Mistakes of Fact When Using Lethal Force in International Law: Part II

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If a state believes that it is the target of an ongoing or imminent armed attack and uses force to repel that attack, but it later turns out that it was mistaken and that there either was no such attack or that there was no necessity to respond to it, is that use of force in putative self-defence ipso facto a violation of Article 2(4) of the Charter? Or would the state’s mistake excuse it?

There are three possible options in answering this question:

  • An honest mistake of fact would excuse the state, as in ICL;
  • An honest and reasonable mistake would excuse the state, as in IHL and IHRL;
  • No mistake, however honest and reasonable, would excuse the state – it violated the prohibition on the use of force, and would have to provide reparation for any injury caused.

Any one of these options is plausible in principle. In particular, I do not think that the text of Article 51 of the Charter is entirely dispositive of the issue.

It’s true that Article 51 permits self-defence ‘if an armed attack occurs/ dans le cas où un Membre des Nations Unies est l’objet d’une agression armée’ and that one could therefore say that the existence of an armed attack is an objective fact and a necessary predicate for any self-defence claim. But we routinely do far greater violence to far clearer texts than it would take to incorporate a mistake of fact doctrine into the law of self-defence. The big question is whether we should, not whether we could.

One thing is however clear – that one mistake of fact standard operates in one area of the law doesn’t mean that other areas of the law have to use that same standard. Indeed, we have already seen how ICL uses a purely subjective standard, whereas IHL and IHRL also additionally require objective reasonableness.

A detour to English criminal law

A particularly instructive example in that regard is a 2008 English case, Ashley v Chief Constable of Sussex [2008] UKHL 25. In that case a police officer shot dead a naked and unarmed individual in that person’s own home at night, thinking, in the darkness of the house, that this individual was about to attack him. The officer’s belief was informed in part by an erroneous briefing by his superior that the victim was very dangerous. The officer was acquitted criminally on the application of a purely subjective mistake of fact doctrine – he honestly believed that he had to act in self-defence to repel an imminent threat. The issue for the House of Lords in Ashley was whether the same standard should apply when the officer was sued civilly in tort. It was faced with the same set of three choices that I have outlined above (para. 16, per Lord Scott):

In para 37 of his judgment the Master of the Rolls identified three possible approaches to the criteria requisite for a successful plea of self-defence, namely, (1) the necessity to take action in response to an attack, or imminent attack, must be judged on the assumption that the facts were as the defendant honestly believed them to be, whether or not he was mistaken and, if he made a mistake of fact, whether or not it was reasonable for him to have done so (solution 1); (2) the necessity to take action in response to an attack or imminent attack must be judged on the facts as the defendant honestly believed them to be, whether or not he was mistaken, but, if he made a mistake of fact, he can rely on that fact only if the mistake was a reasonable one for him to have made (solution 2); (3) in order to establish the relevant necessity the defendant must establish that there was in fact an imminent and real risk of attack (solution 3). It was common ground that, in addition, based on whatever belief the defendant is entitled to rely on, the defendant must, in a civil action, satisfy the court that it was reasonable for him to have taken the action he did. Of the three solutions the Court of Appeal held that solution 2 was the correct one. On this appeal the Chief Constable has contended, as he did below, that solution 1 is the correct one. The respondents have not cross-appealed in order to contend that solution 3 should be preferred.

It was common ground among the Law Lords that the correct criminal law standard was the first, purely subjective one, and also that there was no need for the civil law standard to follow the criminal law one (para. 3, per Lord Bingham; para. 17, per Lord Scott; para. 53, per Lord Rodger; para. 76, per Lord Carswell; para. 87, per Lord Neuberger). As Lord Scott then explained in detail (para. 18):

