The Use of Nerve Agents in Salisbury: Why does it Matter Whether it Amounts to a Use of Force in International Law?

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Over the past few days, there has been discussion of whether the attempt to murder Sergei Skripal and his daughter, in the UK, by the use of a nerve agent amounts to an unlawful use force by Russia in breach of Art. 2(4) of the United Nations Charter and customary international law (see posts by Marc Weller, Tom Ruys, and Ashley Deeks). There is agreement that if the action was attributable to Russia, it would amount to a breach of at least some obligation under international law. Marc Weller, points out that the act would amount to an unlawful intervention and a violation of the territorial sovereignty of the UK. Marko argues that these acts would also be a violation of the human rights of the individuals concerned. However, the British Prime Minister characterised the act as an unlawful use of force. What I wish to do in this post is to ask why this categorisation might matter in international law. What exactly are the implications, as a matter of law, of characterising the act as a use of force? This was an issue that was raised in the comments to Marc Weller’s post and some of the points I make below have already been made in that discussion though I expand on them. As discussed below, this characterisation might have far reaching implications in a number of areas of international law, extending beyond the possibility of self-defence, to the possibility of countermeasures, the law relating to state responsibility, the qualification of a situation in the law of armed conflict, and international criminal law. I accept that many of the points discussed below are not clear cut, and some are even contentious. However, I think that having a catalogue of the possible consequences of the arguments relating to the use of force helps us to see more clearly what is at stake when we make these arguments.  

Use of Force, Armed Attack & Self Defence

The first point is an obvious one, saying that an act is a use of force under international law potentially opens the door to a response in self-defence. Or to put it the other way round, for something to be an armed attack which would justify a response in self-defence, it would have to be a use of force. As the International Court of Justice stated in the Nicaragua case (ICJ Reps., 1986) : “it will be necessary to distinguish the most grave forms of use of force (those constituting armed attacks) from other less grave forms.”  (para. 191, emphasis added). In this particular context, one might ask whether this is an important point given that we are unlikely to see the UK taking military action on the territory of Russia anytime soon. However, if the UK were contemplating a cyber operation in response to the alleged Russian act (as has been mooted here and here), it may well need to consider whether that cyber operation amounts to a use of force and whether that use of force can be justified on the basis of self-defence. That would depend, in the first place, on whether the use of the nerve agents was an armed attack which would in the first place require one to categorise it as a use of force. And it would also depend on a determination that a use of force in self defence is necessary to bring an attack to an end or to prevent a future imminent one, and is proportionate to the armed attack (on which see Tom Ruys’ post).

Use of Force, Countermeasures and Circumstances Precluding Wrongfulness in the Law of State Responsibility

Second, if the use of the nerve agent was a use of force by Russia then it may not be justified as a countermeasure or reprisal under the law of state responsibility (Art. 50(1) ILC Articles on State Responsibility). Now, to be clear, Russian is not even admitting to undertaking the acts and is therefore clearly not seeking to preclude the wrongfulness of the act as a countermeasure in response to a perceived UK breach of international law. However, I note this point because in arguing about the principles at stake here one ought to be attentive to the implications of the argument being made.

It is worth noting that the two points made above arguably pull in different directions. To say that a low-level use of force like the attempting killing in Salisbury is not a use of force caught by the prohibition of force is to say that it can be lawful to do it as a countermeasure. So those like Marc Weller who take the view that these sorts of acts are below a threshold for the use of force can be said to take a permissive approach with respect to this type of low level activity, in that the view taken would allow states to do the act more often than would otherwise be the case. Permissive because it would open up the possibility that such acts may potentially be justified by circumstances precluding wrongfulness.

However, the pull in the opposite direction is this: to say, as Tom Ruys does, that the act is a use of force is to open up the possibility that self defence measures to be taken in response to it. And that position may be said to be a permissive one too with regard to the use of force.

Use of Force and Jus Cogens (and Circumstances Precluding Wrongfulness Again)

A third consequence of characterizing an act such as that in Salisbury (or any other low-level forcible act) as a use of force is that it is then arguably a beach of a jus cogens norm. This is because there is widespread acceptance that there is a jus cogens norm in the area of the prohibition of the use of force, even though there is not always agreement on how to formulate it. Whether one considers that the act in Salisbury amounted a violation of a norm of jus cogens would of course depend on what the jus cogens norm is. It may be argued that it is the prohibition of aggression that is the jus cogens norm and not the prohibition of the use of force itself. It may also then be argued that like the armed attack criterion in the law of self-defence, characterization of an act as an act of aggression requires a gravity threshold [see this piece by Antonios Tzanakopoulos and me, where we note that the General Assembly Definition of Aggression, GA Res 3314 (1974) seems to include a gravity threshold in Art. 2]. However, if one takes a broader view of the prohibition of aggression then any use of force which is a breach of the UN Charter, i.e a breach of the prohibition that cannot be justified under the recognized exceptions, is an act of aggression by the state concerned.

