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; . . .
The Alien Enemy Act of 1798 authorized the President to deport any alien or to license him to reside within the United States at any place to be designated by the President. 1 Though critical of the measure, many persons conceded its constitutionality on the theory that Congress’s power to declare war carried with it the power to treat the citizens of a foreign power against which war has been declared as enemies entitled to summary justice. 2 A similar statute was enacted during World War I 3 and was held valid in Ludecke v. Watkins. 4
During World War II, in Ex parte Quirin, the Court unanimously upheld the power of the President to order to trial before a military tribunal German saboteurs captured within the United States. 5 Chief Justice Stone found that enemy combatants, who without uniforms come secretly through the lines during time of war, for the purpose of committing hostile acts, are not entitled to the status of prisoners of war but are unlawful combatants punishable by military tribunals. Because this use of military tribunals was sanctioned by Congress, the Court has found it unnecessary to decide whether
the President may constitutionally convene military commissions 'without the sanction of Congress’s in cases of 'controlling necessity.' 6