As to assault and battery and self-defence, every person has the right in principle not to be subjected to physical harm by the intentional actions of another person. But every person has the right also to protect himself by using reasonable force to repel an attack or to prevent an imminent attack. The rules and principles defining what does constitute legitimate self-defence must strike the balance between these conflicting rights. The balance struck is serving a quite different purpose from that served by the criminal law when answering the question whether the infliction of physical injury on another in consequence of a mistaken belief by the assailant of a need for self-defence should be categorised as a criminal offence and attract penal sanctions. To hold, in a civil case, that a mistaken and unreasonably held belief by A that he was about to be attacked by B justified a pre-emptive attack in believed self-defence by A on B would, in my opinion, constitute a wholly unacceptable striking of the balance. It is one thing to say that if A’s mistaken belief was honestly held he should not be punished by the criminal law. It would be quite another to say that A’s unreasonably held mistaken belief would be sufficient to justify the law in setting aside B’s right not to be subjected to physical violence by A. I would have no hesitation whatever in holding that for civil law purposes an excuse of self-defence based on non existent facts that are honestly but unreasonably believed to exist must fail. This is the conclusion to which the Court of Appeal came in preferring solution 2.

But Lord Scott was not content simply with embracing solution 2 (honest and reasonable mistake). Rather, his ultimate preference was for solution 3 (no mistake can be an excuse), at para 20:

I would start with the principle that every person is prima facie entitled not to be the object of physical harm intentionally inflicted by another. If consent to the infliction of the injury has not been given and cannot be implied why should it be a defence in a tort claim for the assailant to say that although his belief that his victim had consented was a mistaken one nonetheless it had been a reasonable one for him to make? Why, for civil law purposes, should not a person who proposes to make physical advances of a sexual nature to another be expected first to make sure that the advances will be welcome? Similarly, where there is in fact no risk or imminent danger from which the assailant needs to protect himself, I find it difficult to see on what basis the right of the victim not to be subjected to physical violence can be set at naught on the ground of mistake made by the assailant, whether or not reasonably made. If A assaults B in the mistaken belief that it is necessary to do so in order to protect himself from an imminent attack by B, or in the mistaken belief that B has consented to what is done, it seems to me necessary to enquire about the source of the mistake. If the mistake were attributable in some degree to something said or done by B or to anything for which B was responsible, then it seems to me that the rules relating to contributory fault can come into play and provide a just result. If the mistake were attributable in some degree to something said to A by a third party, particularly if the third party owed a duty to take care that information he gave was accurate, the rules relating to contributions by joint or concurrent tortfeasors might come into play. But I am not persuaded that a mistaken belief in the existence of non-existent facts that if true might have justified the assault complained of should be capable, even if reasonably held, of constituting a complete defence to the tort of assault. However, and in my view, unfortunately, solution 3 has not been contended for on this appeal, its pros and cons have not been the subject of argument, and your Lordships cannot, therefore, conclude that it is the correct solution. But I would, for my part, regard the point as remaining open.

As far as I know, the question of whether English law should move to solution 3 (i.e. mistake of fact in self-defence is never an excuse in the civil context) remains open post Ashley, I imagine because such cases are very rare.

Which solution for the jus ad bellum?

The case is instructive because many of the same considerations that weighed on the Law Lords in fashioning a civil mistake of fact standard and justifying their departure from the criminal law one are of relevance for jus ad bellum self-defence. If a state mistakenly but honestly and reasonably believes it is being attacked, and then acting in that error causes injury to another state, why shouldn’t it be required to provide reparation for the injury caused? Unless the injured state through its own behaviour contributed to the mistake (cf. Art. 39 ASR), why should it be expected to wholly shoulder the burden of the injury? It would in particular be hard to justify a purely subjective mistake of fact standard in self-defence (solution 1), since doing so could easily incentivize resort to unilateral force. States today, especially powerful ones, are hardly too hesitant in relying on self-defence. When deciding on whether to use force in putative self-defence states need to reflect critically on the intelligence information available to them, which is frequently limited and fallible, and should not simply be allowed to invoke an honest belief that an attack against them was ongoing or was imminent.

