Article I, Section 8, Clause 11:
[The Congress shall have Power . . .] To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water; . . .
Military law to the exclusion of constitutional limitations otherwise applicable is the rule in the areas in which military operations are taking place. This view was assumed by all members of the Court in Ex parte Milligan, 1 in which the trial by a military commission of a civilian charged with disloyalty in a part of the country remote from the theater of military operations was held invalid. Although unanimous in the result, the Court divided five-to-four on the ground of decision. The point of disagreement was over which department of the government had authority to say with finality what regions lie within the theater of military operations. The majority claimed this function for the courts and asserted that an area in which the civil courts were open and functioning, and in which there were no hostilities, does not qualify. 2 The minority argued that the question was for Congress’s determination. 3 The entire Court rejected the Government's contention that the President's determination was conclusive in the absence of restraining legislation. 4
Similarly, in Duncan v. Kahanamoku, 5 the Court declared that the authority granted by Congress to the territorial governor of Hawaii to declare martial law under certain circumstances, which he exercised in the aftermath of the attack on Pearl Harbor, did not warrant the supplanting of civil courts with military tribunals and the trial of civilians for civilian crimes in these military tribunals at a time when no obstacle stood in the way of the operation of the civil courts, except, of course, the governor's order.