McManus v. O'Sullivan et al. 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 who was being held in federal custody. The case arose when a prisoner, McManus, was arrested in the state of New York and held in federal custody. McManus then filed a petition for a writ of habeas corpus in the state court, seeking to be released from federal custody. The state court granted the writ, and the federal government 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 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 should not be allowed to 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 McManus v. O'Sullivan et al., arguing that the Court should not have reversed the decision of the Supreme Judicial Court of Massachusetts. He argued that, under Article IV Section 2 Clause 1 of the Constitution (the "Full Faith and Credit Clause"), a state court's judgment must be given full faith and credit by other states unless there is some valid reason to deny it. In this case, he argued, there was no such valid reason; thus, he believed that Massachusetts' judgment should stand as final and binding on all parties involved. Furthermore, Justice Field noted that if a party believes they were wronged by a state court's ruling then they can appeal to higher courts for relief instead of relying on another state to reverse or modify it - which would set an undesirable precedent for future cases involving interstate disputes.