McCormick v. Knox 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 the petitioner, John McCormick, was arrested by federal officers in the state of Pennsylvania and held in federal custody. The state court issued a writ of habeas corpus to the federal officers, ordering them to produce McCormick before the court. The federal officers refused to comply with the writ, arguing that the state court did not have the authority to issue such a writ. 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 power to interfere with the federal government's authority to detain a prisoner. The Court also noted that the writ of habeas corpus was a fundamental right, and that the state court could not interfere with the federal government's exercise of that right. In conclusion, 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 power to interfere with the federal government's authority to detain a prisoner.

Justice Field delivered the dissenting opinion in McCormick v. Knox, arguing that the majority's decision was contrary to established precedent and would lead to a dangerous expansion of federal power. He argued that Congress had no authority under Article I of the Constitution to pass legislation regulating labor disputes between private parties, as it had done with this case. Furthermore, he contended that if such regulation were allowed, then there would be no limit on what Congress could do in terms of interfering with state laws and regulations regarding matters traditionally within their jurisdiction. Justice Field concluded by noting his belief that allowing such an expansive interpretation of congressional powers would ultimately weaken states' rights and undermine our system of dual sovereignty between state and federal governments.