On February 27, 1863, a Congressional conference committee finalized the Habeas Corpus Suspension Act. Habeas Corpus is the right of a prisoner to challenge the basis of his confinement.
The Constitution prohibits Congress from suspending this right, except in times of rebellion or invasion. On that basis, Congress granted military officials acting on authority of the President the right to detain prisoners indefinitely or until the end of the Civil War.
The act was controversial and several House members attempted to leave the chamber when it was introduced. Nonetheless, the act passed. It was used to detain, among others, former Ohio Congressman Clement Vallandigham, who had been publicly supportive of the Confederacy. In 1866, the Supreme Court declared the use of military tribunals against civilians unconstitutional.