 United States Court of Appeals
         FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued May 3, 2012                 Decided October 16, 2012

                        No. 11-1257

                  SALIM AHMED HAMDAN,
                       PETITIONER

                             v.

                UNITED STATES OF AMERICA,
                       RESPONDENT


              On Petition for Review from the
    United States Court of Military Commission Review


    Joseph M. McMillan argued the cause for petitioner.
With him on the briefs were Harry H. Schneider Jr., Charles
C. Sipos, Angela R. Martinez, Abha Khanna, Adam
Thurschwell, and Jahn C. Olson.

   J. Wells Dixon, Shayana D. Kadidal, and Pardiss
Kebriaei were on the brief for amicus curiae Center for
Constitutional Rights in support of petitioner.

     David C. Lachman was on the brief for amicus curiae
International Legal Scholars Terry D. Gill and Gentian Zyberi
in support of petitioner.
                              2
     John S. Summers and Michael J. Newman were on the
brief for amicus curiae Professor David W. Glazier in support
of petitioner.

     Gene C. Schaerr and Kimball R. Anderson were on the
brief for amicus curiae Constitutional Law Scholars in
support of petitioner.

    Jonathan Hafetz and David Cole were on the brief for
amicus curiae Japanese American Citizens League, et al. in
support of petitioner.

     John F. De Pue, Attorney, U.S. Department of Justice,
argued the cause for respondent. With him on the brief were
Lisa O. Monaco, Assistant Attorney General for National
Security, Jeffrey M. Smith, Attorney, Edward S. White,
Captain, JAGC, U.S. Navy Appellate Counsel, and Francis A.
Gilligan, Appellate Counsel, Office of the Prosecutor for
Military Commissions.

    Before: SENTELLE, Chief Judge, KAVANAUGH, Circuit
Judge, and GINSBURG, Senior Circuit Judge.

     Opinion for the Court filed by Circuit Judge
KAVANAUGH, with whom Chief Judge SENTELLE joins except
as to footnote 6, and with whom Senior Judge GINSBURG
joins except as to footnotes 3, 6, and 8.

    Concurring Opinion filed by Senior Circuit Judge
GINSBURG.

     KAVANAUGH, Circuit Judge: The United States is at war
against al Qaeda, an international terrorist organization. Al
Qaeda’s stated goals are, among other things, to drive the
United States from posts in the Middle East, to devastate the
State of Israel, and to help establish radical Islamic control
                              3
over the Greater Middle East. Al Qaeda uses terror to
advance its broad objectives. Al Qaeda terrorists do not wear
uniforms, and they target American civilians and members of
the U.S. Military, as well as U.S. allies. After al Qaeda’s
attacks on the United States on September 11, 2001, Congress
authorized the President to wage war against al Qaeda. That
war continues.

     In war, when the United States captures or takes custody
of alien enemy combatants or their substantial supporters, it
may detain them for the duration of hostilities. Moreover, the
United States may try unlawful alien enemy combatants
before military commissions for their war crimes. See Hamdi
v. Rumsfeld, 542 U.S. 507, 518-24 (2004); Ex parte Quirin,
317 U.S. 1, 26-45 (1942).

    This case raises questions about the scope of the
Executive’s authority to prosecute war crimes under current
federal statutes.

    This particular dispute involves the military commission
conviction of Salim Hamdan, an al Qaeda member who
worked for Osama bin Laden. In 2001, Hamdan was captured
in Afghanistan. He was later transferred to the U.S. Naval
Base at Guantanamo Bay, Cuba.

    Hamdan was not just detained at Guantanamo as an
enemy combatant. He was also accused of being an unlawful
enemy combatant and was tried and convicted by a military
commission for “material support for terrorism,” a war crime
specified by the Military Commissions Act of 2006. See 10
U.S.C. § 950t(25); see also 10 U.S.C. § 950v(b)(25) (2006)
(previous codification of same provision).         Hamdan’s
conviction was based on actions he took from 1996 to 2001 –
before enactment of the Military Commissions Act. At the
time of Hamdan’s conduct, the extant federal statute
                               4
authorized and limited military commissions to try violations
of the “law of war.” 10 U.S.C. § 821.

     As punishment for his war crime, Hamdan was sentenced
by the military commission to 66 months’ imprisonment, with
credit for some time already served. Hamdan’s sentence
expired in 2008. Although the United States may have
continued to detain Hamdan until the end of hostilities
pursuant to its wartime detention authority, see Hamdi, 542
U.S. at 518-22, Hamdan was transferred in late 2008 to
Yemen and then released there. Even after his release,
Hamdan has continued to appeal his U.S. war crimes
conviction.

     This appeal presents several issues. First, is the dispute
moot because Hamdan has already served his sentence and
been released from U.S. custody? Second, does the Executive
have authority to prosecute Hamdan for material support for
terrorism on the sole basis of the 2006 Military Commissions
Act – which specifically lists material support for terrorism as
a war crime triable by military commission – even though
Hamdan’s conduct occurred from 1996 to 2001, before
enactment of that Act? Third, if not, did the pre-existing
statute that authorized war-crimes military commissions at the
time of Hamdan’s conduct – a statute providing that military
commissions may try violations of the “law of war,” 10
U.S.C. § 821 – proscribe material support for terrorism as a
war crime?

    We conclude as follows:

     First, despite Hamdan’s release from custody, this case is
not moot. This is a direct appeal of a conviction. The
Supreme Court has long held that a defendant’s direct appeal
of a conviction is not mooted by the defendant’s release from
custody.
                               5
     Second, consistent with Congress’s stated intent and so as
to avoid a serious Ex Post Facto Clause issue, we interpret the
Military Commissions Act of 2006 not to authorize
retroactive prosecution of crimes that were not prohibited as
war crimes triable by military commission under U.S. law at
the time the conduct occurred.         Therefore, Hamdan’s
conviction may be affirmed only if the relevant statute that
was on the books at the time of his conduct – 10 U.S.C. § 821
– encompassed material support for terrorism.

      Third, when Hamdan committed the relevant conduct
from 1996 to 2001, Section 821 of Title 10 provided that
military commissions may try violations of the “law of war.”
The “law of war” cross-referenced in that statute is the
international law of war. See Quirin, 317 U.S. at 27-30, 35-
36. When Hamdan committed the conduct in question, the
international law of war proscribed a variety of war crimes,
including forms of terrorism. At that time, however, the
international law of war did not proscribe material support for
terrorism as a war crime. Indeed, the Executive Branch
acknowledges that the international law of war did not – and
still does not – identify material support for terrorism as a war
crime. Therefore, the relevant statute at the time of Hamdan’s
conduct – 10 U.S.C. § 821 – did not proscribe material
support for terrorism as a war crime.

     Because we read the Military Commissions Act not to
retroactively punish new crimes, and because material support
for terrorism was not a pre-existing war crime under 10
U.S.C. § 821, Hamdan’s conviction for material support for
terrorism cannot stand. We reverse the judgment of the Court
                                6
of Military Commission Review and direct that Hamdan’s
conviction for material support for terrorism be vacated. 1

                                 I

     In 1996, Salim Hamdan traveled from his native Yemen
to Pakistan and then to Afghanistan to participate in jihad. In
Afghanistan, Hamdan attended an al Qaeda training camp. At
the camp, Hamdan received weapons training, met Osama bin
Laden, and listened to bin Laden’s lectures.

     Later in 1996, Hamdan became an al Qaeda driver. His
duties included transporting personnel, supplies, and weapons
between an al Qaeda guesthouse and al Qaeda’s al Farouq
training camp in Afghanistan. Eventually, Hamdan became
Osama bin Laden’s personal driver and bodyguard.

