Ex parte McNiel was a United States Supreme Court case that dealt with the issue of habeas corpus. The case involved a man named McNiel who was arrested and held in custody without a warrant. McNiel filed a petition for a writ of habeas corpus, which is a legal document that requires a person to be brought before a court to determine if they are being held lawfully. The Supreme Court ruled that the writ of habeas corpus should be granted, and that McNiel should be released from custody. The Supreme Court held that the writ of habeas corpus should be granted because the arrest was made without a warrant. The Court reasoned that the writ of habeas corpus was a fundamental right that should be protected, and that the arrest was made without due process of law. The Court also held that the writ of habeas corpus should be granted because the arrest was made without probable cause. The decision in Ex parte McNiel established the principle that the writ of habeas corpus should be granted when a person is arrested without a warrant and without probable cause. This decision has been cited in numerous cases since then, and it has been used to protect the rights of individuals who are arrested without due process of law.

In Ex Parte McNiel, the Supreme Court was asked to decide whether a writ of habeas corpus should be granted in order for an individual to be released from custody. The majority opinion held that the writ should not be granted because it had been issued by a state court without jurisdiction over the matter. However, Justice Field dissented and argued that while he agreed with the majority's conclusion on jurisdictional grounds, he believed that there were other reasons why habeas corpus relief should have been provided in this case. He noted that if Congress had intended to limit federal courts' power to issue such writs then they would have done so explicitly rather than leaving it up to interpretation. Furthermore, Field argued that denying habeas relief could lead individuals who are wrongfully imprisoned or detained into lengthy legal battles as they attempt to prove their innocence before being released from custody; thus making justice more difficult and expensive for those seeking redress through judicial means.