties should be interpreted according to its plain language, not by what one party may have intended or expected when they entered into it. In this case, he believed that the contract clearly stated that if either party failed to perform their obligations under it then all rights of ownership would revert back to the original owner; thus, there could be no transfer of title from one party to another without full performance on both sides. Furthermore, Justice Field noted that while Congress had authority over public lands within its jurisdiction, such power did not extend beyond those boundaries and so could not override an existing private agreement between two individuals as in this case.