It has for centuries been lawful to kill enemy commanders, on the principle that “a man who is dead renews no war”, a thought that comforted Cromwell as he viewed the body of Charles I. The outcry in the 1970s over comical CIA plots to murder Fidel Castro by sending him exploding cigars and poison pens led Congress to ban political assassinations under Executive Order 12333: “No person employed by or acting on behalf of the United States government shall engage in, or conspire to engage in, assassination.” This comports with the Fifth Amendment to the US constitution, which protects “any person” (not just US citizens) from being “deprived of life . . . without due process of law”.

Until 9/11, the legal position was clear: in war, active combatants could kill and be killed, subject to rules governing surrender, use of banned weapons, etc. But “war law” applied only to conflicts between armed forces of opposing states, invoking the right of self-defence. Confrontations with insurgents, rioters and terrorists were governed by human rights law, which requires state use of force against serious criminals to be reasonable in the circumstances. This is more restrictive – after three IRA bombers were shot dead on Gibraltar in 1988, the European Court held that the UK had denied them the right to life because MI5 had jumped to mistaken conclusions. In the case of known members of terrorist organisations, the “reasonable force” requirement exercises a necessary and humane restraint over the trigger-happiness of “special forces” and drone targeters. This is why the US, Russia and Israel pretend they are bound only by the law of war, which allows suspects to be killed without much compunction.

The states that deploy drones argue that they are operating under war law, where human rights are less relevant. As Harold Koh, legal adviser to the US state department, puts it: “The US is in an armed conflict with al-Qaeda . . . and may use force consistent with its inherent right to self-defence . . . including by targeting persons such as high-level al-Qaeda leaders who are planning to attack us.” This bald statement prompts many questions. How can you have “an armed conflict” without an enemy state? What criteria are used for putting names on the secret death list: is it enough to be sympathetic to terrorism, married to a terrorist, or anti-American? To provide shelter or give funds to terrorist groups? What is the required degree of proof? There are no accountability mechanisms – no inquests, sometimes not even a casualty list (although the US usually announces and celebrates when it hits a “high-value target”).

In drone warfare, there is no fairness or due process to enable the potential victim, his relatives or any outside body to challenge the accuracy of the information on which the targeting decision has been made. The Senate foreign relations committee reported in 2009 that the Pentagon’s approved list of “prioritised targets” contained 367 names and had been expanded to include 50 Afghan drug lords suspected of donating money to the Taliban. Suppose the suspicion was unreasonable, or the donation had been at gunpoint, or of a negligible amount? What the Pentagon is doing is secretly sentencing people to death for an unproven crime.

The Israeli Supreme Court is the only tribunal to have confronted the legality of targeted kill­ing, at a time (2008) when 234 victims had been members of Hamas and a further 153 had been civilians who got in the way. The court contented itself with comments about limiting the targets to dangerous terrorists and issued Polonius-like precautionary precepts: “well-based information is needed”; “innocent civilians are not to be harmed”; “careful verification is needed before an attack is made”. In reality, innocent civilians very often are killed, and “verification” always seems careful to the minds of the targeters.

Israeli officials seem morally content to risk civilian lives: after a one-tonne bomb was dropped on Gaza City in 2002, killing many civilians in order to assassinate the Hamas military leader Salah Shehadeh, an inquiry merely noted “shortcomings” in evaluation of information. This was a case of manslaughter by gross negligence. The CIA’s anxiety to kill the al-Qaeda leader Ayman al-Zawahiri led to a drone attack in 2006 on a village in Pakistan where he was mistakenly thought to be hiding, and 18 civ­ilians were killed. There was no explanation, no accountability and no compensation for what the CIA calls a “decapitation strike”.