     In August 1996, Osama bin Laden publicly declared war
on the United States. That declaration came after various al
Qaeda terrorist attacks, including the 1993 bombing of the
World Trade Center. In 1998, bin Laden issued a fatwa
calling for the indiscriminate killing of Americans, including
American civilians. Hamdan was fully aware of bin Laden’s
public statements targeting the United States.



    1
        Our judgment would not preclude detention of Hamdan until
the end of U.S. hostilities against al Qaeda. Nor does our judgment
preclude any future military commission charges against Hamdan –
either for conduct prohibited by the “law of war” under 10 U.S.C.
§ 821 or for any conduct since 2006 that has violated the Military
Commissions Act. Nor does our judgment preclude appropriate
criminal charges in civilian court. Moreover, our decision concerns
only the commission’s legal authority. We do not have occasion to
question that, as a matter of fact, Hamdan engaged in the conduct
for which he was convicted.
                              7
     In August 1998, al Qaeda operatives bombed U.S.
Embassies in Kenya and Tanzania, killing 257 people,
including 12 Americans. Hamdan was generally aware that
such an attack was planned. Around the time of the attack,
Hamdan assisted Osama bin Laden in evacuating from
Kandahar and moving around Afghanistan.

     Later in August 1998, asserting the President’s Article II
power of self-defense, President Clinton ordered the U.S.
Military to bomb targets in Afghanistan in an attempt to kill
bin Laden. Bin Laden narrowly avoided being killed in that
military action.

    In October 2000, at the direction of bin Laden and senior
al Qaeda leaders, al Qaeda bombed the U.S.S. Cole off the
coast of Yemen, killing 17 Americans and injuring many
others. Around that time, Hamdan returned to Afghanistan
from Yemen.

     In August 2001, Hamdan drove bin Laden to various
planning meetings in Afghanistan. Several days before
September 11, 2001, bin Laden told Hamdan that they had to
evacuate their compound because of an impending operation.
Hamdan drove bin Laden to Kabul. They later moved to a
series of locations around Afghanistan.

     On September 11, 2001, al Qaeda attacked the United
States, killing thousands of civilians and causing massive
long-term damage to the American economy and way of life.

     In the days following the attacks of September 11, 2001,
Congress passed and President George W. Bush signed the
Authorization for Use of Military Force. That law authorized
the President
                               8
    to use all necessary and appropriate force against those
    nations, organizations, or persons he determines planned,
    authorized, committed, or aided the terrorist attacks that
    occurred on September 11, 2001, or harbored such
    organizations or persons, in order to prevent any future
    acts of international terrorism against the United States
    by such nations, organizations or persons.

Pub. L. No. 107-40, 115 Stat. 224 (2001).

    Consistent with the 2001 Authorization for Use of
Military Force, President Bush directed the use of force to kill
or capture and detain al Qaeda operatives, and where
appropriate to try unlawful al Qaeda combatants who had
committed war crimes. On October 7, 2001, as part of the
overall operation, President Bush ordered U.S. troops into
Afghanistan to wage war against al Qaeda there, as well as
against the Taliban government that was in control of
Afghanistan and had been supporting and harboring al Qaeda.

     On November 13, 2001, the President issued an executive
order establishing military commissions to try al Qaeda
members and aiders and abettors who had committed war
crimes as defined under the “laws of war” or other “applicable
laws.” Military Order of Nov. 13, 2001, 66 Fed. Reg. 57,833;
57,833-34. The executive order did not purport to rely solely
on the President’s constitutional authority; rather, it cited two
separate statutes as congressional authorization for the
President to employ military commissions: the 2001
Authorization for Use of Military Force and 10 U.S.C. § 821,
the long-standing statute that authorized military commissions
to try violations of the “law of war.”

    In November 2001, Hamdan was captured in Afghanistan
while driving toward Kandahar. The car he was driving
                               9
contained two anti-aircraft missiles. Also in the car was an al
Qaeda-issued document that authorized the bearer to carry a
weapon in Afghanistan. Hamdan’s captors turned him over to
U.S. authorities. He was later transferred to Guantanamo
Bay, Cuba, and the U.S. Military detained him there as an
enemy combatant.

     At Guantanamo, Hamdan not only was detained as an
enemy combatant but also was eventually charged with one
count of conspiracy and was to be tried before a military
commission as an unlawful enemy combatant who had
committed war crimes. 2         Hamdan raised various legal
objections to the prosecution, and the case ultimately wound
its way to the Supreme Court. The Supreme Court held that
the military commission rules then in place contravened
statutory limits because the rules did not comply in certain
respects with statutory restrictions contained in 10 U.S.C.
§ 836. See Hamdan v. Rumsfeld, 548 U.S. 557, 613-35
(2006). The Court split 4-3 on and thus did not decide a
separate issue: whether conspiracy was a cognizable charge in
a military commission under the “law of war” for purposes of
10 U.S.C. § 821. Compare Hamdan, 548 U.S. at 595-612
(Stevens, J., plurality opinion) (conspiracy is not a law of war
crime), with id. at 697-706 (Thomas, J., dissenting)
(conspiracy is a law of war crime). (Justice Kennedy did not
address that issue; Chief Justice Roberts did not participate in
the case.)


    2
       Generally speaking, enemy soldiers or combatants are
considered unlawful enemy combatants when they, for example,
join or support an organization waging unlawful war or they
commit specific “acts which render their belligerency unlawful.”
Ex parte Quirin, 317 U.S. 1, 31 (1942). For purposes of the war
against al Qaeda, this concept is now defined by statute. See 10
U.S.C. § 948a.
                              10
    In the Hamdan case, several Justices specifically invited
Congress to clarify the scope of the President’s statutory
authority to use military commissions to try unlawful alien
enemy combatants for war crimes. See Hamdan, 548 U.S. at
636 (Breyer, J., concurring); id. at 636-37 (Kennedy, J.,
concurring).

     In the wake of the Supreme Court’s decision in Hamdan,
Congress enacted a new military commissions statute. See
Military Commissions Act of 2006, Pub. L. No. 109-366, 120
Stat. 2600. Of particular relevance here, Congress expanded
military commissions beyond prosecuting violations of the
generic “law of war,” spying, and aiding the enemy, which
were the crimes listed by statute at the time. See 10 U.S.C.
§§ 821, 904, 906. Of most importance here, Congress
alleviated some of the uncertainty highlighted in Hamdan
about the phrase “law of war” in 10 U.S.C. § 821 by listing a
large number of specific war crimes that could be charged by
military commission, including conspiracy and material
support for terrorism. See § 3(a), 120 Stat. at 2630. (In 2009,
Congress enacted a new Military Commissions Act; that law
did not make changes relevant to this case. See Pub. L. No.
111-84, 123 Stat. 2574.)

    After passage of the 2006 Military Commissions Act,
Hamdan was charged anew before a U.S. military commission
on one charge of conspiracy and one charge, containing eight
specifications, of material support for terrorism.

    At his military commission trial, Hamdan was acquitted
of conspiracy but convicted of five specifications of material
support for terrorism. In August 2008, he was sentenced to
66 months’ confinement and credited for having already
served most of that time.
                             11
     When his sentence ended later in 2008, the war against al
Qaeda had not ended. Therefore, the United States may have
continued to detain Hamdan as an enemy combatant. See
Hamdan, 548 U.S. at 635; Hamdi v. Rumsfeld, 542 U.S. 507,
518-24 (2004). But in November 2008, Hamdan was
transferred by the U.S. Military to Yemen, and he was then
released on or about January 8, 2009, in Yemen.

