On July 14, the en banc D.C. Circuit ruled in al Bahlul v. United States that "plain error" review applied to Bahlul's ex post facto challenge to his military commission convictions for conspiracy, material support, and solicitation--and then upheld the first of those charges under such deferential review (while throwing out the latter two). One of the potentially unintended consequences of the Court of Appeals' decision is to bring to the forefront a critically important constitutional question that, until now, the commissions have been able to duck: Does Article III apply to (and, thus, prohibit) military commission trials for offenses against the domestic--but not international--laws of war? In Ex parte Quirin, the Supreme Court held that Article III does not apply to "offenses committed by enemy belligerents against the law of war," which is why the war crimes trials after World War II--and the 9/11 trial at Guantánamo--haven't raised Article III questions. But Quirin was necessarily (and logically) limited to violations of the international laws of war, and, as the government now concedes, standalone conspiracy (the only remaining charge against al Bahlul) is not such an offense. Thus, unless the panel invalidates al Bahlul's conviction on one of the other three remaining grounds, it will have to resolve the Article III question. In a new amicus brief filed today (which I co-authored), the National Institute of Military Justice argues that, even if "conspiracy" is an offense against the "U.S. common law of war," Article III mandates that such offenses be tried before civilian, rather than military, federal courts. [NIMJ had filed similar briefs before the original three-judge panel andthe en banc court; the new brief responds to government's arguments and Judge Kavanaugh's en banc concurrence.] As the brief explains, "no U.S. court has ever upheld the jurisdiction of a military tribunal to try an offense that that court believed to be a violation of the domestic—but not international—laws of war. As the Supreme Court has made clear, Article III demands far more historical evidence—and far more compelling prudential justifications—before Congress may depart from its mandate." Oral argument is scheduled for October 22.