When can states use lethal drone strikes on terrorists operating abroad? There is little consensus between government lawyers and academics on when international law will permit unmanned aerial vehicles to target individuals. And the need for a common position was given added impetus late last month when a QC announced an international investigation. From a legal point of view, though, it does not matter whether a state uses a drone, a manned aircraft or its ground forces to kill a legitimate target; the laws of war are not the laws of cricket and there is nothing ‘unsporting’ in not putting your pilots at risk.

The barrister who announced he would be investigating the use of drones in counter-terrorism and counter-insurgency is Ben Emmerson QC, of Matrix chambers. In August 2011, Emmerson was appointed as an unpaid special rapporteur on counter-terrorism and human rights by the UN Human Rights Council. While Emmerson is a distinguished practitioner and was runner-up in the recent election for the UK seat on the European Court of Human Rights, the UN Human Rights Council has a much less impressive record. It was set up by the UN General Assembly in 2006 to replace the by-then discredited UN Commission on Human Rights, and many of its 47 member states have questionable human rights records. Europe and the US have just 13 seats on the council, with the rest taken by states from Africa, Asia, Latin America and the Caribbean. Pakistan is a member; the UK is not.

Emmerson said his main objective was to examine evidence that ‘drone strikes and other forms of remote targeted killing have caused disproportionate civilian casualties in some instances’. His team of experts was planning to focus on up to 25 case studies from Pakistan, Yemen, Somalia, Afghanistan and what he referred to as the ‘OPT’ (occupied Palestinian territories). All findings would be put to the states concerned. What, though, was the point of conducting investigations into the actions of the US, the UK and Israel – the three countries currently using drone strikes – when, as Emmerson freely admitted, he had ‘not yet formed any view on the difficult legal questions which arise’?

Whatever the legal framework, Emmerson told me, all states were required to minimise civilian casualties and ‘conduct independent investigations where there is plausible evidence of the commission of a war crime’. That applied if the correct framework was international human rights law, which bans targeted killing unless strictly necessary as a matter of immediate self-defence. It also applied under the US view, as summarised by Emmerson, that western democracies were ‘engaged in a global war against a stateless enemy’, without geographical boundaries or limit of time.

And it applied to what Emmerson described as the third way of analysing the issue: that foreign states could come to the aid of government forces in putting down an internal armed conflict. Such intervention would clearly be lawful if it took place at the government’s request, he explained. What was far less clear was whether that government’s tacit consent or acquiescence was sufficient, and whether this could be inferred from the state’s failure to tackle insurgent groups operating on its own territory.

An analysis of this and related issues has been developed by Sir Daniel Bethlehem QC, legal adviser to the Foreign and Commonwealth Office between 2006 and 2011. In a recent note for the American Journal of International Law, Bethlehem sets out 16 principles governing the so-called jus ad bellum - the threshold for resorting to armed force - in the case of an imminent or actual armed attack by terrorists and insurgents, or ‘non-state actors’ as lawyers prefer to call them.

The Bethlehem principles start off uncontroversially: ‘States have a right of self-defence against an imminent or actual armed attack by non-state actors.’ Bethlehem then becomes a little more controversial, suggesting that armed action in self-defence may be taken against those providing ‘material support essential to the attacks’ provided there is a ‘strong, reasonable and objective basis’ for that conclusion.

He then attempts a definition of ‘imminent’, on which there is little international consensus. And finally Bethlehem suggests that a state acting in self-defence against a non-state actor within the territory of a third state does not need that state’s consent if the third state is either colluding with – or unable to restrain – the non-state actor’s armed activities.

There is much more detail, of course, and Bethlehem stresses that he is putting forward proposals for debate rather than a summary of existing law. But his principles are closely attuned to government thinking and provide a principled, practical way of fighting international terrorism. Rather than looking at what may have gone wrong, they provide a template for getting it right next time.