     After his release, Hamdan nonetheless continued to
appeal his U.S. military commission conviction. On appeal to
the en banc Court of Military Commission Review, Hamdan
argued (i) that Congress lacked authority under Article I of
the Constitution to make material support for terrorism a war
crime triable by military commission; (ii) that in any event,
the 2006 Military Commissions Act, which listed material
support for terrorism as a war crime, could not be
retroactively applied to him because his conduct occurred
from 1996 to 2001; and (iii) that the statute in effect at the
time of his alleged conduct – 10 U.S.C. § 821, which limited
military commissions to violations of the “law of war” – did
not authorize prosecution of material support for terrorism as
a war crime. In 2011, the Court of Military Commission
Review affirmed the conviction. See United States v.
Hamdan, 801 F. Supp. 2d 1247 (C.M.C.R. 2011) (en banc).

     By statute, Hamdan has an automatic right of appeal to
this Court. See 10 U.S.C. § 950g.

                              II

     We must first address the issue of mootness – that is,
whether this appeal is moot because Hamdan has been
released from U.S. custody. Although the parties agree that
the appeal is not moot, mootness is a jurisdictional question
that we must independently consider. See United States v.
                                12
Juvenile Male, 131 S. Ct. 2860, 2864-65 (2011); Sibron v.
New York, 392 U.S. 40, 50 n.8 (1968).

     This case is a direct appeal of a military commission
conviction. In the criminal context, a direct appeal of a
criminal conviction is not mooted by a defendant’s release
from custody. See Sibron, 392 U.S. 40. The Supreme Court
has so ruled in part because of the collateral legal
consequences of a conviction – namely, the possibility that
the defendant could commit or be tried for a new offense, the
punishment for which could take account of a past conviction.
Those collateral consequences are of course present in
virtually all criminal cases (other than, for example, when the
defendant has died after the conviction and thus obviously
cannot commit a new offense).             The same collateral
consequences are present in military commission conviction
cases. See, e.g., MANUAL FOR MILITARY COMMISSIONS, Rules
1001(a)(2), 1001(b)(1)(A) (2012) (in military commission
sentencing, the prosecution may “introduce evidence of
military or civilian convictions, foreign or domestic, of the
accused” as an aggravating factor); 18 U.S.C. § 3553(a)(1)
(sentencing courts shall take into account “the history and
characteristics of the defendant”). 3 Applying the relevant
Supreme Court precedent, we therefore conclude that a direct
appeal of a military commission conviction is likewise not
mooted by the defendant’s release.

    To be sure, that principle generally does not apply to the
habeas context where a detainee is challenging the basis for
executive detention. Such a habeas case is sometimes moot

    3
       In his concurring opinion, Judge Ginsburg calls for a change
to existing Supreme Court mootness doctrine. In doing so, Judge
Ginsburg suggests that Hamdan is in Yemen and has little chance
of landing in future trouble in the U.S. legal system. Maybe.
Maybe not.
                             13
after the detainee’s release. See Spencer v. Kemna, 523 U.S.
1, 8-14 (1998); Gul v. Obama, 652 F.3d 12, 17 (D.C. Cir.
2011). In our recent habeas decision in Gul, where a former
Guantanamo detainee objected to a military detention
determination after his release, this Court dismissed the case
as moot.

     But Hamdan is not just a military detainee; he has been
convicted of a war crime by military commission. Therefore,
our recent decision in Gul does not control here. Rather, this
case is controlled by the principle that a direct appeal of a
conviction is not mooted by the defendant’s release from
custody.

    This case is not moot.

                             III

     Under a law now codified at 10 U.S.C. § 821, Congress
has long authorized the Executive to use military
commissions to try war crimes committed by the enemy. See
Ex parte Quirin, 317 U.S. 1 (1942). That statute authorizes
military commissions to try violations of the “law of war” – a
term, as we explain below, that has long been understood to
mean the international law of war. See Hamdan v. Rumsfeld,
548 U.S. 557, 603, 610 (2006) (plurality); id. at 641
(Kennedy, J., concurring); Quirin, 317 U.S. at 27-30, 35-36.
Two other longstanding statutes separately authorize military
commission prosecutions for spying and aiding the enemy.
See 10 U.S.C. §§ 904, 906. 4


    4
      The “aiding the enemy” proscription in 10 U.S.C. § 904,
which was first codified in the Articles of War of 1806, see
WILLIAM WINTHROP, MILITARY LAW AND PRECEDENTS 102-03,
981 (rev. 2d ed. 1920), generally requires breach of a duty of
                                14
     After the Supreme Court’s 2006 decision in Hamdan,
Congress enacted a new military commissions statute that,
among other things, clarified the scope of the Executive’s
authority to try war crimes. See Military Commissions Act of
2006, Pub. L. No. 109-366, 120 Stat. 2600. Of particular
relevance here, Congress expanded military commissions
beyond trying violations of the generic “law of war,” spying,
and aiding the enemy. Congress instead also listed a large
number of specific war crimes that could be tried by military
commission, including conspiracy and material support for
terrorism. See id. § 3(a), 120 Stat. at 2630 (now codified at
10 U.S.C. § 950t).

     Hamdan argues that Congress lacked authority under
Article I of the Constitution – namely, the Define and Punish
Clause – to define material support for terrorism as a war
crime subject to trial by a U.S. military commission.5
Hamdan maintains that Congress’s authority under the Define
and Punish Clause is limited to proscribing offenses that are
already illegal under international law. And Hamdan contends


loyalty as well as aid to the enemy. See Hamdan, 548 U.S. at 600-
01 n.32 (plurality) (“aiding the enemy may, in circumstances where
the accused owes allegiance to the party whose enemy he is alleged
to have aided, be triable by military commission”). The breach of
loyalty requirement is made explicit in the 2006 Military
Commissions Act, which re-codified the crime. 10 U.S.C.
§ 950t(26) (“Any person subject to this chapter who, in breach of
an allegiance or duty to the United States, knowingly and
intentionally aids an enemy of the United States, or one of the co-
belligerents of the enemy, shall be punished as a military
commission under this chapter may direct.”) (emphasis added).
     5
       The Define and Punish Clause provides that Congress has the
power: “To define and punish Piracies and Felonies committed on
the high Seas, and Offences against the Law of Nations.” U.S.
CONST. art. I, § 8, cl. 10.
                                15
that material support for terrorism is not a recognized
international-law war crime. The Government responds that
Hamdan’s focus on the Define and Punish Clause alone is
misplaced. According to the Government, the Declare War
Clause and other war clauses in Article I, as supplemented by
the Necessary and Proper Clause, independently authorize
Congress to establish military commissions to try an enemy’s
war crimes. And the Government further contends that
Congress’s broad authority under the Declare War Clause is
not constrained by the evolving and often difficult to discern
standards of international law. Therefore, as the Government
sees it, Congress has authority to make material support for
terrorism a war crime triable by military commission.

