Trying citizens in military courts is unconstitutional when civilian courts are still operating. Trial by military tribunal is constitutional only when there is no power left but the military, and the military may validly try criminals only as long as is absolutely necessary. Decided April 3, 1866 (argued March 5, 1866). Opinion of the Court: David Davis → go there Topic: Other areas. Ex parte Milligan, 71 U.S. 2 (1866), is a landmark decision of the U.S. Supreme Court that ruled that the use of military tribunals to try civilians when civil courts are operating is unconstitutional. In Ex parte Milligan, the Court was unwilling to uphold former President Abraham Lincoln's assertion of power to use military commissions to try civilian opponents of the war, part of the administration's controversial plan to deal with Union dissenters during the American Civil War. Justice David Davis, who delivered the majority opinion, stated that "martial rule can never exist when the courts are open" and confined martial law to areas of "military operations, where war really prevails", and when it was a necessity to provide a substitute for a civil authority that had been overthrown. Chief Justice Salmon P. Chase and three associate justices filed a separate opinion concurring with the majority in the judgment, but asserting that Congress had the power to authorize a military commission, although it had not done so in Milligan's case. Wiki link