Hall et al. 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 Hall family over the ownership of a parcel of land in the state of California. The Halls claimed that they had purchased the land from the Mexican government prior to the Mexican-American War, and thus had a valid title to the land. The United States argued that the land had been acquired by the United States as part of the Treaty of Guadalupe Hidalgo, and thus the Halls had no valid claim to the land. The Supreme Court ultimately sided with the United States, ruling that the Halls had no valid claim to the land. The Court held that the Treaty of Guadalupe Hidalgo had extinguished any prior claims to the land, and that the Halls had no valid title to the land. The Court also held that the Halls had failed to prove that they had purchased the land from the Mexican government prior to the Mexican-American War. As a result, the Court ruled in favor of the United States and held that the Halls had no valid claim to the land.

In Hall v. United States, the Supreme Court was tasked with deciding whether a federal statute that allowed for the sale of public lands to settlers who had improved them could be applied retroactively. The majority opinion held that Congress did have the power to pass such a law and it could apply retroactively, but Justice Field dissented from this decision. He argued that although Congress has broad powers when it comes to disposing of public lands, they do not extend so far as allowing laws like this one which would take away vested rights without due process or compensation. Furthermore, he noted that if such laws were allowed then there would be no limit on what Congress might decide in future cases regarding land ownership and property rights more generally. In his view, these kinds of decisions should only be made prospectively rather than retrospectively since doing otherwise would create an unfair situation where people's expectations are suddenly changed without warning or recourse for those affected by it.