There is no bar to this Nation’s holding one of its own citizens as an enemy combatant. In Quirin, one of the detainees, Haupt, alleged that he was a naturalized United States citizen. 317 U.S., at 20. We held that “[c]itizens who associate themselves with the military arm of the enemy government, and with its aid, guidance and direction enter this country bent on hostile acts, are enemy belligerents within the meaning of … the law of war.” Id., at 37—38. While Haupt was tried for violations of the law of war, nothing in Quirin suggests that his citizenship would have precluded his mere detention for the duration of the relevant hostilities. See id., at 30—31. See also Lieber Code, ¶153, Instructions for the Government of Armies of the United States in the Field, Gen. Order No. 100 (1863), reprinted in 2 Lieber, Miscellaneous Writings, p. 273 (contemplating, in code binding the Union Army during the Civil War, that “captured rebels” would be treated “as prisoners of war”). Nor can we see any reason for drawing such a line here.
A citizen, no less than an alien, can be “part of or supporting forces hostile to the United States or coalition partners” and “engaged in an armed conflict against the United States,” Brief for Respondents 3;
such a citizen, if released, would pose the same threat of returning to the front during the ongoing conflict.
In light of these principles, it is of no moment that the AUMF does not use specific language of detention.
Because detention to prevent a combatant’s return to the battlefield is a fundamental incident of waging war, in permitting the use of “necessary and appropriate force,” Congress has clearly and unmistakably authorized detention in the narrow circumstances considered here.
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