In any event, if one takes the view that the act is a use of force and a breach of a norm of jus cogens then certain consequences would follow. First circumstances precluding wrongfulness under the law of state responsibility may not pleaded as a defence to a breach of a jus cogens norm (Art 26, ILC  Articles on State Responsibility) except of course self defence and consent, which would already be ruled out for one to classify it as an act of aggression. Second if the breach is a serious one, because it is gross or systematic, there are certain consequences that follow under the law of state responsibility (Arts. 40 & 41 ILC Articles on State Resp.) – states must cooperate to bring it to an end and must not recognize the situation created by it.

Use of Force, Erga Omnes Breaches and Third Party Countermeasures

In the comments to Marc Weller’s post, Martin Dawidowicz points out a fourth consequence of determining that the act is a use of force, which is that it would be a breach of an erga omnes norm – one owed to the international community as a whole – such that third states can invoke Russia’s responsibility. More pertinently, such a breach would then permit (in Martin’s view, and mine too) third party countermeasures as a matter of customary international law. This would mean that states other than the UK can impose countermeasures (sanctions consisting of acts that would otherwise breach international law) on Russia. If the act is a breach of a jus cogens norm, then it would automatically be a breach of an erga omnes norm. However, it could be argued that the prohibition of the use of force is itself an erga omnes norm even if that prohibition standing on its own, as distinct from the prohibition of aggression, is not a jus cogens norm.

Use of Force and International Armed Conflicts (and International Criminal Law)

A fifth consequence of charactering the act as a use of force is that it brought into effect, even if only briefly, an international armed conflict between the UK and Russia. Some, like Federica Paddeu, in the comments to Marc’s post, and Charlie Dunlap have frowned on this possibility. I have previously discussed (here, here, and here) the argument that a use of force on the territory of a state without the consent of that state brings into effect an armed conflict between the two states thus leading to the applicability of the law of international armed conflicts in relation to that conflict between the two states. I won’t repeat the argument here. However, I would like to address the concerns that have been expressed by some in relation to this argument. As Rob Lawless points out in his comment to Marc’s post, there seems to be an assumption that if there is an international armed conflict this, on its own, provides authorization to use force on the territory of another state. But it does not! We need to be clear about the different functions of the jus ad bellum and the jus in bello. The former is about whether and when a state is entitled to resort to force. The latter is about the law that applies during an armed conflict. All that the applicability of the law of armed conflict does is to trigger for participants in an armed conflict, the prohibitions, protections and (in an international armed conflict) permissions that might exist in that body of law. Nothing in the law of armed conflict justifies or excuses a violation of the prohibition of the use of force on the territory of another state. Only the exceptions to the prohibition that are contained in the Charter would justify or excuse such a violation.  Thus, the only thing that authorizes the use of force in another state is the jus ad bellum. For the UK to use force against Russia, it would need to show that its actions are being conducted in self defence (since it would, in practice, be impossible for the Security Council to authorize such an action).

I take the point that this legal nuance may be lost on the public. But it is one that lawyers have an obligation to explain to policy makers.

As Ryan Goodman and Alex Whiting have pointed out, if there was an armed conflict between Russia and the UK then the possibility of treating the attempted killings as war crimes arises. This has implications not only for the ICC as the suggest but also has implications for domestic prosecutions whether in the UK or elsewhere of persons who are alleged to be responsible for the acts. Assuming these persons are Russian officials who were acting in their official capacity, the question whether they would have immunity ratione materiae from prosecution would arise. If the acts are war crimes then there is a strong argument that there is no immunity ratione materiae (see this article by Sangeeta Shah and me  and see also the International Law Commission’s  Draft Art. 7 on the Immunity of Officials from Foreign Criminal Jurisdiction adopted in 2017)


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Rob Lawless says

March 17, 2018

Prof. Akande,

Thank you for this very helpful post.

In the last section, you discuss (among other things) the nature of the law of armed conflict. You state that the existence of an international armed conflict does not "provide[] authorization to use force on the territory of another state." Rather, it only "trigger[s] for participants in an armed conflict, the prohibitions, protections and (in an international armed conflict) permissions that might exist in that body of law."

