Ex Parte Milligan
An 1866 Supreme Court decision, Milligan ex parte, 71 U.S. (4 Wall.) 2, 18 L.Ed. 281, recognized that a civilian and citizen of a state that is not invaded by hostile forces during wartime is not subject to the jurisdiction of a COURT-MARTIAL.
In 1864, Lambdin P. Milligan, a civilian, was arrested in Indiana for conspiracy, insurrection, and other crimes arising from his alleged involvement in organizing a secret military unit in the state to assist the Confederacy. His arrest and detention were made pursuant to the orders of General Alvin P. Hovey, commander of the military district of Indiana. He was brought to trial before a military commission in Indianapolis, convicted, and sentenced to death. Milligan applied for a writ of HABEAS CORPUS to the Supreme Court, challenging the jurisdiction of the military commission to try and sentence him.
The Court acknowledged that Article III, Section 2, Clause 3 of the Constitution—which provides "that the trial of all crimes, except in cases of IMPEACHMENT, shall be by jury"—and other constitutional provisions safeguarded this right. It recognized, however, that in times of war, various civil liberties and the right to challenge illegal detention by a writ of habeas corpus may be suspended. MARTIAL LAW might be imposed, however, only where an actual invasion of enemy forces effectively stopped the operation of the civil government.
The military argued that the designation of Indiana as a military district with a commander because of the constant threat of invasion by Confederate troops justified the imposition of martial law. The military commission, therefore, had lawful jurisdiction under the "laws and usages of war." The Court rejected this argument. The state of Indiana had not opposed federal authority, its civil and criminal courts continued to operate during the war, and Milligan was a civilian who was not connected to the military. Although civil liberties and habeas corpus could be suspended in wartime, to permit the military commission to determine the fate of Milligan, a civilian, in a state which was loyal to the Union, and where there was only a mere threat of invasion and the courts were open, would usurp the powers of the courts in violation of the Constitution. The Court decided that the military commission had no jurisdiction over Milligan and therefore ordered Milligan's release.