United States v. McMasters 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 state court. The case arose when a prisoner, McMasters, was held in a state prison in Pennsylvania. McMasters sought a writ of habeas corpus from the federal court, claiming that he was being held in violation of the Constitution. The Supreme Court held that the federal court did not have the power to issue a writ of habeas corpus to a state court. The Court reasoned that the writ of habeas corpus was a remedy that was available only to federal courts, and that the federal court did not have the power to interfere with the state court's decision. The Court also noted that the writ of habeas corpus was a remedy that was available only in cases where the prisoner was being held in violation of the Constitution. The Court concluded that the federal court did not have the power to issue a writ of habeas corpus to a state court, and that the prisoner must seek relief from the state court.

In United States v. McMasters, the Supreme Court was asked to decide whether a federal court had jurisdiction over a case involving an alleged violation of state law in which both parties were citizens of the same state. The majority opinion held that federal courts did not have jurisdiction in such cases and dismissed the appeal. However, Justice Field dissented from this decision on two grounds: firstly, he argued that Congress had given federal courts concurrent jurisdiction with state courts over all civil actions arising under any laws made by Congress; secondly, he argued that even if there was no express grant of authority for such cases, it should be implied as necessary to protect individuals' rights against violations of their constitutional rights or other acts prohibited by law. He concluded his dissent by stating that "the power conferred upon [federal] Courts is ample to enable them to enforce obedience" when needed and thus should not be denied simply because both parties are citizens of the same state.