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A federal appeals court in Washington today reversed the conviction of Salim Hamdan, who worked as a driver for al Qaeda and Osama bin Laden, saying a military commission improperly applied a law that wasn't meant to be retroactive.

Hamdan was convicted under the Military Commission Act of 2006 for "material support of terrorism." He served his time and has since been released in Yemen.

But a unanimous three-judge panel said today that Hamdan's conduct occurred before the Military Commissions Act of 2006, and that the federal statute in effect at the time of his conduct from 1996-2001 did not authorize prosecution for "material support for terrorism."

"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," Judge Brett Kavanaugh wrote, "Hamdan's conviction for 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."

Hamdan was captured in 2001 in Afghanistan in a car that contained two anti-aircraft missiles and an al Qaeda-issued document. He was turned over to US authorities and later transferred to Guantanamo Bay, Cuba, and detained as an enemy combatant.

He was sentenced to 66 months, with credit for time served. In late 2008 he was transferred to Yemen and was released there, but continued to appeal his conviction.

The opinion was released on a day that KSM's lawyers were defending him in a military court in Guantanamo Bay Cuba.

"The decision is a loss for the commission system itself, in which, without any dissent, the trial judge and Court of Military Commission Review held that it was simply beyond question that material support was a recognized violation of the international laws of war. " says Stephen I. Vladeck, a professor of law at the American University Washington College of law. "That view received exactly zero votes from a very conservative panel of a court that has not exactly been sympathetic to claims by Guantanamo detainees.

"It's hard to imagine a stronger rebuke of the quality (or lack thereof) of the legal reasoning employed by the military commission or by the Court of Military Commission Review-and that repudiation may be where today's decision has the greatest ramifications," he said.