Koh says that drone strikes are an exercise in self-defence under Article 51 of the UN Charter. But Article 51 applies only to attacks (or imminent attacks) by other states, not by terrorist groups. Nobody has yet noticed the irony of squeezing terrorism into this war-law paradigm. Because the Geneva Conventions and customary rights must apply to terrorist and law enfor­cer alike, if it is lawful to kill Osama Bin Laden, al-Zawahiri and Hamas commanders, then it must be lawful for them to kill their opposite numbers – Barack Obama and Binyamin Netan­yahu, generals, allies. (Even the Queen, as head of a co-belligerent state, may qualify.) Those who take the lives of innocent civilians in order to spread terror deserve to be treated like dangerous criminals and shot down when necessity requires, not dignified in law as if they were warriors matched in combat with great states.
What is the position under human rights law? It would obviously be a breach of the right to life if terrorist sympathisers were targeted to deter others, or killed in circumstances where it was possible to arrest them. It would be reasonable to kill terrorists on missions to blow up civilians, or engaged in conspiracies to kill them. But the record of drone attacks demonstrates that often individuals are targeted when they constitute no clear or present danger.

Drone killings in tribal areas of Pakistan and in Yemen have taken the lives of targets who are armed and in conspiratorial meetings, but others have merely been attending weddings or funerals or emerging from hospitals or mosques. In Pakistan, there have been cases where pro-government leaders, their families and even army soldiers have been killed by mistake in drone attacks that have severely damaged US relations with a politically tense, nuclear-armed nation that is not at war with the US.

There was little protest in the US until last year, when a drone strike in Yemen targeted a US citizen, Anwar al-Awlaki, rumoured to be al-Qaeda’s leader in that area. The rockets were fired at his pick-up truck, in which he might have been picked up rather than bombed. Obama’s lawyers said that the Fifth Amendment could not avail a US citizen who joined an enemy force. This is correct as far as it goes, but the Fifth Amendment must entitle a citizen or his family to know whether he is on a death list and to apply to have himself taken off it. When al-Awlaki’s father sought judicial review, the judge told him he did not have standing. If a father does not have standing to challenge a targeted killing, who does?

The Obama administration seems to have given the CIA carte blanche to choose targets, subject to the approval of Koh, a law professor, now an executioner. Those who press the Hellfire buttons in Nevada do not pause to consider whether their targets are engaged in combatant missions or not. But there is no point speculating about the criteria for listing or executing: these are secret CIA prerogatives, beyond the jurisdiction of the courts or the provisions of the Freedom of Information Act.

The battlefield utility of drone technology is such that it will be used widely in future conflicts, and by states much less scrupulous than the US and Israel (Syria and Iran, for example). Drones will become more compact, and more difficult to detect or shoot down – already there are plans for bird- and even insect-sized drones, capable of crawling inside homes or squatting on window ledges to listen and send “kill” messages to their bigger brethren without any “pilot” in Nevada pressing a button.

There is an urgent need for the US to make its drone operations more principled, first, by moving responsibility from the CIA to the department of defence, which is more accountable and bound by the Geneva Conventions. Second, there must be transparency in respect of both the target list and criteria for listing, and an opportunity for those listed to surrender or seek judicial review of whether the evidence against them proves they are an active combatant. Third, rules of engagement must exclude any killing if civilians are likely to be present, and finally, rules must prevent killing of a target who can be captured or arrested.

There is a sense that international law has failed: the UN Charter, the conventions and the norms of the courts have not provided satisfactory guidance for waging asymmetric warfare. Hence the silence of states and the recent earnest request, by the UN’s human rights commissioner, for urgent clarification of the law. The way forward may be to find a way back, to reasonable force and proportionality. At present, many drone killings can only be described as summary executions – the punishment of the Red Queen (“sentence first, trial later”), which denies the right to life, the presumption of innocence and the right to a fair trial.

Geoffrey Robertson QC’s full legal analysis of drone warfare is in his fourth edition of “Crimes Against Humanity” (Penguin, September 2012). Also in the New Statesman's Drones issue: Chris Woods on the legality of drones, Jemima Khan's interview with former Pakistani president Pervez Musharraf and Michael Brooks on the science that makes drones work