Lambdin Purdy Milligan was an American lawyer and farmer who was the subject of Ex parte Milligan 71 U.S. 2 (1866), a landmark case by the Supreme Court of the United States. He was known for his extreme opinions on states' rights and his opposition to the Lincoln administration's conduct of the American Civil War.

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 this particular case, the Court was unwilling to give former President Abraham Lincoln's administration the power of military commission jurisdiction, 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.