Ex parte Milligan, 71 U.S. (4 Wall.) 2 (1866), was a landmark decision of the US Supreme Court that ruled the application of military tribunals to citizens when civilian courts are still operating is unconstitutional.
Justice David Davis, who delivered the majority opinion, stated that "martial rule can never exist when the courts are open" and confined martial law to areas of "military operations, where war really prevails", and when it was a necessity to provide a substitute for a civil authority that had been overthrown