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.
The landmark case stemmed from a trial by a military commission of Lambdin P. Milligan (for whom the case is named), Stephen Horsey, William A. Bowles, and Andrew Humphreys that convened at Indianapolis on October 21, 1864. The charges against the men included, among others, conspiracy against the U.S. government, offering aid and comfort to the Confederates, and inciting rebellion. On December 10, 1864, Milligan, Bowles, and Horsey were found guilty on all charges and sentenced to hang
https://en.wikipedia.org/wiki/Ex_parte_Milligan