Morgan v. United States was a case heard by the United States Supreme Court in 1875. The case involved a dispute between the United States and the Morgan Iron Company over the ownership of a parcel of land in the state of California. The Morgan Iron Company 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 Morgan Iron Company had no valid claim to the land. The Court reasoned that the Treaty of Guadalupe Hidalgo was a valid treaty between the United States and Mexico, and that the United States had acquired the land through the treaty. The Court also held that the Morgan Iron Company had no valid claim to the land because it had purchased the land from the Mexican government after the United States had acquired the land through the treaty. The Court's decision in Morgan v. United States established the principle that the United States had acquired the land through the Treaty of Guadalupe Hidalgo and that the Morgan Iron Company had no valid claim to the land. This decision has been cited in numerous subsequent cases involving the ownership of land acquired by the United States through treaties with foreign governments.

In Morgan v. United States, the Supreme Court was asked to decide whether a defendant could be convicted of two separate offenses when they were committed in one act. The majority opinion held that the defendant could indeed be convicted of both offenses as long as each offense required proof of an element which the other did not. Justice Field dissented from this decision and argued that it violated fundamental principles of justice by allowing for multiple punishments for a single act or omission. He reasoned that if Congress had intended to impose such punishment, it would have done so explicitly rather than leaving it up to judicial interpretation. Furthermore, he argued that punishing someone twice for one crime is contrary to common sense and morality since no person should suffer more than once for any given wrong-doing regardless of how many different statutes are broken in its commission.