South Boston Iron Company v. United States was a case heard by the United States Supreme Court in 1885. The case involved a dispute between the South Boston Iron Company and the United States government over the ownership of a certain piece of land. The South Boston Iron Company had purchased the land from the state of Massachusetts in 1871, but the United States government claimed that the land was part of a military reservation and thus belonged to the federal government. The Supreme Court ultimately sided with the United States government, ruling that the land was part of a military reservation and thus belonged to the federal government. The Court held that the United States government had the right to take possession of the land, as it was part of a military reservation and thus was subject to the exclusive jurisdiction of the federal government. The Court also held that the South Boston Iron Company had no right to the land, as it had not acquired title to the land from the United States government. The decision in this case established the principle that the United States government has the right to take possession of land that is part of a military reservation, even if the land has been purchased by a private party. This principle has been applied in numerous cases since then, and is still in effect today.

In South Boston Iron Company v. United States, the Supreme Court was asked to decide whether a contract between the two parties that had been made in violation of an existing statute could be enforced by either party. The majority opinion held that it could not, as such contracts were void and unenforceable due to their illegality. However, Justice Field dissented from this opinion on the grounds that while Congress had certainly intended for its statutes to be followed and obeyed, there should still remain some degree of protection for those who have entered into contracts in good faith prior to learning about any violations or irregularities with them. He argued that if no such protection existed then individuals would face significant financial losses simply because they were unaware of a law at the time they signed a contract - something which he felt was unjustified given their lack of fault or negligence in entering into it.