Ex parte Loring is 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, William Loring, was held in federal custody in Massachusetts. Loring sought a writ of habeas corpus from the state court, claiming that he was being held in violation of the Constitution. The Supreme Court held that the state court did not have the authority to issue the writ, as the power to issue writs of habeas corpus was exclusively vested in the federal courts. The Court reasoned that the power to issue writs of habeas corpus was a federal power, and that the state court did not have the authority to interfere with the federal government's power to detain prisoners. The Court also noted that the state court's writ of habeas corpus would have interfered with the federal government's power to enforce the laws of the United States. The Court concluded that the state court did not have the authority to issue the writ of habeas corpus, and that the prisoner must be released from federal custody.

In Ex Parte Loring, the Supreme Court was asked to decide whether a writ of habeas corpus should be issued in order for an individual to challenge his arrest and detention. The majority opinion held that the writ should not issue because it would interfere with military operations during wartime. Justice Field dissented from this decision, arguing that no matter what the circumstances are, individuals have a right to challenge their arrests and detentions through habeas corpus proceedings. He argued that if Congress had intended otherwise they would have passed legislation suspending or limiting these rights during times of war. Furthermore, he noted that even when martial law is declared by executive authority alone there must still be some form of judicial review available so as not to leave citizens without any recourse against arbitrary power exercised by government officials.