Was the killing of Osama bin Laden really a war crime?

We'll probably never know the accuracy of all the details in Seymour Hersh's alternative account of the killing of Osama bin Laden. But Hersh's version has enough verisimilitude that it calls for reconsidering what has always been the most troubling legal question, even under the official version of the event: Was the shooting of bin Laden proportionate and therefore justified under international law? Or was it, to put the matter bluntly, a war crime?

Recall that, when the White House first broke the story, it incorrectly stated that bin Laden had been reaching for an AK-47 when he was shot. Were that true, the killing would have been legal under the U.S. interpretation of international law. Since Congress had declared war on al-Qaida after Sept. 11, bin Laden was a combatant — and it's permissible to shoot an armed combatant in wartime. True, you have to accept that the struggle against al-Qaida is really a war, and that the battlefield extends to the whole world — propositions that many non- American international lawyers dispute. But at least within the official U.S. version of the laws of war, the killing would not have been problematic.

When official word came down that bin Laden had in fact been unarmed, another legal justification became necessary. The laws of war require proportionate force to be used against the enemy. One variant of the story has it that bin Laden was shot first in the body, disabling him, then in the head to make sure he was dead. If this had been true, the second shot or shots sound uncomfortably like a coup de grace — which would be illegal, as the first shots would've rendered bin Laden out of combat under the laws of war.

The government's argument seemed to be that the shots to bin Laden's head were legally justified because bin Laden might have had a button for a suicide bomb or a remotely triggered explosive device on his person. Under the circumstances, shooting him in the body wouldn't have guaranteed the safety of the shooters. Shooting him in the head would therefore arguably have been justified, because it would have been proportionate to the amount of force needed to defeat the enemy while preserving the safety of the U.S. troops.

If this theory sounds tenuous to you, you're not alone. Nevertheless, it furnished the fig leaf the Barack Obama administration needed so that the president didn't find himself bragging about a killing that was unlawful even under the U.S. interpretation of the laws of war.

Hersh's account — which cites both named and unnamed sources — differs in three important ways, raising new legal questions. First, Hersh says that the U.S. knew bin Laden was being held in the Abbottabad compound as a prisoner of the Pakistani ISI, that country's intelligence service. The U.S. knew that his ISI guards would be gone from the compound — and as a result, Hersh implies, the U.S. knew with a high degree of likelihood that there would be no guns in the compound and no resistance.

If the U.S. in fact knew that bin Laden was effectively a prisoner — not to mention that he was unarmed and unprotected - - then killing him would almost certainly have violated the laws of war. A former combatant who is "hors de combat" — rendered incapable of fighting — isn't a lawful target for killing. A prisoner is a perfect example. (To make matters worse, Hersh also reports that bin Laden was known to be an invalid.) Of course, Seal Team Six couldn't be 100 percent certain that bin Laden was unarmed and without a kamikaze device — and who knows what orders they received. But according to Hersh, the chain of command knew bin Laden was effectively an unarmed, unprotected prisoner.

The second difference in Hersh's account is the claim that killing bin Laden, not taking him captive, was a condition set by senior Pakistani generals for allowing the mission. According to Hersh, neither the Pakistanis nor the Saudis wanted bin Laden talking. But the motive is irrelevant. If the U.S. agreed in advance to kill a captive bin Laden, then the whole operation takes the form of a targeted and premeditated extrajudicial killing. That would be fine according to the American interpretation of the laws of war, like any other drone strike - - so long as bin Laden was a combatant, not a Pakistani prisoner.

The final twist is Hersh's version of the killing itself. According to his sources, the SEALs were led directly to bin Laden's room by an ISI operative, then opened fire massively and immediately, killing bin Laden instantly. Under the laws of war, you're allowed to ambush somebody. There's no obligation to give an enemy the chance to surrender. But the description supports the theory that bin Laden was a prisoner of the ISI being handed over for execution.

Does it matter? Does anyone care whether killing bin Laden was lawful at all? "By law, we know what we're doing inside Pakistan is a homicide," a former SEAL told Hersh. "Each one of us, when we do these missions, say to ourselves, 'Let's face it. We're going to commit a murder.' "

Perhaps there are circumstances where breaking the laws of war is morally justifiable. By today's legal standards, the bombings of Hiroshima and especially of Nagasaki are difficult to justify, given the essentially civilian nature of the targets. Yet many Americans no doubt still believe that Truman was right to use nuclear weapons to shorten World War II and save countless American lives.

Maybe killing bin Laden was one of those circumstances. If it were known for certain that bin Laden had been shot like a fish in a barrel, it's still highly unlikely that there would be much protest about it in the U.S. But given that we're trying the 9/11 planners in Guantanamo for gross breaches of the laws of war, and that they will presumably be executed once convicted, it's worth taking a moment to investigate war crimes. And who we think should be punished for committing them.

— Noah Feldman is a professor of constitutional and international law at Harvard and the author of six books, most recently "Cool War: The Future of Global Competition."