[Sina Etezazian serves as regional coordinator for the Digest of State Practice at the Journal on the Use of Force and International Law. He is also a PhD candidate at Monash Law School, where he is researching the necessity and proportionality criteria for the exercise of self-defense in international law.]
The lawfulness of conducting air strikes against the Islamic State Group (IS) in Syria is attracting increasing scrutiny from legal commentators. This scrutiny has intensified markedly (for example, see here, here, here, and here) since the UK’s targeting of alleged IS terrorists using drones and France’s joining the air campaign to bomb IS positions in Syrian territory. The extent to which air strikes would meet the necessity and proportionality requirements in the exercise of the right to self-defense under Article 51, however, remains less explored.
This post does not aim to consider the issue of the permissibility of engaging in unilateral forcible measures against unattributable attacks by private groups. However, even assuming that the lawful exercise of the right of self-defense extends to action against irregular forces, it can be argued that the air campaign in Syria goes beyond the necessity and proportionality conditions of defensive force.
First, the operation in Syria would appear to act in direct contradiction to the legal obligations attached to the “no choice of means” criterion of necessity. As I have explained before (see here and here), “no choice of means” — or, as most legal writers have referred to it, the “last resort” — as a condition inherent in the necessity requirement, denotes that self-defense is available to the victim state only when measures not involving force are unlikely to be practicable and effective to cease an actual armed attack (or prevent an impending attack, supposing that one accepts the idea of anticipatory self-defense). This implies that if measures other than force are likely to be practicable in redressing the wrong caused by the attacker, the victim state may not be entitled to use force under Article 51.
An exploration of state practice since the establishment of the UN would suggest that, in several instances (see here, here, and here), the claimant state highlighted its alleged failed attempts to convince the territorial state to suppress the activities of the non-state entities acting from that state, so as to prove that its self-defense action against those entities had satisfied the necessary requirement. Therefore, whatever the legal merit of the actions themselves (and regardless of whether, in practice, the responding states authentically used force outside an inter-state context), adherence to the “no choice of means” requirement can be distilled from state practice during the UN-era.
Conversely, most states carrying out air strikes in Syria did not even consider cooperating with the Syrian government in suppressing the activities of IS militants in Syria. The United States, for example, explicitly rejected a request for such cooperation, maintaining that it is “not looking for the approval of the Syrian regime.” In its letter to the Security, Canada likewise stated that “in expanding our airstrikes into Syria, the government has now decided we will not seek the express consent of the Syrian government.” The approach taken by US and Canadian officials appears to be in clear violation of the necessity condition of defensive action, mainly because the US and Canada have not provided an explanation of why cooperating with the Syrian government seems impracticable to settle the problem. The use of force in Syria, accordingly, hardly seems compatible with the concept of “no choice of means” that states have shared during the UN-era.
As for proportionality, the air campaign in Syria may be seen to have contravened the geographical requirement inherent in proportionate self-defense. Under the contemporary jus ad bellum regime, defensive action must conform to three criteria to determine its proportionality with regard to a primary objective of halting the attack: effects on civilians, the geographical scope and temporal duration of the conflict (Judith Gardam, Necessity, Proportionality and the Use of Force by States (2004) 155–187). The second of these criteria, usually called the geographical criterion of proportionality, means that forcible self-defensive measures must be limited to the region of the attack that they are designed to repel. In other words, any coercive action that occurs far from the initial attack is likely to constitute a disproportionate use of force (Christopher Greenwood, ‘Self-Defence and the Conduct of International Armed Conflicts’ in International Law at a Time of Perplexity, Yoram Dinstein (ed) (1989) 276–278).
Observance of the geographical criterion of proportionality has been required by both state practice and ICJ jurisprudence (see examples from state practice in Gardam, Necessity, Proportionality and the Use of Force by States, 162–167). For example, in the Armed Activities case, the Court refuted Uganda’s claim of self-defense against attacks from the private groups based in the Eastern Democratic Republic of Congo (DRC). More concretely, Uganda asserted that a string of attacks that had been mounted by those private groups across its border had justified Uganda’s right to use force in self-defense. However, Uganda had taken airports and towns in the DRC, which were located “many hundred kilometers” from Uganda’s border. This extensive forcible response gave rise to the majority judgment observing that the measures undertaken by Ugandan forces were disproportionate to those alleged cross-border attacks (Armed Activities on the Territory of the Congo (Democratic Republic of the Congo v Uganda) (Judgment)  ICJ Rep 223, 223).
However, some commentators have occasionally argued for the diminishing role of the geographical criterion in the assessment of proportionate self-defense, particularly when the situation encompasses the use of force against non-state actors. Thus, in the words of Tams and Devaney:
[R]ecent practice suggests that geographical factors that may be considered relevant to the proportionality of inter-state self-defence are of limited relevance: hence states hit by terrorist attacks on their home soil have asserted a right to respond against terrorists at their base – and even where their conduct was not generally accepted, the fact that the self-defence operation had carried the fight against terrorism into far-away, remote countries seemed to be a factor of limited relevance (Christian J Tams and James G Devaney, ‘Applying Necessity and Proportionality to Anti-Terrorist Self Defence’ (2012) Israel Law Review 94, 104).