Normative questions aside, one could probably muster some support in existing practice and jurisprudence for all of these options. The easiest one to defend would be solution 3, i.e. that mistake of fact is never an excuse for violating Article 2(4) of the Charter. The ILC dealt with this problem in a few spare sentences in the ASR commentary (at 329-330):

A State which resorts to countermeasures based on its unilateral assessment of the situation does so at its own risk and may incur responsibility for its own wrongful conduct in the event of an incorrect assessment. In this respect there is no difference between countermeasures and other circumstances precluding wrongfulness [i.e. including self-defence]. (emphasis added)

In Oil Platforms, the ICJ held that the US could not discharge its burden of proving that Iran had committed armed attacks against it. In doing so, the Court framed its inquiry as one into objective fact, rather than as to what the US honestly or reasonably believed. Similarly, when it responded to the US argument that ‘that it considered in good faith that the attacks on the platforms were necessary to protect its essential security interests,’ the Court held that ‘the requirement of international law that measures taken avowedly in self-defence must have been necessary for that purpose is strict and objective, leaving no room for any “measure of discretion”.’ (para. 73)

The ILC and the ICJ thus do not seem to allow for invoking any kind of mistake of fact doctrine in the self-defence context. While neither addressed the merits or demerits of a mistake of fact doctrine in any detail, they both clearly seem to see the justifiability of self-defence in purely objective terms.

The USS Vincennes and Iran Air Flight 655

But there are examples that may cast doubt on such a position. Let’s look at only one, and a particularly appropriate one at that. On 3 July 1988 a US warship, the Vincennes, after exchanging fire with several Iranian small craft, shot down a civilian Iranian airliner by mistake. The commander of the warship believed that Iran Air Flight 655 was actually a military plane on a hostile mission (on the facts, the mistake may have been honest, but would be difficult to label as reasonable).

The US justified its action before the Security Council by invoking self-defence and called the downing of the airliner ‘a terrible human tragedy’ (S/19989). At the Security Council meeting at which the incident was debated, then-Vice President Bush committed the US to paying ex gratia compensation to the victims of the crash ‘strictly as a humanitarian gesture, not as a matter of legal obligation.’ (S/PV.2818, at 58).

Several states other than Council members were also allowed to participate in the debate that followed. Of the participating states, the greatest number (10) deplored the incident as a tragedy or an error while welcoming the ex gratia payment of compensation, (UK, S/PV.2819, 5-7; Italy, S/PV.2819, 21-25; France, S/PV.2819, 26-28; Argentina, S/PV.2819, 33; Japan, S/PV.2819, 39-40; Germany, S/PV.2819, 43-45; Senegal, S/PV.2819, 47; Pakistan, S/PV.2820, 12; India, S/PV.2820, 23-25; Brazil, S/PV.2820, 38). None of these states expressly said that an honest and reasonable mistake of fact in self-defence would serve as some kind of an excuse, but that seems to have been part of the sub-text – certainly none of them condemned the US action as a violation of international law. One more state, Zambia, somewhat more openly appeared to accept error as a possible excuse in principle (S/PV.2820, 8).

An equal number of states condemned the US action directly and (more or less) forcefully. Many of these states – i.e. those not aligned with it in the Cold War – demanded that the US withdraws its military presence from the Gulf, which they saw as a major prior cause of the incident. Some of these states condemned the US action in purely political terms, without invoking international law –  Algeria, S/PV.2819, 51; Cuba, S/PV.2820, 27-28; Romania, S/PV.2820, 30-31; and somewhat more ambiguously, with perhaps an implicit legal argument there, the Soviet Union: ‘how can one regard as self-defence the destruction of a passenger airliner flying within an established air corridor many thousands of kilometres away from the borders of the Power that is supposedly defending itself.’ (S/PV.2819, at 18).

