Against the al-Nashiri backdrop, the government has now filed its brief on the merits in United States v. Hamdan (the first post-conviction appeal under the Military Commissions Acts of 2006 and 2009), which is set to be argued to a D.C. Circuit panel (Chief Judge Sentelle and Judges Ginsburg and Kavanaugh) on May 3. The central issues in Hamdan are whether Congress had the power (1) to subject the offense of "providing material support to terrorism" to trial by military commission; and (2) to apply that offense retroactively. There's also a claim that the MCA violates equal protection to the extent that it only applies to non-citizens.
The government's brief is worth reading in full, especially since it deviates in some fairly significant ways from the arguments relied upon by the Department of Defense before the commission itself and the Court of Military Commission Review. As relevant here, the brief relies quite heavily on the claim that Congress has the power to subject to trial by military commission offenses that are violations of the "U.S. common law of war," and then endeavors to explain why the provision of material support finds support in that common law. [As I read it, the argument is not that the crime itself is a common-law offense, but that Congress in the MCA could find sufficient support for the offense in past practice--which would also then bolster the government's defense on the ex post facto issue.] As I explain below, such a focus may ameliorate one of the flaws in the government's position, but it (significantly) exacerbates another.
Presumably, the emphasis on the domestic common-law argument reflects the (in my view correct) realization by the Justice Department that (1) there is insufficient support for the argument that "providing material support for terrorism" is a violation of the law of nations that Congress could proscribe under the Define and Punish Clause and subject to military jurisdiction; and (2) whether the precedent comes from domestic or international law, there is a serious ex post facto problem to the extent that such an offense wasn't triable by a military commission until the MCA was enacted, hence the focus on past U.S. practice.
The problem with the "U.S. common law of war" argument, though, is that it raises a brand-new constitutional issue, i.e., whether Congress may subject offenses against the "U.S. common law of war" (as opposed to offenses against the law of nations) to trial by a non-Article III court, such as the military commissions at issue here. As I've written about previously, the Grand and Petit Jury Clause requirements of Article III and the Fifth and Sixth Amendments are the principal constitutional constraints on military jurisdiction. Thus, servicemembers can be subjected to trial by court-martial entirely because the Fifth Amendment includes an express exception for "cases arising in the land or naval forces," and a similar exception has been read into the Sixth. But for non-servicemembers, the exception is not nearly as categorical. Instead, as Chief Justice Stone explained in Ex parte Quirin (my emphasis),
An express exception from Article III, § 2, and from the Fifth and Sixth Amendments, of trials of petty offenses and of criminal contempts has not been found necessary in order to preserve the traditional practice of trying those offenses without a jury. It is no more so in order to continue the practice of trying, before military tribunals without a jury, offenses committed by enemy belligerents against the law of war.
The question, of course, is what Quirin meant by "offenses committed by enemy belligerents against the law of war." But critically, in ascertaining whether the charged offenses qualified as such, the Quirin Court looked to the international law of war, and not to the "U.S. common law of war" that the government now invokes in Hamdan. Indeed, Chief Justice Stone specifically tied Congress's authority to create the jurisdiction at issue in Quirin to the Define and Punish Clause (which incorporates international law), not Congress's various "war-making" powers (which would presumably be the indirect source of such common law).
I don't mean to oversell the point; Quirin itself has lots of warts, and whatever else may be said about Chief Justice Stone's analysis, it is hardly a treatise when it comes to the scope of the exception to Article III and the Fifth and Sixth Amendments that justified the assertion of military jurisdiction there. But the more the government in Hamdan seeks to rely on the "U.S. common law of war" as the basis for Congress's power to define "material support" as an offense triable by a military commission, as opposed to the law of nations, the weaker the precedential footing becomes for the assertion of military--as opposed to civilian--jurisdiction. As such, the more the government relies on the "U.S. common law of war" argument, the more it's also forcing the D.C. Circuit (and, perhaps, the Supreme Court) to revisit just when Congress is allowed to subject particular offenses and offenders to trial by military courts, an issue that may already be on its way to the Court through other channels.