     We do not decide that antecedent question. Even
assuming arguendo that Congress had authority under its
various Article I war powers to establish material support for
terrorism as a war crime in the Military Commissions Act of
2006, 6 we conclude that the Act did not authorize retroactive
    6
       Judge Kavanaugh alone concurs in this footnote. Because
the question of Congress’s Article I power to make material support
for terrorism a war crime has been thoroughly briefed and argued,
because that question is logically antecedent to the ex post facto
issue, and because of the importance of deciding wartime cases in a
way that provides clear guidance, Judge Kavanaugh believes it
appropriate to address the antecedent question – as the Supreme
Court itself did in resolving similar antecedent issues in both
Hamdi and Hamdan. See Hamdi, 542 U.S. at 516-24, 533-34
(resolving several “threshold” questions, including whether enemy
combatants may be detained for the duration of hostilities, before
concluding that the procedures used to detain Hamdi were
insufficient); Hamdan, 548 U.S. at 593-94 (resolving antecedent
question whether relevant statutes generally authorized military
commissions, before concluding that the commission in Hamdan’s
case contravened separate statutory limits).          Here, Judge
Kavanaugh would conclude that Congress has authority under
                                 16
prosecution for conduct that was committed before the Act’s
enactment and was not prohibited by U.S. law at the time the
conduct occurred. Here, Hamdan’s conduct occurred from
1996 to 2001 – before enactment of the Military Commissions
Act. And as we will explain, the federal statute in effect at
the time of Hamdan’s conduct – 10 U.S.C. § 821 – did not
authorize prosecution for material support for terrorism.

                                 A

    As is clear from the text of the Military Commissions Act
of 2006, Congress was quite concerned about the ex post


Article I, § 8 to establish material support for terrorism as a war
crime that, when committed by an alien, may be tried by military
commission. Although material support for terrorism is not yet an
international-law war crime, Congress’s war powers under Article I
are not defined or constrained by international law. The Declare
War Clause and the other Article I war powers clauses do not refer
to international law, unlike the Define and Punish Clause.
Moreover, Congress has long prohibited war crimes beyond those
specified by international law. See 10 U.S.C. § 904 (aiding the
enemy); id. § 906 (spying); cf. Quirin, 317 U.S. 1. The U.S.
Constitution does not give the international community – either
directly, or indirectly through the vehicle of international law – a
judicially enforceable veto over Congress’s exercise of its war
powers. Put simply, the United States may be a leader in the
international community, not just a follower, when Congress
authorizes war against a terrorist organization or makes crimes such
as material support for terrorism war crimes triable by military
commission. To be sure, it is often prudent for Congress and the
President to coordinate closely with the international community
and pay careful attention to international law when authorizing war
and enacting war crimes. But those policy factors, political
realities, and international-law considerations are not constitutional
constraints incorporated into the Article I war powers clauses and
thereby enforceable in U.S. courts.
                                17
facto implications of retroactively prosecuting someone under
the Act for conduct committed before its enactment.
Congress tried to deal with any ex post facto problem by
declaring in the text of the statute that “[t]he provisions of this
subchapter codify offenses that have traditionally been triable
by military commissions. This chapter does not establish new
crimes that did not exist before its enactment, but rather
codifies those crimes for trial by military commission.”
§ 3(a), 120 Stat. at 2624. The Act continued: “Because the
provisions of this subchapter (including provisions that
incorporate definitions in other provisions of law) are
declarative of existing law, they do not preclude trial for
crimes that occurred before the date of the enactment of this
chapter.” Id.

    As Congress well understood when it appended this
unusual statement to the statute, the U.S. Constitution bars
Congress from enacting punitive ex post facto laws. See U.S.
CONST. art. I, § 9, cl. 3 (“No Bill of Attainder or ex post facto
Law shall be passed.”). Among other things, the Ex Post
Facto Clause bars laws that retroactively punish conduct that
was not previously prohibited, or that retroactively increase
punishment for already prohibited conduct. See Collins v.
Youngblood, 497 U.S. 37, 42 (1990); Calder v. Bull, 3 U.S.
386 (1798) (opinion of Chase, J.). The Ex Post Facto Clause
thus prevents Congress and the Executive from retroactively
applying a federal criminal statute to conduct committed
before the statute was enacted.

     As Congress itself recognized in the statutory text,
retroactive prosecution by military commission could
similarly raise serious constitutional issues, at the very least.
As stated in the statutory text, however, Congress believed
that the Act codified no new crimes and thus posed no ex post
facto problem. As we explain below, Congress’s premise was
                               18
incorrect. The statute does codify some new war crimes,
including material support for terrorism. The question for ex
post facto purposes is this: If Congress had known that the
Act was codifying some new crimes, would Congress have
wanted the new crimes to be enforced retroactively? To begin
with, the statutory text reveals a tight causal link between (i)
Congress’s belief that the statute codified only crimes under
pre-existing law and (ii) Congress’s statement that the statute
could therefore apply to conduct before enactment. That
causal link suggests that Congress would not have wanted
new crimes to be applied retroactively. The Executive Branch
agrees with that interpretation of the statute, stating:
“Congress incorporated ex post facto principles into the terms
of the MCA itself.” Brief for the United States at 66. At a
minimum, we know that the statutory text does not
contemplate or address the possibility of retroactively
applying new crimes, leaving us with at least something of an
ambiguity. And courts interpret ambiguous statutes to avoid
serious questions of unconstitutionality. See Rapanos v.
United States, 547 U.S. 715, 738 (2006) (plurality opinion of
Scalia, J.) (constitutional avoidance where statute “raises
difficult questions” of constitutionality); Cherokee Nation of
Oklahoma v. Leavitt, 543 U.S. 631, 646 (2005) (avoiding an
interpretation that “may violate the Constitution”). To avoid
the prospect of an Ex Post Facto Clause violation here, we
interpret the Military Commissions Act of 2006 so that it does
not authorize retroactive prosecution for conduct committed
before enactment of that Act unless the conduct was already
prohibited under existing U.S. law as a war crime triable by
military commission. In this case, therefore, Hamdan’s
conviction stands or falls on whether his conduct was
prohibited by the pre-existing statute, 10 U.S.C. § 821, at the
time he committed the conduct. 7

    7
        To be clear, we do not here decide whether or how the Ex
                                 19
                                  B

     Before enactment of the Military Commissions Act in
2006, U.S. military commissions could prosecute war crimes
under 10 U.S.C. § 821 for violations of the “law of war.” The
Government suggests that at the time of Hamdan’s conduct
from 1996 to 2001, material support for terrorism violated the
“law of war” referenced in 10 U.S.C. § 821. It is true that in
the text of the Military Commissions Act of 2006, Congress
declared its belief that material support for terrorism was a
pre-existing crime under the law of war and thus under 10
U.S.C. § 821. See § 3a, 120 Stat. at 2624. But exercising our
independent review, as we must when considering the ex post
facto implications of a new law, see Calder v. Bull, 3 U.S.
386 (1798) (opinion of Chase, J.); Marbury v. Madison, 5
U.S. 137 (1803), we conclude otherwise. Material support for
terrorism was not a war crime under the law of war referenced
in 10 U.S.C. § 821 at the time of Hamdan’s conduct.