Seven more states expressly used legal language to condemn the US action, if without much consistency. Thus, Nepal ‘would have preferred [to an ex gratia payment] an unqualified apology and the granting of full compensation to the Government of the Islamic Republic of Iran and the members of the bereaved families’ (S/PV.2819, at 8); for Yugoslavia ‘this [was an] irresponsible and incomprehensible act, the shooting down of an Iranian passenger airliner by United States naval forces in the Gulf – which is in contravention of international law’ (S/PV.2819, at 14-15); China ‘the rules of international law that prohibit military actions and acts of violence that threaten the safety of international civil aviation and its normal operation must be strictly observed. … We are of the view that it is only right and proper that compensation be paid to the bereaved families’ (S/PV.2819, at 29-30); United Arab Emirates: ‘The United States therefore bears full responsibility for the tragedy because its forces did not make sure that their target was not a military one’ (S/PV.2819, 55); Libya: attack a ‘barbaric act of aggression’ (S/PV.2820, 16); Syria: ‘This tragic accident has shocked the conscience of mankind and is a violation of the human right to life, liberty and freedom of movement’ (S/PV.2820, 21); Nicaragua: strongly worded political condemnation, but also refers to the rules Chicago Convention the breach of which cannot be resolved by ex gratia compensation ‘but through recognition of the rights of the offended country and the victims’ relatives’ (S/PV.2820, 33).

After the debate the Security Council unanimously adopted resolution 616 (1988), in which it expressed its deep distress at the downing of the Iran Air flight and regret for the loss of civilian life. There was, however, no condemnation in the resolution of the US action in legal terms – which only makes sense since the resolution likely could not have been adopted otherwise in light of the US veto. The Iranian representative expressly acknowledged that the resolution did not protect the legal rights that Iran thought it possessed: ‘The draft resolution under consideration lacks emphasis on the Islamic Republic of Iran’s right under international law to redress and full reparation, a point which is absolutely essential for a position of principle by the Security Council.’ (S/PV.2821, 7).

For his part, the US ambassador reiterated the US position that it had no legal liability:

As the Council is aware, the United States has offered to pay ex gratia compensation to the families of the victims – not as an act of charity, not on the basis of any legal liability but, rather, as a sincere humanitarian gesture. We do so without apology for the action of the Vincennes, which was taken in justifiable self-defence in the context of unprovoked attacks from Iranian forces, which bear a substantial measure of responsibility for the incident. (S/PV.2821, 14)

A subsequent ICAO fact-finding report and resolution did not cast doubt on the honesty of the US mistake. They neither expressly endorsed nor rejected any kind of mistake of fact theory (see (1989) 28 ILM 896). Iran filed a case against the US before the ICJ in which the Court would have had the opportunity to opine on the matter, but the case was ultimately discontinued after a settlement between the parties (Aerial Incident of 3 July 1988).

Conclusion on self-defence and mistake of fact

I confess to being somewhat torn on whether the jus ad bellum should admit of any kind of misstate of fact doctrine. My objectivist instinct is that the answer should be no, for much the same reasons as those articulated by Lord Scott in Ashley. And as I noted above, a purely subjective, honest belief variant of mistake of fact would be capable of far too much abuse. Perhaps a limited, honest and objectively reasonable form of mistake of fact might be appropriate for on-the-spot defensive reactions in situations in which to the state concerned the ‘necessity of self-defence was instant, overwhelming, leaving no choice of means, and no moment of deliberation.’ Perhaps – but even so, unless it somehow contributed to the mistake, I fail to see why a state injured by a putative defensive action should not be entitled to reparation.

In my third, final post I will offer some tentative conclusions on the place of mistake of fact in international law, and some thoughts on how the downing of the Ukrainian airliner by Iran should therefore best be legally assessed.

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Omar Naqib says

January 15, 2020

Hi Marko

Thanks for this and yesterday’s excellent analyses. Throwing in another example: in Land and Maritime Boundary (Cameroon v Nigeria) Cameroon argued that Nigeria had violated Article 2 (4) of the Charter through its military occupation of the Bakassi Peninsula, which was disputed territory . Nigeria responded in the first instance that these acts were lawful self-defence over its territory, but then added in the alternative:

"...even if the Court should find that Cameroon has sovereignty over these areas, the Nigerian presence there was the result of a "reasonable mistake" or "honest belief'. Accordingly, Nigeria cannot be held internationally responsible for conduct which, at the time it took place, Nigeria had every reason to believe was lawful." (para. 311)

The ICJ declined to adjudicate on the jus ad bellum arguments or Nigeria’s responsibility; however it held that by "the very fact of the present Judgment and of the evacuation of the Cameroonian territory occupied by Nigeria, the injury suffered by Cameroon by reason of the occupation of its territory will in all events have been sufficiently addressed" (para.319).

