Ex parte Milligan, 71 U.S. (4 Wall.) 2 (1866), was a United States Supreme Court case that ruled that the application of military tribunals to citizens when civilian courts are still operating is unconstitutional. It was also controversial because it was one of the first cases after the end of the American Civil War.
Read more about Ex Parte Milligan: Background of The Case, Arguments, The Court's Decision, Concurrence