     Analysis of this issue begins by determining what body
of law is encompassed by the term “law of war” in 10 U.S.C.
§ 821. The Supreme Court’s precedents tell us: The “law of
war” referenced in 10 U.S.C. § 821 is the international law of
war. 8 See Hamdan, 548 U.S. at 603 (plurality) (act is law of


Post Facto Clause might apply to this case. As we interpret the
statute, that ultimate constitutional question need not be decided.
      8
        It has been suggested that courts should not use international
law as a free-floating tool to alter how the courts would otherwise
interpret a domestic U.S. statute when the statute does not
incorporate or refer to international law. See Al-Bihani v. Obama,
619 F.3d 1, 5-8 (D.C. Cir. 2010) (Brown, J., concurring in denial of
rehearing en banc); id. at 9-23 (Kavanaugh, J., concurring in denial
of rehearing en banc). But that interpretive issue is not implicated
in this case. As Congress has often done, and as explained in an Al-
Bihani concurrence, Congress here explicitly referred to
                                20
war offense when “universal agreement and practice both in
this country and internationally” recognize it as such)
(internal quotation marks omitted); id. at 610 (analyzing
international sources to determine whether conspiracy was
“recognized violation of the law of war”); id. at 641
(Kennedy, J., concurring) (“the law of war” referenced in 10
U.S.C. § 821 “derives from rules and precepts of the law of
nations” and is “the body of international law governing
armed conflict”) (internal quotation marks omitted); Quirin,
317 U.S. at 29 (“law of war” referenced in 10 U.S.C. § 821 is
a “branch of international law”); id. at 27-28 (The “law of
war” is “that part of the law of nations which prescribes, for
the conduct of war, the status, rights and duties of enemy
nations as well as of enemy individuals.”); see also
Instructions for the Government of Armies of the United
States in the Field (Lieber Code), General Orders No. 100,
arts. 27 & 40 (Apr. 24, 1863) (describing the law of war as a
“branch” of the “law of nations”); O.L.C. Memorandum from
Patrick F. Philbin to Alberto R. Gonzales 5 (Nov. 6, 2001)
(“laws of war” are “considered a part of the ‘Law of
Nations’”); id. at 29 (“the term ‘law of war’ used in 10 U.S.C.
§ 821 refers to the same body of international law now
usually referred to as the ‘laws of armed conflict’”). 9


international law and explicitly incorporated international norms
into domestic U.S. law in 10 U.S.C. § 821 by means of the express
cross-reference to the “law of war.” See id. at 10, 13-15
(Kavanaugh, J., concurring in denial of rehearing en banc)
(explaining that distinction).
     9
        See also Curtis A. Bradley & Jack L. Goldsmith, The
Constitutional Validity of Military Commissions, 5 GREEN BAG 2d
249, 256 (2002) (“As noted above, President Bush’s Military
Order, 10 U.S.C. § 821, and Supreme Court precedent all indicate
that the jurisdiction of military commissions extends (at least) to
violations of the international laws of war.”); Maj. Michael A.
                                21
     We turn, then, to the question whether material support
for terrorism is an international-law war crime.


Newton, Continuum Crimes: Military Jurisdiction Over Foreign
Nationals Who Commit International Crimes, 153 MIL. L. REV. 1,
21 (1996) (“Therefore, the entire scope of history and American
jurisprudence compel the conclusion that Article 21 grants
jurisdiction only over violations of the international laws of war.
The text of Article 21 leads to the same conclusion.”); Ruth
Wedgwood, Al Qaeda, Terrorism, and Military Commissions, 96
AM. J. INT’L L. 328, 334 (2002) (“This statutory language” in 10
U.S.C. § 821 “acknowledges that the jurisdiction of military
commissions is defined by the norms of the customary law of
nations, namely, the law of war.”).
     Even outside the context of 10 U.S.C. § 821, the term “law of
war” in the U.S. Code and precedent generally refers to the
international law of war. See Madsen v. Kinsella, 343 U.S. 341, 354
(1952) (The “law of war” includes that part of “the law of nations”
which “defines the powers and duties of belligerent powers.”);
Prize Cases, 67 U.S. 635, 667 (1863) (“The laws of war, as
established among nations, have their foundation in reason, and all
tend to mitigate the cruelties and misery produced by the scourge of
war.”); Hearings on H.R. 2498 (UCMJ) Before the H. Comm. on
Armed Servs., 81st Cong. 959 (1949) (Representative Overton
Brooks, Chairman, House Subcommittee No. 1 on Armed Services:
“What is a law of war?” Colonel John P. Dinsmore: “‘Law of war’
is set out in various treaties like the Geneva convention and
supplements to that.” Representative Brooks: “International law.”
Colonel Dinsmore: “Yes, sir.”); U.S. ARMY JAG, LAW OF WAR
HANDBOOK 20 (Maj. Keith E. Puls ed., 2005) (identifying
“customary international law” – that is, “the ‘unwritten’ rules that
bind all members of the community of nations” during war as one
of the two major sources of the law of war, along with conventional
international law); MANUAL FOR COURTS-MARTIAL UNITED
STATES, at I-1 (2012) (“The sources of military jurisdiction include
the Constitution and international law. International law includes
the law of war.”).
                              22
     It is true that international law establishes at least some
forms of terrorism, including the intentional targeting of
civilian populations, as war crimes. See, e.g., Rome Statute of
the International Criminal Court art. 8(2)(b), July 17, 1998,
2187 U.N.T.S. 90; Geneva Convention Relative to the
Protection of Civilian Persons in Time of War (Geneva IV),
art. 33, Aug. 12, 1949, 6 U.S.T. 3516, 75 U.N.T.S. 287;
COMMISSION OF RESPONSIBILITIES, CONFERENCE OF PARIS
1919, VIOLATION OF THE LAWS AND CUSTOMS OF WAR 17
(Clarendon Press 1919) (the Allied Nations condemned
Germany for “the execution of a system of terrorism” after
World War I).

     But the issue here is whether material support for
terrorism is an international-law war crime. The answer is
no. International law leaves it to individual nations to
proscribe material support for terrorism under their domestic
laws if they so choose. There is no international-law
proscription of material support for terrorism.

     To begin with, there are no relevant international treaties
that make material support for terrorism a recognized
international-law war crime. Neither the Hague Convention
nor the Geneva Conventions – the sources that are “the major
treaties on the law of war” – acknowledge material support
for terrorism as a war crime. See Hamdan, 548 U.S. at 604
(plurality); Geneva Convention Relative to the Protection of
Civilian Persons in Time of War (Geneva IV), Aug. 12, 1949,
6 U.S.T. 3516, 75 U.N.T.S. 287; Hague Convention (IV)
Respecting the Laws and Customs of War on Land and Its
Annex, Oct. 18, 1907, 36 Stat. 2277.

     Nor does customary international law otherwise make
material support for terrorism a war crime. Customary
international law is a kind of common law; it is the body of
                                 23
international legal principles said to reflect the consistent and
settled practice of nations. See RESTATEMENT (THIRD) OF
FOREIGN RELATIONS LAW OF THE UNITED STATES § 102(2)
(1987) (“Customary international law results from a general
and consistent practice of states followed by them from a
sense of legal obligation”). It is often difficult to determine
what constitutes customary international law, who defines
customary international law, and how firmly established a
norm has to be to qualify as a customary international law
norm. Cf. Sosa v. Alvarez-Machain, 542 U.S. 692 (2004). 10


