Among the many oddities of the U.S.'s first war crimes trial since World War II is this: The war crime for which Salim Hamdan was convicted, material support of terrorism, has never before been considered a war crime.

As the nonpartisan Congressional Research Service put it in a report (PDF) last year, "Defining as a war crime the 'material support for terrorism' does not appear to be supported by historical precedent."

Hamdan was not convicted of being involved in any plots against civilians. Rather, he was found to have supported al-Qaida "through his service as a driver." There is little doubt that chauffeuring and occasionally serving as a bodyguard to Osama bin Laden violates criminal law. But "war crimes" are a different matter.

International law offers a kind of bargain: Soldiers are allowed to do things that would normally be criminal—namely, kill their enemy. At the same time, they can be prosecuted under international law for acts the world has decided are particularly heinous, such as rape, genocide and torture.

The problem with material support is that it’s never been considered one of those acts. "I can't see it being a violation of war to support combat," says David Glazier, a national security law expert at Loyola Law School and former naval officer.

Glazier, who wrote a thoughtful post himself about the Hamdan verdict, thinks the government should have treated Hamdan either as a prisoner of war or as a criminal, and instead has essentially cherry-picked its charge in branding a domestic law, material support, as a war crime.

"If you wish to subject someone to the burdens of the laws of war, they must then also grant them the legal benefits," says Glazier. "If they kill a soldier as a noncombatant, they shouldn't be tried for a war crime, they should be tried under criminal law, for murder."

The U.S.'s first material support laws were passed in the mid-1990s and have long been the focus of criticism themselves. (The laws ban not only support for specific acts of terrorism, but also "support" of any kind for organizations the U.S. has designated as terrorism organizations. The main concern is that it then allows guilt by association. As I reported earlier this year in a piece for Mother Jones, the government has relied heavily on the charge in building criminal cases against alleged terrorists.)

Material support was first labeled a war crime by the controversial Military Commissions Act of 2006, most widely known for stripping Gitmo detainees of their rights to habeas corpus. Early this summer, the Supreme Court ruled that stripping that right was unconstitutional. But the portion of the law enshrining material support as a war crime remains.

The issue is not just academic. Hamdan's lawyers will likely appeal his conviction. When they do that, his lawyers will almost surely argue to a federal judge that because material support wasn't a war crime before 2006, when the Military Commissions Act was passed, he can't be convicted under that law for something he did five years earlier.

The lawyers made that argument during Hamdan's trial at Gitmo. But the judge, Justice Roberts, didn't rule on that point, deciding instead that all of those arguments should be heard in an appeal after the tribunal was over. That means it's going to come up again -- possibly for the Supreme Court to decide.

Sharona Coutts helped research this story.