This reasoning is admittedly tricky to conclude on either way for these purposes. However in my view the tacit acknowledgment that Cameroon suffered injury as a result of the occupation -but that this was remedied in other means by the ICJ’s Judgment- implies that Nigeria’s honest and reasonable belief argument would probably not have succeeded had the jus ad bellum claim been examined on the merits.



Adil Haque says

January 15, 2020

Hi Marko,

Here's a thought:

State A reasonably but mistakenly believes that State B is responsible for an armed attack (or will be responsible for an imminent armed attack) against State A.

Stipulate that State B is not responsible for State A's mistake (never mind how this could happen).

State A uses grave armed force against State B.

May State B use armed force in self-defense against State A?

I would think so. Since only an unlawful armed attack triggers the right of self-defense, this suggests that State A acts unlawfully, despite its reasonable mistake of fact.

What do you think? Where is Federica when we need her?

All the best,


Marko Milanovic says

January 15, 2020

Hi Omar,

Thanks a lot for this, that's an excellent example. I don't know what we can get out of the Court's approach - probably even less than you do. That's because the Court says in the same para: 'The Court will not therefore seek to ascertain whether and to what extent Nigeria's responsibility to Cameroon has been engaged as a result of that occupation.' Both the 'whether' and 'to what extent' (responsibility and mitigation) could take into account the effects of a mistake, its honesty and reasonableness. So this seems less categorical than Oil Platforms.

Marko Milanovic says

January 15, 2020

Hi Adil,

I agree with you that B would have a self-defence claim against A, if the necessity to act was there. As to whether this outcome is squareable with a mistake of fact doctrine - I'd say yes, in the sense that the mistake would only be operating as an excuse, not as a justification. (Federica will join us, I'm sure...) That is, the mistaken attack would still be unlawful even if liability for it was excused.

Federica Paddeu says

January 16, 2020

Great posts, Marko! This is indeed a rather under-explored issue in PIL. It’s been at the back of my mind (and in a folder in my Dropbox) for a few years now, though I’ve not done extensive research on it. All I can add, therefore, are some scattered thoughts on the post and Adil’s question.

I think there’s three related though conceptually distinct issues in your post: (i) whether mistaken self-defence is a defence in PIL; (ii) whether it is a justification or an excuse; and (iii) whether an excused actor would be required to pay compensation.

On (i), I’ve not come across any clear State practice on this point. I’ve seen the question raised in different terms: mistake as a defence to a potential breach of the prohibition of force, where mistake is invoked as an independent defence (and not as ‘mistaken self-defence’). This is how Nigeria pled the defence in Cameroon v Nigeria which Omar mentioned earlier, and how the issue is considered in the literature, including the BIICL report on The Use of Force in relation to Sovereignty Disputes over Land Territory.

On (ii), I agree with your position. In international law, for reasons I explore in my monograph, justifications follow the so-called ‘deeds theory’: namely, the circumstances/facts giving rise to a justification must objectively exist. (Under the ‘reasons theory’ justifications will succeed as long as the actor believes that the justifying circumstances exist.) As a result, mistaken self-defence, if it were acceptable, could not be classified as a justification. The implication is that either mistaken self-defence is no defence at all or, if it is a defence, it must be some other type of defence, such as an excuse.

If we do classify it as an excuse, then it is worth querying what the effect of this classification would be. If excuses, as I have argued elsewhere (including in this blog), exclude the consequences of the wrongful act within the law of responsibility, then this includes both cessation and reparation (so also compensation, on which more below). But it does not (necessarily at least) include other consequences of wrongful acts beyond the law of responsibility such as self-defence. So, as Adil points out, State B would have a right of self-defence against State A despite the existence of an excuse.

