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The threshold question before us is whether the Executive has the authority to detain citizens who qualify as "enemy combatants." There is some debate as to the proper scope of this term, and the Government has never provided any court with the full criteria that it uses in classifying individuals as such. It has made clear, however, that, for purposes of this case, the "enemy combatant" that it is seeking to detain is an individual who, it alleges, was " 'part of or supporting forces hostile to the United States or coalition partners' " in Afghanistan and who " 'engaged in an armed conflict against the United States' " there. Brief for Respondents 3. We therefore answer only the narrow question before us: whether the detention of citizens falling within that definition is authorized.
The Government maintains that no explicit congressional authorization is required, because the Executive possesses plenary authority to detain pursuant to Article II of the Constitution. We do not reach the question whether Article II provides such authority, however, because we agree with the Government's alternative position, that Congress has in fact authorized Hamdi's detention, through the AUMF.
Our analysis on that point, set forth below, substantially overlaps with our analysis of Hamdi's principal argument for the illegality of his detention. He posits that his detention is forbidden by 18 U. S. C. §4001(a). Section 4001(a) states that "[n]o citizen shall be imprisoned or otherwise detained by the United States except pursuant to an Act of Congress." Congress passed §4001(a) in 1971 as part of a bill to repeal the Emergency Detention Act of 1950, 50 U. S. C. §811 et seq., which provided procedures for executive detention, during times of emergency, of individuals deemed likely to engage in espionage or sabotage. Congress was particularly concerned about the possibility that the Act could be used to reprise the Japanese internment camps of World War II. H. R. Rep. No. 92-116 (1971); id., at 4 ("The concentration camp implications of the legislation render it abhorrent"). The Government again presses two alternative positions. First, it argues that §4001(a), in light of its legislative history and its location in Title 18, applies only to "the control of civilian prisons and related detentions," not to military detentions. Brief for Respondents 21. Second, it maintains that §4001(a) is satisfied, because Hamdi is being detained "pursuant to an Act of Congress"--the AUMF. Id., at 21-22. Again, because we conclude that the Government's second assertion is correct, we do not address the first. In other words, for the reasons that follow, we conclude that the AUMF is explicit congressional authorization for the detention of individuals in the narrow category we describe (assuming, without deciding, that such authorization is required), and that the AUMF satisfied §4001(a)'s requirement that a detention be "pursuant to an Act of Congress" (assuming, without deciding, that §4001(a) applies to military detentions).