In perhaps its most noteworthy decision, the Chase Court ruled in Ex Parte Milligan (1866) that Langdon Milligan could not be tried by a military tribunal instead of fully-functioning civil courts. Milligan and several other individuals in the Order of American Knights were arrested on charges of conspiring to steal firearms at a federal arsenal and free Confederate prisoners in Indiana. Milligan was arrested and sentenced to death by a military tribunal. Milligan petitioned a federal court for habeas corpus, asking it to rule that a military tribunal did not have the authority to try Milligan.

The Court unanimously ruled that Milligan should have been tried in a civilian, instead of military, court. In his majority opinion, Justice David Davis reasoned that civil courts were the proper forum when a defendant’s constitutional rights to trial by jury, grand jury indictment, and other protections are present. “Martial rule can never exist where the courts are open, and in the proper and unobstructed exercise of their jurisdiction,” he wrote.

Justice Salmon Chase filed a concurring opinion joined by three other justices that agreed with the result but applied different reasoning. Chase reasoned that Milligan should be released based on statutory, not constitutional, grounds. He wrote that the Habeas Corpus Act of 1863 provided that the proper forum for Milligan’s trial was a civil, not military, court. “The act of Congress of March 3d, 1863, comprises all the legislation which seems to require consideration in this connection,” Chase wrote.