Smith v. Sac County was a United States Supreme Court case that was decided in 1870. The case involved a dispute between the plaintiff, Smith, and the defendant, Sac County, over the ownership of a piece of land. Smith claimed that he had purchased the land from the county in 1867, but the county argued that the sale was invalid because it had not been approved by the county board of supervisors. The Supreme Court ruled in favor of Smith, finding that the county had no authority to invalidate the sale. The Court held that the county had no power to interfere with the sale, as it had been made in accordance with the law and had been approved by the county clerk. The Court also held that the county was not entitled to any compensation for the land, as it had not been legally acquired. The decision in Smith v. Sac County established that a county cannot invalidate a sale of land that has been made in accordance with the law and approved by the county clerk. It also established that a county is not entitled to any compensation for land that has not been legally acquired. This decision has been cited in numerous subsequent cases involving the sale of land.

In Smith v. Sac County, the Supreme Court was asked to decide whether a county in Iowa had the authority to issue bonds for railroad construction. The majority opinion held that it did not have this power and that such an action would be unconstitutional. However, Justice Field dissented from this decision, arguing that counties should have the right to issue bonds if they are authorized by state law and if there is sufficient public interest in doing so. He argued further that states should be allowed some flexibility when it comes to matters of local concern like transportation infrastructure projects since these decisions often require more localized knowledge than could be obtained at a federal level. Ultimately, he concluded that while Congress may regulate interstate commerce through its constitutional powers over foreign affairs and war-making, individual states must retain their own sovereignty when it comes to internal improvements within their borders unless expressly prohibited by Congress or otherwise forbidden by the Constitution itself.