A plurality of the Supreme Court held in 2004 in Hamdi v. Rumsfeld,
164 that the President has the
authority to detain U.S. citizens as enemy combatants pursuant to the AUMF,165 but that the
determination of combatant status is subject to constitutional due process considerations. The
Hamdi plurality was limited to an understanding that the phrase “enemy combatant” means an
“individual who ... 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,”166 but left it to lower courts to flesh out a more precise definition. The U.S. Court of
Appeals for the Fourth Circuit found that the definition continued to apply to a U.S. citizen who
returned to the United States from Afghanistan and was arrested at the airport.167 More recently,
the Fourth Circuit appeared to have expanded the definition of “enemy combatant” to individuals
arrested in the United States on suspicion of planning to participate in terrorist acts without
necessarily having engaged in hostilities in Afghanistan, but this ruling was part of a judgment
that was thereafter vacated by the Supreme Court. (See discussion of Al-Marri, supra.)
In theory, the executive branch could detain a citizen as an enemy belligerent and argue that the
definition of “unprivileged enemy belligerent” provided in the 2009 MCA, which does not
explicitly limit its definitional scope to aliens, bolsters the detention authority already possessed
by virtue of the AUMF. Constitutional due process would apply, and the citizen could petition for
habeas corpus to challenge his detention. However, under the 2006 MCA the citizen-combatant
would not be able to assert rights based on the Geneva Convention in support of his contention
that he is not an enemy belligerent. In that sense, U.S. citizens could be affected by the 2006 and
2009 MCAs even though they do not directly apply to U.S. citizens.