United States v. Hartwell was a United States Supreme Court case that dealt with the issue of whether a federal court had the power to issue a writ of habeas corpus to a prisoner held in military custody. The case arose when the United States government sought to have a prisoner, William Hartwell, released from military custody. Hartwell had been arrested by the military for desertion and was being held in a military prison. The government argued that the federal court had the power to issue a writ of habeas corpus to order Hartwell's release. The Supreme Court held that the federal court did not have the power to issue a writ of habeas corpus to a prisoner held in military custody. The Court reasoned that the writ of habeas corpus was a civil remedy and that the military was not subject to civil law. The Court also noted that the writ of habeas corpus was a remedy for those who were unlawfully detained and that the military had the power to detain those who had violated military law. The Court's decision in United States v. Hartwell established that the federal court did not have the power to issue a writ of habeas corpus to a prisoner held in military custody. The decision also established that the military had the power to detain those who had violated military law. This decision has been cited in numerous subsequent cases and has been used to support the idea that the military is not subject to civil law.

In United States v. Hartwell, the Supreme Court was asked to decide whether a vessel that had been seized by the government for violating customs laws could be sold without first obtaining a judicial order of condemnation. The majority opinion held that such vessels could be sold without an order of condemnation, as long as there were reasonable grounds to believe they had violated customs laws. Justice Field dissented from this decision and argued that it would lead to arbitrary seizures and sales of property by executive officers who lacked any legal authority or oversight in making these decisions. He further argued that allowing such seizures and sales would violate both due process rights under the Fifth Amendment and basic principles of fairness enshrined in common law traditions which required some form of judicial review before private property can be taken away from its owner.