On (iii), excuses preclude all consequences of the law of responsibility from arising. This includes compensation as a form of reparation. Identifying a legal obligation to provide compensation even though a State is excused requires accepting that excuses can operate as partial defences (and so exclude some but not all consequences of wrongful acts). This is certainly possible, but: (a) there’s no State practice and (b) we need to justify a partial-excuse-not-excluding-compensation on something other than the desire to compensate the injured party, for otherwise the argument would simply assume its conclusion. (I suggest some plausible arguments in my book on this issue.)

An alternative would be to base compensation on the obligation referred to in Article 27(b) of ARSIWA (compensation for material loss). Compensation here would only be for material loss caused, in the sense of actual harm. ARSIWA Commentary notes that ‘material loss’ is a narrower concept than ‘damage’, moral damage (of the relatives of the victims, for example) would be excluded. The same problems arise here: no practice (though note that some of the literature has argued that there is practice in favour of this obligation in relation to state of necessity – I interpret those same instances of practice differently though) and the lack of a compelling rationale (to say that compensation is required because the act is wrongful throws us back into (ii) and the question of partial excuses).

So, an excuse of mistaken self-defence would not exclude the possibility of self-defence against the excused actor by the victim and does not actually provide a legal basis for compensation (on the contrary, it undercuts the basis for compensation). I wonder what the point would be of allowing an excuse to the mistaken actor in these circumstances… Perhaps, as you say Marko, it is simply better to say it’s a wrongful act and roll with the consequences?

Marko Milanovic says

January 16, 2020

Thanks a lot for that Federica!

Adil Haque says

January 17, 2020

Hi Marko,

Of course, that makes perfect sense. You opened by asking whether mistaken self-defense would be "a violation of Article 2(4) of the Charter? Or would the state’s mistake excuse it?" So I suppose the first question (illegality) stuck in my mind, even though you went on to focus on the second (excuse).

Be well,


Eleni Methymaki says

January 22, 2020

Dear Marko,

This comes with a bit of a delay but many thanks again for the thought-provoking posts!

As already discussed briefly off the blog, another potentially interesting example would be the 1904 Dogger Bank incident, in which the Russian fleet mistakenly took British trawlers to be Japanese torpedo boats and, with the Russian-Japan War ongoing, opened fire that resulted in loss of life, injuries and destruction of property. A Commission of Inquiry was established that produced a report (AJIL vol 2, 1908, 929-36) which said some interesting things in paras 8 and 13, as well as in paras 15-17 for the precautions taken by Admiral Rojdestvensky. In this case, Russia offered both (full, I think) compensation and an apology to the UK for the mistake. My understanding from accounts of the event is also that initially the UK was insisting on the punishment of the commanding officers but after the Commission of Inquiry’s findings re the precautions and general attitude of the Admiral they abandoned such claims.

As you've already noted, the Commission of Inquiry did not make its findings on the basis of legal principles, which makes sense as this was a Commission established under the 1899 Hague Convention tasked only with “elucidating the facts of the case by means of an impartial and conscientious investigation’ (Art 9 Hague Conv) and its report was supposed to be ‘limited to a statement of facts, and [to have] in no way the character of an Arbitral Award’ (Art 14). Exceptionally though the Dogger Bank Commission of Inquiry had a mandate that went beyond the establishment of the facts to also apportioning blame and responsibilities on the basis of the established facts (see Art II of the special agreement). And the Commission did that, finding Russia responsible, saying at the start of its report: ‘By making known the prevailing opinion of the commission on each important or decisive point …they consider that they have made sufficiently clear the causes and the consequences of the incident in question, as well as the deductions which are to be drawn from them with regard to the question of responsibility’. The question would then arise on what basis the Commission apportioned responsibility.

Anyway, sorry for the long comment. All this to say thanks again for the great posts on such an interesting topic!

Best wishes,