The Legality of Using Force to Deter Chemical Warfare

Most commentators agree that last week’s missile strikes on Syria by the United States, the United Kingdom and France violated international law and the jus ad bellum, the conditions under which States may resort to the use of force in their international relations.

Clearly, the strikes did not fall within the exceptions to the overall prohibition on the use of force in Article 2 (4) of the UN Charter. Syria did not consent to the use of force on its territory; the strikes were not authorized by the UN Security Council; Syria’s alleged repeated use of chemical weapons on its own population did not constitute an armed attack that would trigger a right to self-defence for any of the three countries who carried out the strikes. Furthermore, to the extent the strikes should be perceived as a countermeasure in response to Syria’s unlawful use of prohibited weapons, and thus its violation of international law, it must be noted that armed countermeasures are prohibited under the jus ad bellum. As for the contentious claim that the strikes could somehow be justified as a humanitarian intervention without a UN mandate, the vast majority of states and legal scholars remain highly sceptical of such a right. Even Harold Hongju Koh, one of the staunchest defenders of the legality of humanitarian intervention, does not find that the strikes fall within the boundaries of permissible use of force.

There are, of course, many similarities between the recent missile strikes on Syria and last year’s U.S. strike on a Syrian air base after allegations of a more or less similar chemical attack. For present purposes, one of the commonalities between the two has been the extent to which the legal debate has concentrated on the issue of humanitarian intervention. Koh’s analysis of the recent strikes is a case in point (for references to the discussion last year, see Ryan Goodman’s “What Do Top Legal Experts Say About the Syria Strikes?”; Ashley Deeks’ “How Does the Syria Situation Stack up to the ‘Factors’ that Justified Intervention in Kosovo?” and Harold Hongju Koh’s “Not Illegal: But Now the Hard Part Begins”.)

However, as argued at greater length elsewhere, the strikes do not actually fit very well with our traditional understanding of what a humanitarian intervention is or should be. Although the strikes were launched in response to atrocious acts committed against Syrian civilians, they did not appear to be motivated by an overarching desire to put a halt to the war in Syria and therefore put a stop to the suffering of Syrian civilians. In fact, the public justifications for the strikes (here, here, and here)  strongly indicate that the purpose of the strikes was to merely deter the Syrian regime from continuing its use of chemical weapons.

It seems, therefore, that we should be discussing not if the missile strikes represent a lawful humanitarian intervention, but rather if – and when – a state is entitled to use force to deter another state from using chemical weapons in times of armed conflict.

Such a discussion should depart from the understanding that the international norm against any use of chemical weapons in armed conflict is very strong. In his examination of what he calls the “chemical weapons taboo,” Richard Price, a professor of international relations at the University of British Columbia, notes how the objection to chemical warfare originally seems to be derived from the practical difficulty of controlling the release of chemical substances and the accompanying risk that both combatants and civilians would be exposed to through the chemicals. Thus, chemical weapons conflict with the International Humanitarian Law (IHL) principle of distinction that requires parties to a conflict not to use weapons that cannot distinguish between military and civilian objectives. The lack of distinction, however, does not seem to account for all of the objections to chemical weapons. In the seminal Tadíc case, the International Criminal Tribunal for the former Yugoslavia (ICTY) used chemical weapons as an example of a weapon that is inherently inhumane (see Prosecutor v. Dusko Tadic, Decision on the Defence Motion for Interlocutory Appeal on Jurisdiction, IT-94-1 (2 October 1995), paras. 120-124.).

In international law, the prohibition against chemical warfare is found in a declaration from 1899, a 1925 protocol and the 1993 Chemical Weapons Convention (CWC). It is widely held that the prohibition against using chemical weapons reflects customary international law in both international and non-international armed conflicts. The use of chemical weapons in all types of armed conflict is also listed in Art. 8 2 (b) in the 1999 Rome Statute as a war crime within the jurisdiction of the International Criminal Court (ICC).

Last year, in a post on this site, Michael Schmitt and Lt. Col. Chris Ford argued that the April 2017 American strike on Syria could pave the way for changes to the international regulation of the use force. They noted that we may be

“witnessing … the slow and rather painful birth of a nascent right in customary international law allowing States to act forcefully to put an end to the use of particularly repugnant weaponry against a civilian population, or perhaps even one countenancing forceful State responses to other egregious forms of terrorizing and massacring civilian populations in other countries.”

A right to use force to deter the use of chemical weapons would only emerge in customary international law if there is a supportive general practice among states that is accepted as legally relevant (opinio juris). Despite the recent strikes on Syria, state practice remains far too limited to support the conclusion that international law has changed sufficiently to accommodate the kind of use resorted to by the United States, the UK and France. With regard to opinio juris, however, I think it is fair to say that the international reactions to the missile strikes have been surprisingly positive. This was particularly the case with regard to the strike in April last year where even China offered support. (For international reactions to the recent strikes, see here and here.) Although one should always be careful not to read too much into such broad statements of political support, the international reactions to both sets of strikes do underscore the strength of the norm against chemical warfare and the willingness among many states to uphold respect for it.

So, while the use of force to deter chemical warfare is at present unlawful, I don’t think that we ought to rule out that international law may be slowly moving towards the creation a customary right. For that to happen, however, and as Koh rightly points out, states such as the United States need to begin to articulate its legal justification for resorting to the use of military force.

This prompts the question: What might such a right look like?

A sensible place to begin would be to re-examine the criteria for (alleged) lawful non-UN sanctioned humanitarian intervention that has been put forward by the British Government. According to the UK, using force for humanitarian purposes in the absence of UN authorization will be compatible with international law if

1) ‘there is convincing evidence, generally accepted by the international community as a whole, of extreme humanitarian distress on a large scale, requiring immediate and urgent relief’;

2) ‘it must be objectively clear that there is no practicable alternative to the use of force if lives are to be saved’; and

3) ‘the proposed use of force must be necessary and proportionate to the aim of relief of humanitarian need and must be strictly limited in time and scope to this aim.’

Building on these criteria, the relevant factors for determining the potential future legality of using force to deter a state from using chemical weapons could look like the following: First, the use of force must be a response to convincing evidence that a state has already used chemical weapons against civilians in an armed conflict. One could argue, of course, that a potential right to use force to deter chemical warfare ought to be relevant to all uses of chemical weapons and not only to those instances where chemical weapons have been used against civilians/ non-combatants. After all, the legal ban on chemical weapons covers all use of such weapons in armed conflict. Thus far, however, the international condemnation of the chemical attacks in Syria has focused on the civilian casualties of using such weaponry. Furthermore, since a potential right to use force to deter a state from using chemical weapons would be an exception to the overall ban on the use of force in Article 2 (4), it should be kept as narrow as possible. This will also explain why any right to use force must await the first confirmed use of chemical weapons. Second, there should be no practicable diplomatic alternative to bringing the chemical warfare to a halt other than the use of force. Resorting to force without a mandate from the Security Council must be the last resort. The third and final condition would be that the use of force is proportionate to what is required for deterring the delinquent state to discontinue its use of chemical weapons. In most cases, this requirement will demand that there are indications that continuing uses of chemical weapons are likely.

At present, however, there is neither the state practice nor the opinio juris required for the creation of new customary law. As long as that is the case, the use of force to deter the use of chemical weapons will remain unlawful.

Image: Drew Angerer/Getty


About the Author(s)

Anders Henriksen

Associate Professor of Public International Law at the Faculty of Law at the University of Copenhagen