Ex Parte Quirin

The military trial went forward, even while the Supreme Court considered their petitions. By 27 July 1942, all the evidence had been submitted at the military trial. The case was closed except for the arguments of prosecutors and defense lawyers. On 28 July 1942, the Supreme Court met in special session to consider the saboteurs' petitions. (This was an ex parte proceeding because the petitioners were not present, having been confined in military prisons.) The Court rejected all eight. Six of the prisoners were executed a little more than a week later.

The Court did not consider the guilt or innocence of the alleged saboteurs. The only issue in the case was the authority of the military tribunal. In an opinion finally released 29 October 1942, Chief Justice Stone, writing for a unanimous Court, cited the president's powers as commander-in-chief to uphold the creation of a military tribunal in times of war. The legitimacy of the tribunal was also supported by congressional legislation authorizing military trials for those accused of violating the law of war. Although they did not have a right to ordinary civilian procedures such as a grand jury hearing or trial by jury, defendants such as the eight German saboteurs do have the right to have the judicial denial of petitions for writs of habeas corpus reviewed by federal appellate courts, including the Supreme Court.

In 1866, in Ex parte Milligan, a case resulting from restrictions on civil liberties during the Civil War, the Supreme Court ruled that military courts have no jurisdiction over civilians in time of war in areas where civil courts remain open. Lambdin P. Milligan was a northerner with Confederate sympathies who attempted to seize arms from federal arsenals and to liberate Confederate soldiers from northern prisons. Now, in Quirin, the Court distinguished its earlier ruling by declaring that the German saboteurs, unlike Milligan, were "enemy belligerents":

The spy who secretly and without uniform passes the military lines of a belligerent in time of war, seeking to gather military information and communicate it to the enemy, or an enemy combatant who without uniform comes secretly through the lines for the purpose of waging war by destruction of life or property, are examples of belligerents who are . . . offenders against the law of war subject to trial and punishment by military tribunals.