Ex parte Fonda 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 a federal prison. The case arose when a prisoner, John Fonda, was held in a federal prison in New York. Fonda sought a writ of habeas corpus from the state court, claiming that he was being held in violation of his constitutional rights. 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 a federal prison. The Court reasoned that the writ of habeas corpus was a federal remedy, and that the state court did not have the power to interfere with the federal government's authority to imprison individuals. The Court also noted that the writ of habeas corpus was a remedy that could only be used to challenge the legality of a person's detention, and not to challenge the conditions of the detention. The Court's decision in Ex parte Fonda established that state courts do not have the authority to issue writs of habeas corpus to prisoners held in federal prisons. This decision has been cited in numerous subsequent cases, and has been used to support the principle that state courts cannot interfere with the federal government's authority to imprison individuals.

In Ex Parte Fonda, the Supreme Court was tasked with determining whether a writ of habeas corpus should be issued to release a prisoner from custody. The majority opinion held that the writ should not be granted because it did not appear that there had been any abuse or neglect of duty on behalf of the jailer in keeping him detained. Justice Field dissented, arguing that under existing law and precedent, if an individual is being unlawfully restrained by another person without authority from any court or other competent tribunal then they are entitled to relief via habeas corpus. He argued further that since no evidence had been presented as to why this particular prisoner was being detained, he should have been released upon application for a writ of habeas corpus. In conclusion, Justice Field believed that due process requires more than just mere allegations when someone is seeking their freedom through such means and thus found fault with the majority's decision in this case.