## Facts

- Salim Ahmed Hamdan, a Yemeni national alleged to have worked for Osama bin Laden, was captured in Afghanistan in 2001 and held in U.S. custody.
- After earlier commission proceedings were invalidated, Congress enacted the Military Commissions Act of 2006 (MCA).
- Under the MCA, a military commission at Guantánamo tried Hamdan and convicted him of “material support for terrorism” based on conduct occurring from 1996 through November 2001.
- The Court of Military Commission Review affirmed the conviction.
- Hamdan petitioned for review in the U.S. Court of Appeals for the D.C. Circuit under the MCA’s review provisions.

## Issues

1. Whether “material support for terrorism” was a recognized war crime under the international law of war (or U.S. law-of-war practice) at the time of Hamdan’s alleged conduct (1996–2001), such that it could be tried by military commission.
2. Whether the MCA should be interpreted to authorize retroactive military-commission prosecution for “material support for terrorism” for pre-enactment conduct, consistent with ex post facto limits and fair-notice requirements.

## Decision

- The D.C. Circuit vacated Hamdan’s conviction for material support for terrorism.
- The court held that, at the time of Hamdan’s conduct, material support for terrorism was not an established law-of-war offense triable by military commission.
- Applying constitutional avoidance, the court construed the MCA not to authorize retroactive prosecution for conduct not previously recognized as a war crime.
- The court distinguished the government’s authority to detain enemy combatants from its authority to impose criminal punishment for a newly defined commission offense applied to earlier conduct.

## Legal Principles

- The Ex Post Facto Clause prohibits retroactive criminal punishment; courts avoid statutory interpretations that create serious ex post facto questions.
- A military commission’s criminal jurisdiction is limited to offenses that were established law-of-war violations at the time of the defendant’s conduct when the prosecution is predicated on law-of-war authority.
- Ambiguous criminal statutes are generally not construed to apply retroactively, especially where retroactivity would undermine fair notice.
- Broad references to “violations of the law of war” do not, by themselves, supply the clear content needed to create or retroactively apply a war-crime offense.
- Detention authority in armed conflict does not automatically confer authority to prosecute for retroactively defined war crimes; criminal liability requires clear, preexisting legal prohibition and fair notice.

## Conclusion

The D.C. Circuit set aside Hamdan’s military-commission conviction because material support for terrorism was not an established war crime during the relevant period and the MCA was interpreted to bar retroactive application of that offense to pre-2006 conduct.


Hamdan v. United States, 696 F.3d 1238 (D.C. Cir. 2012)