How has the Supreme Court ruled on military trials of civilians in U.S. history?
Executive summary
The Supreme Court’s long run of decisions has carved a narrow rule: civilians may not be tried by military tribunals when ordinary civilian courts are open and functioning, yet the Court has allowed exceptions for enemy belligerents and has repeatedly delineated the reach of Congress’s Article I power to create military courts — a balancing act evident from Ex parte Milligan through Reid v. Covert and later wartime cases [1] [2] [3].
1. The foundational prohibition: Ex parte Milligan and the post‑Civil War rebuke
In Ex parte Milligan the Court held that military tribunals could not constitutionally try civilians in any jurisdiction where the civil courts were operating, a ruling that established the bedrock principle limiting military jurisdiction over civilians in peacetime [1] [2]; scholars and historical summaries emphasize that Milligan arose from Civil War practice and marked the Court’s rejection of tribunals as substitutes for civilian common‑law processes [4] [5].
2. Wartime exceptions and the Quirin counterpoint
Despite Milligan’s strong language, the Court has carved exceptions for persons treated as enemy belligerents: in Ex parte Quirin and related wartime cases the Court sustained military trials of saboteurs and spies on the theory that unlawful combatants fall outside the civilian‑court rule and may be tried by military commissions under the law of war [2] [1] [5].
3. Civilian dependents and the constitutional floor: Reid v. Covert and Kinsella
In a forceful reaffirmation of civilian procedural rights, Reid v. Covert held that U.S. citizens and civilian dependents could not be tried by court‑martial abroad when their offenses were not within the special necessities of military jurisdiction, insisting that constitutional guarantees like jury trial apply even overseas when civilian courts are available — a holding the Court later reinforced through related decisions such as Kinsella [3] [6] [7].
4. Service‑connection, former service members, and the Toth/O’Callahan line
The Court has also limited military jurisdiction by asking whether an offense is sufficiently “service‑connected”: in O’Callahan the Court ruled that crimes lacking a military nexus could not be prosecuted by court‑martial, and Toth and related opinions barred courts‑martial for former service members whose offenses were not tied to military duties [8] [9]; these cases underscore the Court’s insistence that military justice serves a disciplinary, not general criminal, function [4].
5. Article I authority, judicial deference, and doctrinal tensions
At the same time the Court recognizes Congress’s Article I power “to make Rules for the Government and Regulation of the land and naval Forces” and has acknowledged a distinct military justice system that operates apart from Article III courts, creating recurring tensions between deference to military necessity and protection of civilian constitutional rights [10] [11] [4]; this tension explains divergent rulings and the narrow, fact‑specific contours of exemption for wartime or battlefield contexts [2].
6. Modern aftershocks: post‑9/11 tribunals, enemy combatant jurisprudence, and unresolved lines
The post‑9/11 era revived these doctrines: the Court has accepted government detention and trial power over certain enemy combatants while insisting on some judicial protections (as in Hamdi and related lines summarized by secondary sources), and controversies over Guantánamo and military commissions illustrate that the Milligan‑to‑Covert framework continues to be tested when national security, citizenship, and the availability of civilian courts collide [1] [11]; reporting and institutional histories show the Supreme Court’s approach remains case‑specific and fact‑bound rather than categorical [7] [12].