     10
         Although customary international law, including the
customary international law of war, contains some well-defined
prohibitions at the core, the contours of customary international law
are imprecise. That imprecision provides good reason for Congress
and the Executive, when they want to outlaw violations of
perceived international-law norms, to enact statutes outlawing
specific conduct, rather than simply prohibiting violation of
something as vague as “international law” or “the law of nations”
or the “law of war.” Congress has done so in many recent statutes,
including the Military Commissions Act of 2006. Pub. L. No. 109-
366, 120 Stat. 2600 (2006). See also War Crimes Act of 1996, Pub.
L. No. 104-192, 110 Stat. 2104; Torture Victim Protection Act of
1991, Pub. L. No. 102-256, 106 Stat. 73 (1992); Foreign Sovereign
Immunities Act of 1976, Pub. L. No. 94-583, 90 Stat. 2891.
     At the same time, the imprecision of customary international
law calls for significant caution by U.S. courts before permitting
civil or criminal liability premised on violation of such a vague
prohibition. Cf. Sosa v. Alvarez-Machain, 542 U.S. 692 (2004). A
general prohibition against violations of “international law” or the
“law of nations” or the “law of war” may fail in certain cases to
provide the fair notice that is a foundation of the rule of law in the
United States. Therefore, as the Supreme Court required in an
analogous context in Sosa, and as the plurality suggested in
Hamdan, imposing liability on the basis of a violation of
“international law” or the “law of nations” or the “law of war”
                               24
     But here, the content of customary international law is
quite evident. Material support for terrorism was not a
recognized violation of the international law of war as of 2001
(or even today, for that matter). As we have noted, the
Geneva Conventions and the Hague Convention do not
prohibit material support for terrorism. The 1998 Rome
Statute of the International Criminal Court, which catalogues
an extensive list of international war crimes, makes no
mention of material support for terrorism. See Rome Statute
of the International Criminal Court, July 17, 1998, 2187
U.N.T.S. 90. Nor does the Statute of the International
Tribunal for the Former Yugoslavia, the Statute of the
International Tribunal for Rwanda, or the Statute of the
Special Court for Sierra Leone. See Statute of the
International Tribunal for the Former Yugoslavia, adopted by
S.C. Res. 827, U.N. Doc. S/RES/827 (1993), reprinted in 32
I.L.M. 1159, 1192; Statute of the International Tribunal for
Rwanda, adopted by S.C. Res. 955, U.N. Doc. S/RES/955
(1994), reprinted in 33 I.L.M. 1598, 1602 (includes terrorism
itself as a crime); Statute of the Special Court for Sierra
Leone art. 3(d), Jan. 16, 2002, 2178 U.N.T.S. 138 (same).
Nor have any international tribunals exercising common-law-
type power determined that material support for terrorism is
an international-law war crime.

     Commentators on international law have similarly
explained that material support for terrorism is not an
international-law war crime. See, e.g., ANDREA BIANCHI &
YASMIN NAQVI, INTERNATIONAL HUMANITARIAN LAW AND
TERRORISM 244 (2011) (“there is little evidence” that a
proscription of “material support for terrorism” is “considered

generally must be based on norms firmly grounded in international
law. See Sosa, 542 U.S. at 724-38; Hamdan, 548 U.S. at 602-03 &
n.34, 605 (plurality). In this case, the asserted norm has no
grounding in international law, much less firm grounding.
                              25
to be part of the laws and customs of war”). Nor is the
offense of material support for terrorism listed in the JAG
handbook on the law of war. See U.S. ARMY JAG, LAW OF
WAR HANDBOOK (Maj. Keith E. Puls ed., 2005); see also
Jennifer K. Elsea, The Military Commissions Act of 2006:
Analysis of Procedural Rules and Comparison with Previous
DOD Rules and the Uniform Code of Military Justice 12
(CRS, updated Sept. 27, 2007) (“defining as a war crime the
‘material support for terrorism’ does not appear to be
supported by historical precedent”) (footnote omitted).

     In short, neither the major conventions on the law of war
nor prominent modern international tribunals nor leading
international-law experts have identified material support for
terrorism as a war crime. Perhaps most telling, before this
case, no person has ever been tried by an international-law
war crimes tribunal for material support for terrorism.

     Not surprisingly, therefore, even the U.S. Government
concedes in this case that material support for terrorism is not
a recognized international-law war crime. No treaty that the
Government has cited or that we are aware of identifies
material support for terrorism as a war crime. And the
Government further admits: The “offense of providing
material support to terrorism, like spying and aiding the
enemy, has not attained international recognition at this time
as a violation of customary international law.” Brief for the
United States at 48; see also id. at 55-56 (same).

     To be sure, there is a strong argument that aiding and
abetting a recognized international-law war crime such as
terrorism is itself an international-law war crime. And there
are other similar war crimes. But Hamdan was not charged
with aiding and abetting terrorism or some other similar war
crime. He was charged with material support for terrorism.
                              26
And as the Government acknowledges, aiding and abetting
terrorism prohibits different conduct, imposes different mens
rea requirements, and entails different causation standards
than material support for terrorism. If the Government
wanted to charge Hamdan with aiding and abetting terrorism
or some other war crime that was sufficiently rooted in the
international law of war (and thus covered by 10 U.S.C.
§ 821) at the time of Hamdan’s conduct, it should have done
so.

     The Government latches on to a few isolated precedents
from the Civil War era to prop up its assertion that material
support for terrorism was a pre-existing war crime as of 2001
for purposes of 10 U.S.C. § 821.            There are several
independent reasons that those cases fail to support the
Government’s argument. First, the Civil War cases did not
involve any charges of material support for terrorism.
Instead, several cases involve guerillas who were punished for
taking up “arms” as “insurgents” – that is, for direct attacks
rather than material support. See, e.g., G.O. No. 15, HQ,
Dep’t of the Mississippi (Apr. 3, 1862), 1 OR ser. II, at 472-
76. Others were convicted of “joining, aiding and assisting a
band of robbers and bandits” – in other words, what we would
likely call aiding and abetting, not material support. G.O. No.
19, HQ, Dep’t of the Mississippi (Apr. 24, 1862), 1 OR ser.
II, at 478. In short, those precedents are at best murky
guidance here. Cf. Hamdan, 548 U.S. at 602 (plurality)
(requiring “plain and unambiguous” precedent). Second,
those Civil War commissions were in part military tribunals
governing certain territory – which are a separate form of
military commission subject to a separate branch of law, and
not the kind of law-of-war military commission at issue here.
As others have suggested, their precedential value is therefore
limited. See Hamdan, 548 U.S. at 596 n.27; id. at 608
(plurality) (The “military commissions convened during the
                              27
Civil War functioned at once as martial law or military
government tribunals and as law-of-war commissions.
Accordingly, they regularly tried war crimes and ordinary
crimes together.”) (citation omitted). Third, and perhaps most
to the point, those cases do not establish that material support
for terrorism was a war crime recognized under international
law as of 1996 to 2001 when Hamdan committed his conduct,
which is the relevant inquiry under 10 U.S.C. § 821. The
Government contends that those Civil War precedents
illuminate what it calls the “U.S. common law of war” – not
the international law of war. But the statutory constraint here
imposed by 10 U.S.C. § 821 is the international law of war.
As the Government told the Supreme Court in Quirin, “This
‘common law of war’ is a centuries-old body of largely
unwritten rules and principles of international law which
governs the behavior of both soldiers and civilians during
time of war.” Brief for the United States at 29, in Quirin, 317
U.S. 1 (emphasis added) (citation omitted). To be sure, U.S.
precedents may inform the content of international law. But
those Civil War precedents fail to establish material support
for terrorism as a war crime under the international law of war
as of 1996 to 2001. And even the Government admits that
material support for terrorism was not an international-law
war crime as of 1996 to 2001.

     In short, material support for terrorism was not an
international-law war crime under 10 U.S.C. § 821 at the time
Hamdan engaged in the relevant conduct.

                             ***

    Because we read the Military Commissions Act not to
sanction retroactive punishment for new crimes, and because
material support for terrorism was not a pre-existing war
crime under 10 U.S.C. § 821, Hamdan’s conviction for
                            28
material support for terrorism cannot stand. We reverse the
decision of the Court of Military Commission Review and
direct that Hamdan’s conviction for material support for
terrorism be vacated.

                                              So ordered.
     GINSBURG, Senior Circuit Judge, concurring:       I join
the decision of the Court but, with respect to its holding Mr.
Hamdan’s appeal of his criminal conviction by military
commission is not moot, I do so only because precedent so
dictates. I write separately to explain the unfortunate state of
that precedent, which requires us to review the conviction of a
man long since transferred to Yemen who, even if his
conviction were overturned and he were to hear of it, would
not be affected in any way by the result. Because today’s
decision has no actual consequence for Mr. Hamdan, his case
is moot in fact, though, curiously, not in law.

