Pierce v. Cox was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue a writ of habeas corpus to a prisoner held in federal custody. The case arose when a prisoner, Pierce, was held in federal custody in the state of Georgia. Pierce sought a writ of habeas corpus from the state court, which the court granted. The federal government then appealed the decision to the Supreme Court. The Supreme Court held that the state court did not have the authority to issue a writ of habeas corpus to a prisoner held in federal custody. The Court reasoned that the writ of habeas corpus was a federal prerogative, and that the state court did not have the authority to interfere with the federal government's power to detain a prisoner. The Court also noted that the writ of habeas corpus was a fundamental right, and that the state court should not be allowed to interfere with the federal government's power to protect this right. The decision in Pierce v. Cox was significant because it established that the federal government had the exclusive authority to issue writs of habeas corpus to prisoners held in federal custody. This decision has been cited in numerous subsequent cases, and it has been used to support the idea that the federal government has the exclusive authority to protect the fundamental right of habeas corpus.

In Pierce v. Cox, the Supreme Court was asked to decide whether a state court had jurisdiction over a case involving citizens of different states. The majority opinion held that it did not have such authority and dismissed the appeal. However, Justice Field dissented from this decision on two grounds: first, he argued that Congress had granted federal courts exclusive jurisdiction in cases between citizens of different states; second, he maintained that even if Congress hadn't done so explicitly, they could still do so under their power to regulate interstate commerce. He concluded by stating his belief that allowing state courts to hear these types of cases would lead to confusion and uncertainty for litigants who might be subject to conflicting judgments depending on which court heard their case first.