During the Civil War, the United States was torn by conflicting loyalties. The question of States' rights versus national supremacy was finally decided by a military conflict which left 600,000 Americans dead. Persons living in the so-called border States along the northern boundary of the Confederacy, associated in business and friendship with persons in States just to the South, were particularly divided in their attitude toward the war. Maryland teetered on the brink of secession throughout the conflict; West Virginia was created as a State out of pro-Union counties in northwestern Virginia. Kentucky was a battleground, as was Missouri, where many people owned slaves.
Because of these unsettled conditions, President Abraham Lincoln ordered the border States placed under military rule as part of the war emergency. Each of the States was organized as a military district, with military courts established to hear cases related to espionage, rebellious action, and the giving of aid and comfort to the Confederacy. Regular State and federal courts were made subordinate to the military district courts in these matters. Peacetime guarantees of due process were suspended in many cases, and arrested persons were denied the use of writs of habeas corpus which required that a detained person be brought before a court to hear the reason why he or she was imprisoned.
In 1861, early in the war, Chief Justice Roger B. Taney and the Court overturned President Lincoln's emergency actions in a case entitled Ex parte Merryman. Subsequently, Congress passed the Habeas Corpus Act in 1863, which gave Lincoln broad discretionary power, including the power to suspend habeas corpus when and where he felt he should.
Lambden P. Milligan was arrested in Indiana in 1864 upon the orders of one General Hovey, commander of the military district of Indiana. He was accused of conspiring against the United States and "giving aid and comfort to its enemies" the South. He was tried in a military court created under Lincoln's orders. The court convicted Milligan of various acts of treason and disloyalty and sentenced him to death by hanging on May 19, 1865. Millligan appealed to the federal courts, claiming that his right to a trial by jury had been denied and that the proceedings of the military court were unconstitutional. Lower federal courts refused to issue an order to bring the case into the regular federal court system and the case was finally appealed to the Supreme Court.
The cased addressed questions about suspending the rights of citizens in time of war and the legitimate authority of the President to suspend writs of habeas corpus. What limits did the Constitution contain concerning the suspension of civil liberties of citizens? Did the National Government have the authority to substitute trial by military courts created in States outside the area of military operations? Did the military courts have constitutional power and authority to try and punish Milligan?
For Milligan: Congress under no circumstances has the authority to deprive citizens of due process. The Constitution's due process guarantees, other than habeas corpus, are absolute and not in any way qualified by circumstances or situation. Milligan should have been tried in the regular courts of the United States. The trial was illegal and the sentence should be overturned.
For the United States: A nation at war must take extraordinary measures to insure the safety of its citizens and government. Milligan was engaged in acts subversive to good order and was giving aid and comfort to the States in rebellion. The Constitution directly states that the writ of habeas corpus may be suspended in time of war. Extraordinary times and great danger require extraordinary measures and actions. Milligan, a convicted traitor, should hang.
In a narrow 5–4 decision, Justice David Davis delivered the opinion of the Court overturning Milligan's conviction. He was joined in a concurring opinion by Chief Justice Chase, and Justices James M. Wayne, Noah H. Swayne, and Samuel F. Miller.
Under no circumstance, Davis wrote, "does either the President or Congress have the legal right to impose military justice on civilians outside an actual war zone…"
Davis affirmed the primacy of the Bill of Rights, regardless of the circumstances, noting: "[B]y the protection of the law, human rights are secured; withdraw the protection, and they [human rights] are at the mercy of wicked rulers, or the clamor of an excited people…The Constitution of the United States is a law for rulers and people, equally in war and in peace, and covers with the shield of its protection all classes of men, at all times, and under all circumstances.
"One of the plainest constitutional provisions was…infringed when Milligan was tried by a court not ordained and established by Congress, and not composed of judges appointed during good behavior…," the Court declared.
Davis then moved to the circumstances surrounding the case: "It is difficult to see how the safety of the country required martial law in Indiana. If any of her citizens were plotting treason, the power of arrest could secure them until the government was prepared for the trial…It is not easy to see how [Milligan] can be treated as a prisoner of war, when he lived in Indiana for the past twenty years, was arrested there, and had not been…[a] resident of any of the States in rebellion."
The Milligan decision clearly limited the powers of the President and Congress in time of war. The authority of military courts during the time of war was never tested in cases involving trials in the South by such courts after the military occupation of the Southern States during Recontruction.