United States v. Armijo et al. was a United States Supreme Court case that dealt with the issue of whether the United States had the right to seize property from Mexican citizens in the newly acquired territory of New Mexico. The case arose from a dispute between the United States and the heirs of a Mexican citizen, Francisco Armijo, who had owned a large tract of land in New Mexico. The United States had seized the land in 1848, following the Mexican-American War, and had subsequently sold it to a third party. The Armijo heirs sued the United States, claiming that the seizure of the land was illegal and that they were entitled to compensation for the land. The Supreme Court ultimately ruled in favor of the United States, holding that the United States had the right to seize the land from Mexican citizens in the newly acquired territory of New Mexico. The Court reasoned that the United States had acquired the land through a treaty with Mexico, and that the treaty gave the United States the right to seize the land from Mexican citizens. The Court also held that the Armijo heirs were not entitled to compensation for the land, as the United States had acted in good faith in seizing the land.

In United States v. Armijo et al., the Supreme Court was asked to decide whether a Mexican citizen could be convicted of murder in a U.S. court for killing another Mexican citizen on Mexican soil during the war between Mexico and the United States. The majority opinion held that such an act constituted an offense against both countries, and thus could be tried in either country's courts. However, Justice Field dissented from this decision, arguing that it violated international law by allowing one nation to try citizens of another nation for acts committed within their own borders without any agreement or treaty providing jurisdiction over those persons or offenses. He argued further that if two nations are at peace with each other then they should not have jurisdiction over crimes committed by citizens of one country against citizens of another unless there is some form of mutual agreement granting them such authority; otherwise it would lead to dangerous conflicts between nations as each sought to assert its power over foreign nationals who had broken laws within their respective territories