Jack Balkin and Jonathan Hafetz have thoughts here and here.

There are a number of complicated questions to unpack:

1. Does the Constitution guarantee some minimum amount of habeas protection? Justice Scalia says no in his dissent in INS v. St. Cyr. Others say yes.

2. If it does require some minimum habeas protection, what does the constitutional minimum look like? The 1789 version of common law habeas that applied in the King’s Bench? Or something more robust?

3. Does constitutional habeas, if it exists, extend to aliens?

4. When can Congress suspend constitutional habeas and for how long?

5. Relatedly, does due process inform what constitutes “constitutional habeas” and, at the same time, limit Congress’s suspension power?

Balkin avoids addressing due process and equal protection arguments against the MCA, noting they require much more heavy lifting than a blog post allows. I’m undeterred by his common sense, however, and below I raise a set of preliminary questions about these claims:

1. Equal Protection. One argument against the MCA’s suspension provisions is that they apply to aliens, but not to American citizens, thereby violating the aliens’ Fifth Amendment equal protection rights. However, its an open question whether equal protection applies extraterritorially to nonresident aliens. It would odd if it did, since foreign policy by nature must prefer and protect the interests of citizens at the expense of those beyond our borders.

That intuition is confirmed by case law. In the Insular Cases, which arose out of legal challenges to Congress’s disposition of conquered territories acquired in the Spanish-American War, Justice Brown said that “the power to acquire territory by treaty implies not only the power to govern such territory, but to prescribe upon what terms the United States will receive its inhabitants, and what their status shall be.”

To be sure, the Court suggested that there “are certain principles of natural justice inherent in the Anglo-Saxon character which need no expression in constitutions or statutes to give them effect or to secure dependencies against legislation manifestly hostile to their real interests.” And in Rasul v. Bush, the plurality indirectly referred to this dictum (by citing Justice Kennedy’s reliance on it in his United States v. Verdugo-Urquidez concurrence), suggesting the plurality believed some fundamental unenumerated constitutional rights may apply to Guantanamo.

Even so, by recognizing that Congress is entitled to prescribe the “status” of noncitizens in treaty-acquired territory, the Insular Cases imply that heightened equal protection review (i.e. strict scrutiny)—which presumes that classifications are strongly disfavored—doesn’t apply in Guantanamo. Moreover, the Insular Cases’ emphasis on the rights of “inhabitants” suggests that those rights attach not to territory but to pre-annexation residents of the territories—suggesting that even minimum “rational basis” equal protection principles might not apply to those who merely transit through or are forcibly detained in Guantanamo, but who have not voluntarily chosen to reside there. Finally, past Courts have repeatedly cast doubt on claims that the Constitution extends to nonresident enemy aliens in In re Yamashita and Johnson v. Eisentrager.

The equal protection claims of lawful resident aliens are equally problematic. Beginning with the Alien Enemy Act of 1798, Congress has granted resident aliens far fewer rights against executive detention than citizens. And, at a minimum, any equal protection claim must contend with the Court’s own haphazard approach to strict scrutiny of resident alienage classifications, which does not apply to deportation proceedings or to certain “constitutional prerogatives” of a democratic government (see, e.g., Sugarman v. Dougall), among which includes the conduct of foreign relations. See Harisiades v. Shaughnessy, 342 U.S. 580, 588–589 (1952) (“any policy toward aliens is vitally and intricately interwoven with … the conduct of foreign relations, the war power, and the maintenance of a republican form of government. Such matters are so exclusively entrusted to the political branches of government as to be largely immune from judicial inquiry or interference”).

2. Due Process. Another argument against the MCA is that it violates due process by denying persons an independent pre-trial judicial hearing determining whether they are a citizen or an alien.

In Verdugo, Justice Kennedy, writing in concurrence, suggested that due process is among the principles of “natural justice” that apply to aliens abroad. After Yamashita and Eisentrager, that may be dubious when applied to alien prisoners of war. But Hamdi v. Rumsfeld held that due process also requires that a “citizen-detainee seeking to challenge his classification as an enemy combatant must receive notice of the factual basis for his classification, and a fair opportunity to rebut the Government’s factual assertions before a neutral decisionmaker.” That may imply that not only citizen-detainees but detainees asserting citizenship status have a due process right to an independent hearing on their classification.

The force of this argument, though, is undercut by the fact that the Hamdi Court took care to underscore that habeas hadn’t been suspended before reaching the due process claim—suggesting, in turn, that the Court believed suspension authorizes detention without any process.

3. The Non-punishment/Punishment Distinction. While it grants limited appellate review of Combatant Status Review Tribunal’s determination of status, the MCA and Detainee Treatment Act forbid any review of the CSRT’s factual findings, which the MCA purports to make conclusive.

It might be argued that while the Suspension Clause authorizes Congress to suspend challenges by citizens or aliens to detention (the issue in Hamdi), a remedy must remain available to challenge the threshold factual determinations of who may be subject to military trial. The theory is that the Suspension Clause allows the executive to preventatively detain people without judicial process—but not to put them on trial without some minimum process.

The theory finds some support in Ex parte Milligan, which held that suspension doesn’t entitle the executive to try and punish citizens in an unlawful way. See, e.g., Ex parte Milligan, 71 U.S. 2, 125 (1868) (“Unquestionably, there is then an exigency which demands that the government, if it should see fit … should not be required to produce the persons arrested in answer to a writ of habeas corpus. The Constitution goes no further. It does not say after a writ of habeas corpus is denied a citizen, that he shall be tried otherwise than by the course of the common law.”). See also Hamdi v. Rumsfeld, 542 U.S. 507, 593 (2004) (Thomas, J., dissenting) (the “punishment-nonpunishment distinction harmonizes” the suspension precedents). Milligan recognizes, in effect, that Congress can’t suspend the use of a habeas to challenge certain features of executive trials.

However, Quirin, Yamashita, and Eisentrager have subsequently limited Milligan’s application to enemy combatants, by rejecting claims that enemy alien combatants and enemy citizen combatants are generally protected by the Bill of Rights or federal statutory law. However, a narrow bit of Milligan may survive. The combatants in Quirin and Eisentrager admitted they fell within the class of individuals that the political branches had determined to try; instead, they challenged the constitutionality of the trials themselves. Hence these cases don’t squarely extinguish a claim that due process requires a minimal, non-suspendable, habeas review of facts on which jurisdiction of military commission trials are premised.

To sum up, its arguable, based on a synthesis of Milligan and subsequent cases, that Congress (1) cannot limit detainee’s use habeas to challenge his citizenship status when—and only when—(2) the detainee is to be put on trial before a military court, and (3) citizenship of the detainee precludes the military court’s legal jurisdiction to try and punish. This argument is probably best framed as a due process right that belongs to citizens, that informs the constitutional core of habeas, and that imposes an external limit on Congress’s suspension power.

The claim is complicated a number of questions, including about how to define the constitutional core of habeas. Some argue that core extends no farther than eighteenth century common law practice, which forbid courts from determining the facts giving rise to detention. See, e.g., INS v. St. Cyr, 533 U.S. 289, 343 (2001) (Scalia, J., dissenting) (“in this country, until passage of the Habeas Corpus Act of 1867, and in England, the longstanding rule had been that the truth of the custodian’s return could not be controverted”). If that were true, then Milligan was wrong and the MCA—which permits detainees to appeal of pure questions of law to the D.C. Circuit–does not “suspend” the constitutional “core” of habeas at all.

The upshot: The Military Commission Act is not patently unconstitutional—but it is hardly on uncontrovertible constitutional footing, either.