I am particularly interested in the "permissions" you speak about. Do these include "permissions," "authorizations," and/or "rights" of the participants in the armed conflict to engage in hostilities? This question has been debated by many, including a very interesting back and forth between Prof. Haque and Lt. Col. Kels. For those interested, here are the links:


I am fascinated by this question because I believe it represents an underlying assumption (often unaddressed) of so many of the legal debates involving the law of armed conflict. I find it remarkable that States, practitioners, and scholars appear divided on this issue... To me, the question seems central to the conceptual coherence of the law of armed conflict and how it interacts with other related regimes of international law (such as the jus ad bellum and international human rights law).


Marty Lederman says

March 17, 2018

Wonderful post, as always, Dapo. Two very minor things:

1. Do you think many (any?) states would agree that if State B uses force against them *and* that establishes an armed conflict for purposes of the GCs (and customary law), it might still be the case that State A may not use force within State B--or may only do so to the extent "necessary" in an Art. 51 sense?

That conclusion is deeply counterintuitive, of course. And if it is right, it leads one to think that the threshold for "armed conflict" should be understood to be higher than the canonical ICRC "any use of force." But of course the implication of *that* would be that the protections (and immunities) of IHL technically would not apply until one reaches that threshold, which is not a result anyone should want (and is itself counterintuitive), which in turn makes one wonder -- back to my old hobbyhorse! -- whether the better rule would be that IHL (at a minimum) applies to any use of force, and that we should simply cut out the intermediate "is it an armed conflict?" inquiry, which I increasingly tend to think serves little or no useful purpose, and gets us all tangled up in weird answers to other questions.

2. You write that for the UK to able to respond with a lawful cyber operation, Russia's use of the nerve agents would have to have amounted to an "armed attack" *and* the use of force in self defence would have to be "necessary to bring [that] attack to an end or to prevent a future *imminent* one." Do you agree that the future Russian attacks in question would have to be "imminent" (rather than, say, very likely), once Russia has, in fact, engaged in armed attacks and there's no reason to think it will not do likewise in similar situations in the future? I had thought you agreed with me that imminence, as such, was relevant (in assessing necessity) but not required in such a case.

Ralph Janik says

March 17, 2018

Excellent post, many thanks. The big question asked already after the ICJ decision remains open: How can/should states react to actions amounting to the use of force short of an armed attack?

Matthew Small says

March 18, 2018

@Prof. Lederman

In re point 1 of your comment, two points regarding the relevance of blurring or clarifyint certain contours of jus ad bellum and jus in bello.

1. Judge Weeramantry very likely agrees with you on the beneficial nature of blurring the line between an "armed conflict" and "any use of force" insofar as jus in bello is concerned. As he noted in his dissenting opinion to the Advisory Opinion on the Legality of the Threat or Use of Nuclear Weapons, "jus ad bellum only opens the door to the use of force ... whoever enters that door must function subject to the jus in bello."

This simplification also seems in keeping with the goals of international humanitarian law, and provides that protection in the broadest circumstances. In this instance, I would think there is broad support for the blurring of this particular line.

2. The ability of a state to respond to a use of force in it's territory with a use of force of its own--without more--does sound intuitive, but here perhaps a clearer line between jus ad bellum and jus in bello is appropriate and beneficial.

If the existence of an armed conflict, a designation in international humanitarian law, has an impact on the lawfulness of the resort to force, perhaps that is not desirable, as it seems to weaken the modern cornerstone of jus ad bellum, Article 2(4) of the Charter.

If jus ad bellum is kept entirely separate and distinct from jus in bello, then regardless of a use of force against State A and the designation of an armed conflict, State A would still need to open the door to its own use of force using self-defense as authorization (or chapter VII authorization by the UNSC) rather than automatically having its use of force automatically justified by the armed conflict designation.

In other words, perhaps there ought to be an extra justification for the use of force: specifically when a use of force has already happened, but the victim state cannot justify its own use of force as "necessary" as you said.

This can appear counterintuitive in some ways, as you noted, but it also is intuitive if you consider the prohibition on the use of force as only having two exceptions, and that each state should have to justify its resort to force within that purposefully constrained legal framework.

In this instance, then, perhaps it is more beneficial to clarify the line between jus ad bellum and jus in bello?

Chrysanthi Samara says

April 1, 2018

Very useful and enlightening post. Indeed it is crucial the proper characterization of an act under international law, given the consequences that may arise. Apart from the legal aspects of the attempt that took place in the UK, we should also take into consideration its political and diplomatic dimensions. The tension between UK and Russia is escalating, with Nikki Haley, US Ambassador to the UN, saying before the UN Security Council that "the United States stands in absolutely solidarity with Great Britain".