De Haro v. United States was a case heard by the Supreme Court of the United States in 1866. The case involved a dispute between two parties over ownership of land located on San Juan Island, Washington Territory. De Haro claimed that he had purchased the land from its original owner and held title to it, while the United States argued that they had acquired title through an executive order issued by President James Buchanan in 1859 which established military reservations on both San Juan and Lopez Islands. The court ultimately ruled in favor of De Haro, holding that his claim was valid under Spanish law as well as American common law principles governing adverse possession claims; thus establishing him as rightful owner of the property at issue. This decision affirmed private landownership rights against government encroachment and set an important precedent for future cases involving similar disputes over public lands within U.S territories or states

In De Haro v. United States, the Supreme Court was tasked with determining whether a Mexican citizen who had been living in California since 1848 could be considered an American citizen and thus entitled to certain rights under the Treaty of Guadalupe Hidalgo. The majority opinion held that he was not an American citizen because he did not meet any of the criteria for citizenship established by Congress at that time. However, Justice Field dissented from this ruling on two grounds: firstly, that there were no laws or treaties preventing him from becoming a naturalized U.S. citizen; and secondly, that his long-term residence in California should have made him eligible for citizenship under common law principles of domicile and allegiance which predated Congressional legislation on the matter. He argued further that denying De Haro's claim would violate both international law as well as due process protections provided by the Fifth Amendment to all persons within U.S territory regardless of their nationality status