      The Supreme Court “presumes” the appeal of a criminal
conviction is not moot unless “it is shown that there is no
possibility that any collateral legal consequences will be
imposed on the basis of the challenged conviction.” Sibron v.
New York, 392 U.S. 40, 57 (1968); see United States v.
Juvenile Male, 131 S. Ct. 2860, 2864 (2011) (per curiam)
(“When the defendant challenges his underlying conviction,
this Court’s cases have long presumed the existence of
collateral consequences”). The Government concedes, as
Hamdan’s counsel contends, that it cannot show there is “no
possibility” Hamdan’s conviction will have a collateral legal
consequence for him. The parties’ mutual desire to have the
court decide this case on its merits is of no moment, however;
an Article III court has an “independent obligation to be sure
[it] ha[s] jurisdiction,” High Plains Wireless, LP v. FCC, 276
F.3d 599, 605 (D.C. Cir. 2002), which here requires us to
determine whether the case has become moot on appeal.

    A criminal conviction may and often does have
consequences beyond the penalties imposed in the sentence.
In Sibron, the Court held the defendant’s appeal of his
conviction was not moot, even though his sentence had
expired during the pendency of the appeal, because that
conviction, left undisturbed, could increase his sentence if he
were later to be convicted of another crime. 392 U.S. at 56;
                                  2
accord United States v. Morgan, 346 U.S. 502, 512-13 (1954)
(“Although the term has been served, the results of the
conviction may persist. Subsequent convictions may carry
heavier penalties”). Similarly, in Carafas v. LaVallee, 391
U.S. 234 (1968), the Court concluded that a continuing civil
disability stemming from a criminal conviction, such as a bar
to voting in state elections or to serving as a juror, also keeps
a criminal appeal from becoming moot. Even an adverse
immigration consequence, including a bar on re-entering
United States, may suffice to keep a case alive and hence to
preserve appellate jurisdiction. See, e.g., United States v.
Hamdi, 432 F.3d 115, 121 (2d Cir. 2005).

    Although, in considering a challenge to a criminal
conviction, * “the Court [has] abandoned all inquiry into the

     *
       Contrary to the Court’s reading of the relevant precedents,
Ct. Op. at 12, the Supreme Court does not distinguish between
direct review of a criminal conviction and a collateral attack on a
criminal conviction, by way of a petition for a writ of habeas corpus
or otherwise. The Supreme Court has, on several occasions,
indicated the Sibron presumption applies in a collateral proceeding
for post-conviction relief. See, e.g., Carafas v. LaVallee, 391 U.S.
234 (1968) (holding habeas challenge to criminal conviction not
moot due to “collateral consequences” of conviction); Lane v.
Williams, 455 U.S. 624, 634 (1982) (Marshall, J., dissenting) (“The
majority recognizes that in habeas corpus challenges to criminal
convictions, the case ‘is moot only if it is shown there is no
possibility that any collateral legal consequences will be imposed
on the basis of the challenged conviction’” (quoting Sibron)).
Spencer v. Kemna, 523 U.S. 1 (1998), and Gul v. Obama, 652 F.3d
12, 19 (D.C. Cir. 2011), were habeas cases that did not apply the
Sibron presumption, but the inapplicability of the presumption did
not depend upon a distinction between direct review and habeas. In
Spencer, the petitioner challenged not a criminal conviction but
rather the revocation of his parole. In Gul, the petitioner challenged
not a criminal conviction but rather his designation as an enemy
                                3
actual existence of specific collateral consequences and in
effect presumed that they exist[],” Sibron, 392 U.S. at 55, the
presumption is rebutted if the alleged collateral consequences
are foreclosed as a matter of law. In Perez v. Greiner, 296
F.3d 123 (2d Cir. 2002), the Second Circuit held moot the
direct appeal of a criminal conviction on the ground there was
“no material possibility that [the defendant] will suffer
collateral consequences on the basis of the challenged
conviction,” id. at 125. The defendant in that case had been
deported when his appeal was heard and was ineligible to re-
enter the country because of an earlier conviction. With the
defendant “permanently barred from this country on a wholly
separate ground, the currently challenged ... conviction
[could] have no meaningful effect on his admissibility and
hence [could not] serve as a possible collateral consequence.”
Id. at 126.

     Hamdan and the Government each point to a collateral
consequence of Hamdan’s conviction. Hamdan claims his
conviction for material support of terrorism makes him
subject to permanent mandatory exclusion from the United
States. See 8 U.S.C. § 1182(a)(3)(B)(i)(I), (V). For its part,
the Government claims Hamdan’s conviction may expose him
to an enhanced sentence if in the future he commits a new
offense and is tried therefore in a civilian or military court of
the United States. See DEP’T OF DEFENSE, MANUAL FOR
MILITARY COMMISSIONS, Rule 1001(b)(1)(A) (2010) (“The
trial counsel may introduce [in a sentencing proceeding]
evidence of [prior] military or civilian convictions, foreign or

combatant. Neither decision rested merely upon the ground the
case sounded in habeas. The present Court’s different view of the
matter is of no moment, however; as explained below, binding
precedent unfortunately but unambiguously dictates the Sibron
presumption applies to the direct review of a criminal conviction,
such as Hamdan presents here.
                                   4
domestic, of the accused”); 18 U.S.C. § 3553(a)(1)
(sentencing court shall consider “the history and
characteristics of the defendant”). The adverse collateral
consequence raised by Hamdan is foreclosed as a matter of
law. The adverse collateral consequence posed by the
Government is so far-fetched that the application of the
Sibron presumption in this case risks making our opinion
merely advisory.

     The adverse immigration consequence alleged by
Hamdan is impossible as a matter of law because Hamdan is
already subject to mandatory exclusion from the United States
regardless whether his conviction stands. The Immigration
and Naturalization Act (INA) provides: “Any alien who ... has
engaged in a terrorist activity ... is inadmissible.” 8 U.S.C. §
1182(a)(3)(B)(i)(I). The Government provided overwhelming
evidence, none of which Hamdan bothers to dispute,
demonstrating that he engaged in “terrorist activity” within
the meaning of the INA, including the provision of material
support for terrorism. * The INA makes Hamdan inadmissible

     *
      The INA defines “[e]ngage in terrorist activity” to include
committing “an act that the actor knows, or reasonably should
know, affords material support ...:

         (aa) for the commission of a terrorist activity;
         (bb) to any individual who the actor knows, or reasonably
                  should know, has committed or plans to commit a
                  terrorist activity; [or]
         (cc) to a terrorist organization ... [designated as such by the
                  Secretary of State] ....”

