As Jack noted earlier this morning, Lawfare's Alex Loomis has a fascinating new paper up on SSRN (for the moment, anyway) about the scope of Congress's Article I power to "define and punish . . . Offences against the Law of Nations." Over the course of 100 pages, Alex provides a powerful historical and analytical defense of Congress's ability to go beyond the strict confines of existing international law when exercising this power, and to prohibit private conduct that violates international law, as well as any private conduct that, while itself not illegal under international law, the United States has a duty to punish under international law. As Alex explains, Congress may also have the power to create entirely new offenses against the law of nations in order to foster changes in customary international law. In the process, Alex's paper is an important addition to a burgeoning body of literature--and a much more nuanced and attractive defense of Congress's power to innovate in this field than, for example, Mike Paulsen's 2009 Yale Law Journal article, which argued that Congress could use the Define and Punish Clause to declare anything it chooses (including, one would think, a failure to carry a minimum level of health insurance) to be a violation of the law of nations.
But it's important to stress what Alex's article does not answer: Although Jack suggested in his post that Alex's analysis is relevant to the ongoing D.C. Circuit litigation in Al Bahlul over the constitutional power of Guantánamo military commissions to try offenses like inchoate conspiracy, it goes only to the Article I question presented therein--whether Congress has the power to define inchoate conspiracy as an offense against the law of nations, as it has done in 10 U.S.C. § 950t(29). A wholly separate question--which Alex's paper does not seek to answer (see, e.g., footnote 357)--is whether Article III nevertheless prohibits a military, as opposed to civilian, court from trying the offense.
I've written at some length elsewhere about why the Article III question is analyically distinct from the Article I question, and won't rehash that analysis here. But an example will hopefully prove the point: Article I questions go to the scope of Congress's regulatory powers in general. Thus, when considering Congress's power to proscribe conduct pursuant to, say, the Commerce Clause, we tend not to care about the forum in which the contested prohibition is enforced. The question is simply whether Congress can reach the relevant conduct or not, period. But if Congress were, pursuant to the Commerce Clause, to subject a particular new offense to trial by a non-Article III federal court, that would raise both the Article I Commerce Clause question and a separate (and independent) question about Congress's power to remit particular disputes to resolution by federal adjudicators unprotected by Article III's salary and tenure rules, and unbounded by Article III's jurisdictional constraints. Thus, the Article III question might turn on whether the court trying the offense is a "territorial court" or whether the underlying dispute involves a "public right"; neither of these has anything to do with the Commerce Clause.
Finally, and critically for present purposes, different standards of review typically govern the Article I and Article III questions, with Congress (rightfully) receiving more deference on the question of whether particular conduct substantially affects interstate commerce than on the question of whether it fits into one of the recognized exceptions to Article III (a matter on which no deference is either afforded or appropriate). Thus, and simply put, the Article I validity of the Commerce Clause statute will never have any direct bearing on the Article III question of whether a non-Article III federal adjudicator may resolve the dispute involving and/or arising under the Article I statute.
What complicates matters in the context of military commissions, and the reason why I suspect so many (including several D.C. Circuit judges) have conflated these two questions, is that the Article I and Article III questions sound similar: The Article I question is whether Congress has the power to define and punish inchoate conspiracy as an offense against the law of nations (or pursuant to its other war powers), whereas the Article III question is, at least according to the Supreme Court, whether inchoate conspiracy fits into the jury-trial exception identified in Ex parte Quirin, which encompasses offenses committed by enemy belligerents against the [international] laws of war." (I've argued that the Article III question should actually be formulated slightly more broadly, but not in a manner that makes a difference here.) In other words, both the Article I and Article III questions are, to some degree, about the content of international law, overlapping to a degree that's just not present in the context of any other non-Article III federal adjudication.
But the argument that Congress has at least some ability to shape the content of international law in the Article I context does nothing to settle the Quirin question, i.e., whether the offense at issue was committed by an enemy belligerent against the international laws of war as they then existed. That's because the deference to which Congress is constitutionally entitled in the Article I context disappears once Congress seeks to depart from Article III--again, because the Article III question is not about Congress's power over the conduct, but rather its power over the forum. And the absence of deference on the Article III question is bolstered by the Supreme Court's far-more-formalistic approach to departures from Article III, as typified by Chief Justice Roberts's opinion for the majority in Stern v. Marshall.
In other words, while the Article I question does (and, in my view, should) allow some flexibility to Congress vis-a-vis the content of international law, the Article III question takes a specific subset of international law (the laws of war) as it finds it, not to protect the rights of the litigants (which, of course, is a contested proposition to begin with in this context), but to protect the Article III courts from excessive diminution of their constitutional authority (an issue that arises without regard to the individual rights of the relevant litigants). That's why, at least under de novo review (which we may not end up with in Al Bahlul), I've always thought that Article III forbids the military commission trial of inchoate conspiracy--whether or not Congress has the power under Article I to define and punish such conduct as an offense against the law of nations. Alex's paper goes a long way toward suggesting that Congress does have that power; it's just worth reminding ourselves why that conclusion doesn't actually settle the dispute in Al Bahlul.