Were US commandos acting lawfully when they killed Osama bin Laden in Pakistan on May 2? If it is lawful to kill a political leader in order to save the lives of his future victims, what about Gaddafi in Libya, Assad in Syria or Mugabe in Zimbabwe? And what about the terrorist leaders who find themselves targeted by the Israelis?
Under US law, the position is tolerably clear. Killing a captive who poses no immediate threat is illegal under military and civilian law, however much blood he may have on his hands. But the Navy Seals who shot bin Laden and members of his entourage were entitled to use proportionate force in self-defence. It is no answer to say they would not have been at any risk if they had not invaded bin Laden's compound.
With the benefit of hindsight, it does not look as if the commandos were in immediate danger. They apparently met little resistance and were not injured. Bin Laden was not said to have been armed.
So could they have captured bin Laden alive and brought him back for trial, as the Israelis did with Eichmann 50 years ago? Almost certainly not. They were surely entitled to proceed on the basis that bin Laden could have set off explosives while he remained alive. That was the assumption made by the police officers in London who killed Jean Charles de Menezes in 2005. Though this was an appalling tragedy and the Brazilian was entirely innocent, the officers were not found to have acted unlawfully. These days, a suspected terrorist cannot be regarded as safe unless he's found alone, stark naked and in an open space.
What, though, if the US had decided to drop a bomb on bin Laden's suburban compound? The question of self-defence would not have arisen.
In that event, the Americans would no doubt have justified their actions as part of the "war on terror". Bin Laden declared war on the US by attacking its territory and killing its people on September 11, 2001. That turned him into a legitimate target in the eyes of the US Attorney General. "It is lawful to target an enemy commander in the field," Eric Holder said after bin Laden was killed.
A moment's thought is enough to expose the difficulties of this analysis. Wars are waged by states, not individuals. Though it may be legitimate to bomb targets in a country with which the US is at war, that hardly applies to a country like Pakistan — with which the US maintains diplomatic relations. US troops are not even meant to operate inside Pakistan without permission, unless there is an overriding threat to national security.
Of course, the position is different once the United Nations is involved. In March, the UN Security Council passed resolution 1973, authorising "all necessary measures" to protect Libyan civilians. The air strike on Gaddafi's compound that killed his youngest son a day before the attack on bin Laden suggests that Nato regards the UN resolution as justification for killing the Libyan leader himself.
There is, inevitably, room for doubt over whether killing Gaddafi is necessary to stop him attacking his people. Some argue that it would be counter-productive. Others pin their faith on the prospect of arresting him and bringing him to trial at the International Criminal Court. All one can say about that is that it has not proved to have been much of a deterrent so far.
Nor was it for bin Laden. Still, says the Left, we should have tried to capture the al-Qaeda leader alive and put him on trial like Milosevic and the Nazis at Nuremberg.
But this misses the point. The wars in former Yugoslavia and Germany were over by the time those responsible were brought to trial. If bin Laden had been captured, his followers would surely have been inspired to launch further attacks on US targets in the hope of persuading the Americans to release him.
International lawyers have always prided themselves on their practicality. In deciding what the law is, they take account of what governments do. They now see states targeting not only international terrorists but also heads of state, who have traditionally enjoyed immunities under international law. Their response tends to be a grudging acceptance of harsh realities coupled with a wish that it did not have to be so.
In the real world, it does. It may not be wise to intervene in other country's wars. We may kill innocent people if we shoot first and ask questions later. But if the law books say it's never lawful to kill tyrants, it's the books that need changing.
The Israelis regard terrorist leaders as ticking bombs that must be neutralised before they cause carnage. Lawyers call that "anticipatory self-defence" — which means you don't have to wait for the other guy to shoot first. Although death must be no ground for rejoicing, that strikes me as a very sound rule of law.