Lambdin Milligan and others were antiwar Democrats living in the North during the Civil War. They were arrested in 1864 in Indiana and charged with aiding the enemy. Under President Abraham Lincoln’s executive order, they were tried and convicted in military courts and sentenced to death by hanging. Milligan challenged the conviction in the U.S. Supreme Court, asking it to decide if the president had sole authority to create military courts and try citizens who could not appeal in a civilian court.
In 1866, after the Civil War ended, the Supreme Court ruled in favor of Milligan. The Court says in Ex parte Milligan that the president does not have authority to try civilians in military courts if civilian courts are open and functioning.