In Ex Parte Milligan (1866) 71 U.S. 2, a landmark decision, a citizen of the United States who had been tried, convicted and sentenced to death by military commission for conspiracy and subversive measures against the federal government, applied for habeas corpus. He had at all times been a resident of the loyal state of Indiana, which was not at the time under occupation by any hostile troops, although it had been previously invaded and was then threatened with invasion.
The Supreme Court held that military authorities were without power to try civilians, relying primarily on the fact that courts-martial dispense with the right of trial by jury guaranteed by Article 3, section 2, clause 3 of the Constitution.
It was settled in Ex parte Milligan, that a state of war, in the absence of some occasion for the declaration of martial law or conditions consequent on military operations, gave no power to the military authorities where the civil courts were open and capable of performing their duties, to disregard their authority or frustrate the exercise by them of their normal and legitimate jurisdiction, it is indeed open to grave doubt whether it was the purpose of Congress, by the words 'except in time of war,' or the cognate words which were used with reference to the jurisdiction conferred in capital cases, to do more than to recognize the right of the military authorities, in time of war, within the areas affected by military operations or where martial law was controlling, or where civil authority was either totally suspended or obstructed, to deal with the crimes specified - a doubt which, if resolved against the assumption of general military power, would demonstrate, not only the jurisdiction of the state courts in this case, but the entire absence of jurisdiction in the military tribunals.
The Supreme Court said that the framers of the Constitution meant to limit the right to jury trial, as guaranteed in the Sixth Amendment, to those persons who, under the Fifth, could be held to answer for infamous crime only on presentment or indictment of a grand jury.
The Supreme Court, in discussing the 5th Amendment, said:
"The discipline necessary to the efficiency of the army and navy, required other and swifter modes of trial than are furnished by the common law courts; and, in pursuance of the power conferred by the Constitution, Congress has declared the kinds of trial, and the manner in which they shall be conducted, for offences committed while the party is in the military or naval service." The Court stated that in any legal sense, 'action,' 'suit' and 'cause' are convertible terms. The court then defined the terms to mean any legal process which a party institutes to obtain his demand or by which he seeks his right. This broad meaning of the word, 'cause,' comports with the interpretation given by other courts and legal writers in the period when our present Constitution was drafted.
Chief Justice Samuel P. Chase, said:
"There are under the Constitution three kinds of military jurisdiction: one to be exercised both in peace and war; another to be exercised in time of foreign war without the boundaries of the United States, or in time of rebellion and civil war within states or districts occupied by rebels treated as belligerents; and a third to be exercised in time of invasion or insurrection within the limits of the United States, or during rebellion within the limits of states maintaining adhesion to the National Government, when the public danger requires its exercise. The first of these may be called jurisdiction under Military Law, and is found in acts of Congress prescribing rules and articles of war, or otherwise providing for the government of the national forces; the second may be distinguished as Military Government, superseding, as far as may be deemed expedient, the local law, and exercised by the military commander under the direction of the President, with the express or implied sanction of Congress; while the third may be denominated Martial Law Proper, and is called into action by Congress, or temporarily, when the action of Congress cannot be invited, and in the case of justifying or excusing peril, by the President, in times of insurrection or invasion, or of civil or foreign war, within districts or localities where ordinary law no longer adequately secures public safety and private rights."