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.