Id. § 1182(a)(3)(B)(iv)(VI). A military commission, the Convening
Authority, and the Court of Military Commission Review each
separately found beyond a reasonable doubt that Hamdan provided
material support generally to al Qaeda, and specifically for an act of
                                5
not for his conviction, which we reverse, * but rather for
having knowingly supported terrorist activities, a historical
fact we cannot reverse. Cf. Fletcher v. Peck, 10 U.S. (6
Cranch) 87, 135 (1810) (“The past cannot be recalled by the
most absolute power”). Nor is it conceivable that an
immigration court, which applies a standard of proof much
more favorable to the Government than does a military
commission, compare 8 U.S.C. § 1229a(c)(2) (“In the
proceeding [for deciding whether an alien is admissible,] the
alien has the burden of establishing ... the alien is clearly and
beyond doubt entitled to be admitted”), with 10 U.S.C. §
949l(4) (in a military commission “the burden of proof to
establish the guilt of the accused beyond a reasonable doubt is
upon the United States”), would find Hamdan in fact had not
provided any of the five types of material support for which
he was convicted and therefore may be admitted to the United

terrorism by, among other things, serving as Osama bin Laden’s
driver and bodyguard from 1996 to 2001, with the knowledge
Hamdan “was protecting the leader of al Qaeda” and was
“facilitating communication and planning used for acts of
terrorism,” United States v. Hamdan, 801 F. Supp. 2d 1247, 1259
(C.M.C.R. 2011); see also id. at 1254, 1258, 1323. Al Qaeda has
been designated as a “foreign terrorist organization” by the State
Department since 1999. See Designation of Foreign Terrorist
Organizations, 64 Fed. Reg. 55,012, 55,012 (Oct. 8, 1999) (initial
designation); 66 Fed. Reg. 51,088, 51,089 (Oct. 5, 2001)
(redesignation).
     *
        Although we reverse Hamdan’s criminal conviction for
material support, we do so not for lack of evidence but rather
because the Military Commissions Act of 2006 does not authorize
retroactive prosecution for an act that was not criminal when done.
There is no comparable bar to retroactivity that prevents the
Government from attaching to those same acts adverse immigration
consequences. See Marcello v. Bonds, 349 U.S. 302, 314 (1955)
(“the prohibition of the ex post facto clause does not apply to
deportation”).
                                6
States. * Because Hamdan is already barred from entering the
United States due to his past involvement in terrorism, his
current conviction has no incremental effect upon his
admissibility and hence the immigration consequence he
proffers cannot serve as a basis for our jurisdiction. Cf. Gul,
652 F.3d at 19—20 (where detention at Guantanamo Bay,
rather than designation as enemy combatant, is the ground for
inadmissibility, immigration consequence of challenge to
designation “too speculative to sustain the exercise of our
jurisdiction”).

    The only other collateral consequence alleged is the
Government’s preposterously hypothetical prospect of an
enhanced sentence if Hamdan is in the future convicted in the
United States for committing another crime. The Supreme
Court held in Sibron that the hypothetical future sentencing
enhancement is sufficient to support the presumption that the
conviction being appealed will have an adverse collateral
consequence and hence to keep the appeal from being moot.
Sibron, 392 U.S. at 56. Subsequent cases, however, cast
doubt upon the continuing validity of Sibron as applied to this
case. In Spencer v. Kemna the Court declined to extend the
Sibron presumption to the appeal of a parole revocation

    *
       In addition to that statutory basis for Hamdan’s permanent
exclusion, Hamdan would not be physically able to re-enter the
country because of his automatic inclusion, as a former
Guantanamo detainee, on the ‘No Fly List.’ See 49 U.S.C. §
44903(j)(2)(C)(v); cf. Gul v. Obama, 652 F.3d 12, 19 (D.C. Cir.
2011) (former detainees are “barred from flights entering the
United States regardless whether a court declares they were
unlawfully detained. An order granting a detainee’s habeas petition
would not mean his exoneration, nor would it be a determination he
does not pose a threat to American interests; it would mean only
that the Government has not proven the detainee more likely than
not ‘materially support[ed]’” terrorism).
                              7
because any collateral consequence in a future sentencing
“was contingent upon [the defendant again] violating the law,
getting caught, and being convicted.” 523 U.S. 1, 15 (1997);
see also Lane v. Williams, 455 U.S. 624, 633 n.13 (1982)
(“The parole violations that remain a part of respondents’
records cannot affect a subsequent parole determination
unless respondents again violate state law, are returned to
prison, and become eligible for parole. Respondents
themselves are able – and indeed required by law – to prevent
such a possibility from occurring”).

     That is, although in Sibron a conviction was presumed to
have adverse consequences for the defendant in a future
hypothetical sentencing, in Spencer the hypothetical
sentencing consequences of a defendant’s parole revocation
were held insufficient to keep his case from being moot
because such consequences are speculative and depend upon
future unlawful conduct by the defendant. Both holdings
were categorical; they did not depend at all upon the
particular defendant’s probability of recidivating. Therefore,
the defendants’ future crimes, apprehension, and conviction
were equally speculative in both cases. It is entirely unclear,
therefore, how the hypothetical sentencing consequences of a
parole revocation could be too speculative to support a
finding of collateral consequences, while the hypothetical
sentencing consequences of a conviction could be concrete
and certain enough to support the presumption of collateral
consequences, and hence Article III jurisdiction, in all
criminal appeals.

     Nonetheless, “[i]f a precedent of [the Supreme] Court has
direct application in a case, yet appears to rest on reasons
rejected in some other line of decisions, the Court of Appeals
should follow the case which directly controls, leaving to [the
Supreme] Court the prerogative of overruling its own
                                8
decisions.” Rodriguez de Quijas v. Shearson/Am. Express,
Inc., 490 U.S. 477, 484 (1989). Accordingly, because
Hamdan’s case is a direct appeal of his criminal conviction
rather than review of a parole revocation as in Spencer, the
Court is bound to hold the Sibron presumption applies and
therefore the hypothetical future sentencing consequences of
Hamdan’s conviction are sufficient to keep his appeal from
being moot.

     Finally, I note that although this is an “appeal of a
criminal conviction,” we have strayed far from the familiar
territory of Sibron and its progeny, which deemed sentencing
consequences the antidote to mootness.          The criminal
conviction in each of those cases was entered in a regularly
constituted civilian court, and the criminal defendant served
time in a domestic prison before being released into the
sovereign territory of the United States. As such, upon his
release the defendant was subject to the criminal laws of the
United States and of the State in which he was located.
Recidivism being common in the United States, it is
unfortunately reasonable to suppose such a defendant may
again be convicted for violating a state or federal law. * By

    *
       Recidivism rates of convicts released from prisons in the
United States are well-known and substantial. See DEP’T OF
JUSTICE, BUREAU OF JUSTICE STATISTICS, RECIDIVISM OF
PRISONERS RELEASED IN 1994 1 (June 2002) (using a sample of
272,111 former prisoners in 15 states, study showed 46.9% were
convicted of another offense within three years of release).
Recidivism rates among Guantanamo detainees are comparatively
speculative, but insofar as they are known, are rather modest. See
DIRECTOR OF NATIONAL INTELLIGENCE, SUMMARY OF THE
REENGAGEMENT OF DETAINEES FORMERLY HELD AT
GUANTANAMO BAY, CUBA 1 (March 1, 2012) (of 599 detainees
released from Guantanamo, 4.7% detained again and “confirmed of
reengaging” in hostilities over a nine-year period). Presuming that
                                9
contrast, Hamdan is presumably in Yemen and is certainly not
in the United States; so far as the record shows, he has never
entered the United States nor expressed any desire to do so;
and he is barred, both legally and physically, from entering
the United States. As a result, the only possible future
sentencing consequence of his conviction by a military
commission would be through extraterritorial application of
our criminal law to a federal crime yet to be committed, or
through a successive prosecution in a military commission for
a future violation of the law of war. It is, to say the least, far
more speculative that Hamdan may find himself again being
sentenced in the United States than it is an domestic criminal
may recidivate and find himself before a domestic criminal
court.




collateral consequences arise from a conviction by a military
commission for violating the law of war and persist after the
convict is released into a foreign country, therefore, hardly seems
justified. Cf. Gul, 652 F.3d at 17 (“not[ing] detention at
Guantanamo and designation as an enemy combatant are recent
phenomena; [therefore a court] ha[s] no basis for inferring they
routinely have collateral consequences”).
