In Castro v. United States, the Supreme Court ruled that a contract between two parties was binding and enforceable even if it violated an existing law. The case involved a dispute over the ownership of land in California which had been granted to one party by Mexico before the Mexican-American War. During this war, Congress passed legislation prohibiting any grants from being made after its commencement date; however, both parties argued that their agreement should be enforced regardless of this law since they entered into it prior to its passage. Ultimately, the court held that contracts are valid and must be honored despite any subsequent laws or regulations which may conflict with them. This ruling established an important precedent for future cases involving contractual disputes where conflicting laws were at issue.

In Castro v. United States, the Supreme Court was asked to decide whether a Mexican citizen who had been living in California since 1853 and became a naturalized U.S. citizen in 1860 could be convicted of treason against the United States for joining an armed rebellion against it during the Civil War. The majority opinion held that he could not because his allegiance to Mexico ended when he became a naturalized U.S. citizen and thus there was no dual loyalty or divided allegiance between two countries at play here; however, Justice Field dissented from this ruling on several grounds including that Congress had never passed any law allowing foreign-born citizens to become full citizens without taking an oath of renunciation of their former country's sovereignty first, which Mr Castro did not do before becoming a U.S Citizen in 1860 so therefore his original citizenship with Mexico still remained intact despite being granted American Citizenship as well and thus making him liable for treason under both countries' laws due to his involvement in the rebellion against America during the civil war conflict