Dair v. United States was a case heard by the United States Supreme Court in 1872. The case involved a dispute between the United States and the Dair family over the ownership of a tract of land in the state of California. The Dair family had purchased the land from the Mexican government in 1845, but the United States claimed that the land had been acquired by the United States through the Treaty of Guadalupe Hidalgo in 1848. The Supreme Court held that the United States had acquired the land through the Treaty of Guadalupe Hidalgo and that the Dair family had no valid claim to the land. The Court also held that the United States had the right to take possession of the land without compensating the Dair family. This decision established the principle that the United States could acquire land from foreign governments without compensating the original owners.

In the case of Dair v. United States, the Supreme Court was tasked with determining whether a contract between two parties could be enforced by one party against another in spite of an intervening act of Congress that rendered it illegal. The majority opinion held that such contracts were unenforceable and thus dismissed the suit brought by Dair against the United States for breach of contract. In his dissent, Justice Field argued that although Congress had passed legislation making such contracts illegal, this did not necessarily mean they should be considered void or unenforceable as well. He reasoned that if a person entered into a contract before any law was enacted to make it illegal then he should still have recourse to seek damages from its breach even after said law is passed. Furthermore, Field argued that allowing these types of contracts to remain enforceable would encourage people to enter into them knowing full well their legality may change at some point in time due to legislative action; however, if those same individuals knew their contractual rights would become nullified upon passage of new laws then there would be no incentive for anyone ever entering into such agreements again in good faith and commerce would suffer greatly as a result.