Against the conflicting nature of legal arguments questioning the recent wave of airstrikes launched by the US in Iraq and Syria, Professor Jason Ralph notes that the intervention is not only legitimate, but also potentially legal.

The United Nations (UN) Charter under Article 2(4) is clear: States must ‘refrain in their international relations from the threat or use of force’. Yet it makes two exceptions within Chapter VII. Article 51 codifies ‘the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations’ and Article 42 enables the UN Security Council (UNSC) to authorise ‘such action by air, sea, or land forces as may be necessary to maintain or restore international peace and security’.

There is arguably a third exception—humanitarian intervention—but this is not mentioned in the Charter and the claim that it is grounded in customary international law is debatable. The much talked about Responsibility to Protect (R2P) norm, which declares a responsibility to intervene when a state manifestly fails to prevent mass atrocities, insists that military intervention must be authorised by the Security Council.

The UNSC has not authorised the current intervention in either Iraq or Syria, so a legal defence of the airstrikes would have to look to the self-defence argument. Indeed, US officials have argued that Iraq has a valid right of self-defence against the Islamic State (IS) in Syria “because the militant group was attacking Iraq from its havens in Syria, and the Syrian government had failed to suppress that threat”.

The frustrating aspect of international law, however, is that it is characterised by a high degree of indeterminacy. There is much scope for interpretive debates and because international society lacks a judiciary of any significance, there is no mechanism for independently judging claims to legality.   Lawyers will argue but politicians will have the scope for deciding which legal argument they sell to their constituency.

For instance, lawyers will question whether the US is threatened by the IS and whether that threat is imminent in a way that justifies a pre-emptive strike. This, however, misses an important element in the current situation where the militant group clearly poses an existential threat for Iraq and where the US is acting in its support under Article 51, which recognises the inherent right to collective self-defence.

But lawyers will also argue that while the US has the approval of the Iraqi government to assist it in its defence against IS, it does not have Syrian consent, and therefore lacks authority to pursue the group in Syria. They will no doubt cite the Nicaragua judgment of the International Court of Justice, which insisted that states had a right to collective self-defence against non-state armed groups only when those groups were sent on aggressive missions by another state. Given that Syria has not sent IS to attack Iraq, lawyers will argue that the US intervention will breach Syria’s sovereignty.

Other lawyers, including those in the US administration, will argue against this “restrictionist” interpretation (see Tom Ruys’ book Armed Attack and Article 51 of the UN Charter). Syria, they will argue, may not have sent IS fighters but has been ‘unwilling or unable’ to stop those fighters. Indeed, the country has manifestly failed to exercise sovereignty responsibly and, thus, it forfeits the right to non-intervention. According to this interpretation, the US has an Article 51 right to assist Iraq to defend itself by pursuing IS fighters in Syria.

Lawyers often disagree, leaving politicians to decide which legal argument they want to present to their constituents, how are we (as voters) to judge their decisions? The oft-cited hazard with “counter-restrictionist” interpretations of Article 51 is that they will create a dangerous precedent. By advancing arguments that enable the US to use force, “counter-restrictionists” risk weakening the power of the law and the restraining effect it has on other states. These arguments, it is claimed, will put international society on a slippery slope to anarchy.

But precedent is only as powerful as the political weight behind it. It will not inform practice if it is politically inconvenient to act upon. Politics plays an important role in judging whether the politician’s decision to adopt a counter-restrictionist legal argument is legitimate. How then does this impact on the current situation?

It is politically significant that the US has managed to unite the region behind its airstrikes against IS in Iraq and Syria. As one would expect, the US is joined by its western allies and regional opponents of the Assad regime. But more than that, Syria’s ally Iran, who was apparently kept informed by the US, equally lacks sympathy for IS; and while Russia will complain about bypassing the Security Council, it is inclined to unite around an anti-IS policy. However fragile, this coalition will add to the claim that the use of force against IS in Iraq, as well as Syria, is legitimate.

However, it is only partly right to conclude that the latest airstrikes in Syria are “illegal but legitimate”. Using this formulation, which is closely associated with NATO action in Kosovo, potentially confuses these strikes with humanitarian intervention, lacking a clear legal base. These strikes are grounded in the law on self-defence, and they are not only legitimate but also potentially legal.

Jason Ralph is Professor of International Relations, University of Leeds, and Marie Curie International Outgoing Fellow, Asia-Pacific Centre for R2P, University of Queensland.