DOD has confirmed that an (as-yet-unidentified) American citizen is being held in U.S. military custody in Syria or Iraq as an enemy combatant. More specifically, the available information asserts that he was a fighter for the Islamic State who was captured in Syria by U.S.-friendly forces (or at least surrendered to those forces), and was then turned over to the U.S.
How significant is this development from a legal perspective?
The Straight Civilian Prosecution Option: Under this option, the individual will be brought back to the United States to face civilian criminal prosecution as rapidly as possible. This option would minimize the legal and political friction that will arise under the other options mentioned below. If the individual was indeed an Islamic State fighter, and there is admissible evidence to that effect, there will be ample charging options (material support, conspiracy, etc.).
The Straight Military Detention Option: The individual might be kept in military detention under color of the law of armed conflict, as an enemy combatant. WE have not had a situation like that (i.e., long-term military detention with a citizen) for many years. It happened previously with Jose Padilla, of course, and Yaser Hamdi. Hamdi's case seems particularly relevant as an analogy, insofar as both cases involve a U.S. citizen said to be a fighter for an enemy force in an overseas location at a time and place when it is quite clear that the law of armed conflict applies. The Supreme Court concluded in Hamdi's case that his citizenship did not relieve him from being subject to military detention in such circumstances, so long as the government could prove he was in fact whom they claimed him to be (since as a citizen he had Fifth Amendment Due Process rights), and so long as the underlying state of armed conflict continued (with qualifications relating to the possibility that the conflict might evolve in murky ways warranting renewed attention down the road). Then again, Hamdi can be distinguished from this particular individual in that the 2001 AUMF relatively clearly covered the Afghan Taliban, whereas the relevance of the 2001 AUMF to the Islamic State is a contested question that courts have never addressed. If this individual is held as an enemy combatant long enough for the machinery of habeas review to get underway--and surely the preliminary steps leading to a petition already are in the works, or soon will be--the administration will find that it has created a situation in which a federal judge might just reject the AUMF interpretive theory that has for several years been the foundation for the anti-IS effort. Thus, even if the citizenship angle does not pose significant legal risk for this approach (and some no doubt will reject my assessment that Hamdi squarely applies in this respect), the AUMF angle is a *very* serious risk.
Another angle to consider here: Where would the individual be held? The options are: inside the United States; GTMO; or continual custody at whatever in-theater base he currently is held. All of these options entail a variety of practical, diplomatic, and political frictions.
The Military Commission Prosecution Option: The military commission prosecution option entails all the same challenges as noted above with the Straight Military Detention Option, plus all the challenges that the existing commission prosecutions have generated. Note that the biggest problem of the latter variety, relevant for this case, is the uncertainty surrounding the availability of inchoate crime charges. Simply put, it is clear you can charge material support and conspiracy in civilian court, but you can't charge material support in the commissions now--and it is quite unclear whether conspiracy will be chargeable (the matter is still under litigation). None of that would matter if this individual can be linked to a specific violation of the laws of war, such as killing a prisoner. But more likely the government can prove no more--and no less--than his membership and support for the Islamic State. The prospects for conviction thus seem manifestly better in a civilian trial. [update: As my colleage Steve Vladeck points out, I should clarify: The Military Commissions Act of 2009 specifies who may be prosecuted before the current military commission system only if they are "alien enemy belligerents" within the meaning of that statute, which of course this individual is not. Trial of this individual in a military tribunal accordingly would require some fancy footwork under 10 USC 821 apart from the MCA 2009, which in turn would at best result in charging options more-clearly limited to accepted categories of war crime in the law of armed conflict, which would seem to exclude the prospect even of a conspiracy charge (since, as I understand it, the current DOJ litigating position is to accept that inchoate conspiracy is not a violation of the laws of war (though they contend it is still chargeable under the MCA 2009)].
The Hybrid Option: Under this option, the individual will remain in military custody for the short-term, subject to interrogation for some period of days or weeks. Then an FBI "clean team" would be brought in, and the model would shift to the civilian prosecution system, with the individual Mirandized (and reinterviewed if possible) and brought to the United States for a criminal trial in federal court. This is the model pioneered by the Obama administrated in situations like Warsame and Khattalah, and it is a clever way of maximizing the potential benefits of short-term interrogation and long-term incapacitation. Because it uses elements of both detention and civilian trial, this model does tend to draw criticism from both ends of the policy spectrum on these issues, yet the nature and scale of that friction has not been debilitating in the past instances. The more interesting challenges in this model have to do with the willingness of the courts to accept what occurred during the military detention phase at step one. That phase entails, by definition, a delay in presentment, and depending on the nature of the pre-Miranda interrogation and also on whether the government puts forward the fruits of the post-Miranda interviews there also may be motions to suppress or even a motion to dismiss the indictment on grounds of outrageous government conduct. Such challenges create pressure on the government to minimize the length of the military detention phase, but to an uncertain extent. Judge Cooper's recent ruling in Khattala on such issues is a very interesting read in that respect. It accepted that particular application of the hybrid model, but goes out of its way to emphasize some very specific facts justifying that outcome; one cannot count on a similar result on just any facts.
Prediction: Some version of the hybrid option, I think. Stay tuned!