A reply to Brassat: The Military Strikes Against the Houthis in Yemen and the ‘Fourth Problem’ of Necessity and Proportionality

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A few days ago Leonie Brassat published an excellent piece on EJIL Talk! which discussed the possible legal bases upon which the current military strikes against the Houthis in the Red Sea and Yemen might be located. The piece centered on the right of self-defence and set out ‘three problems’ which were focused exclusively on the ‘armed attack’ criterion as found in Article 51 of the UN Charter. In responding to Leonie’s piece, I wish to also bring into the analysis a potential ‘fourth problem’ in the form of the reconciliation of the military strikes with the twin customary principles of necessity and proportionality.

This analysis is arguably important for two main reasons. First, much was made in the justificatory discourse of the acting states of the fact that the military strikes were both necessary and proportionate (see, for example, here, here, here, here, here, here, and here). Secondly, while the ‘armed attack’ criterion as found within Article 51 of the UN Charter may still be seen by some as the sine qua non of the right of self-defence, states rarely elaborate upon this particular criterion either in their justifications for defensive military action or their responses to such action, but instead often focus more on the necessity of the action and, to a slightly lesser degree, its proportionality.


Whilst there is no universally accepted interpretation of these twin criteria, satisfying the necessity criterion would arguably require that the acting states on this occasion had no reasonable alternatives to undertaking the military strikes against the Houthis (see Ruys at 95 and the discussion in Henderson at 300-305). Various attempts at non-forcible measures had reportedly been attempted by the acting states, including various more diplomatic overtures towards the Houthis as well as the matter being taken to the UN Security Council. It might, however, be argued that the resort to such non-forcible means is not reasonably required in this instance as they lacked ‘any realistic prospect of success’. In particular, given that the Houthis are designated as a terrorist organisation by several states, attempts to negotiate with them were arguably futile. In addition, the Houthis had made it clear that the attacks would continue as long as Israel’s military operations in Gaza continued, something that neither the US nor the UK clearly had any direct control over.

In her piece, Leonie discusses consent as a possible independent legal basis for the military strikes, but dismisses it due to both the vague and implicit nature of any known statements made in this respect by Yemen’s Presidential Leadership Council (PLC) along with the fact that ‘the US and UK did not justify their strikes in Yemen and the Red Sea as an intervention by invitation under Yemen’s consent’. Yet, from the perspective of the necessity of the action in self-defence, while attempts at negotiating with the Houthis were arguably futile, and any suggestions that there was a requirement to attempt to do so was unreasonable, it might be argued that the acting states should have at least first attempted to seek the consent of – and coordinate military efforts with – the de jure recognised government of Yemen in the form of the PLC. Of course, given the PLC’s lack of control over large parts of the territory of Yemen, along with the fact that it itself was embroiled in a protracted civil war with the Houthis, any attempt to work with the government might also ultimately be seen as futile. However, this does not release the two states from at least first attempting to do so. Despite certain statements made by the PLC that might be construed as providing some form of implicit consent, these were made after the military strikes had commenced and there was no indication that the acting states had reached out to and requested its consent to operate on Yemeni territory prior to their commencement.

In assessing the necessity of the military strikes temporal issues are also of relevance. In particular, it is difficult to justify military action in self-defence as being necessary if the armed attack in which it is in response to is over. In this instance, however, the attacks by the Houthis were clearly of an ongoing nature, having commenced in November 2023 and continuing today. Consequently, on this basis not only is it difficult to characterise the military strikes as armed reprisals, but arguably rendered references by the acting states to the strikes being in response to an ‘imminent threat’ of attack as unnecessary. However, this must be qualified by the fact that while the general stream of Houthi attacks were ongoing, there was, as Leonie highlights, a lack of clarity as to which attack(s) formed the basis for the invocation of the defensive military response and whether they can be construed as ‘armed attacks’ for the purposes of the right of self-defence, as well as the extent to which self-defence was being invoked more for the general protection of shipping in the area and of navigational rights and freedoms. If there was no right of self-defence available, then clearly no amount of force can be justified as necessary under it.

Of further relevance in assessing necessity are the stated aims of the acting states, in this instance being to ‘degrade’ the capabilities of the Houthis to carry out further attacks as well as ‘deterring’ them from doing so. Clearly, the military strikes had the result of destroying missiles, radars, underground storage facilities, command and control centres, and drone sites. In that sense, they can be said to have degraded in some way the ability of the Houthis to carry out attacks, and thereby plausibly had a defensive necessity.

Yet, while one can arguably reconcile deterrence with defence, it is also the case that any military strikes, including those with the result of degrading attack capabilities, may have the reverse effect of making the adversary even more determined. In this instance, there was a clear manifestation of this in both the hardening of the position of the Houthis following the initial round of military strikes against them in January 2024, as well as the rallying of support for the Houthis by Yemen’s domestic population. This response consequently places question marks over both defensive necessity and proportionality of any of the strikes taken subsequently with deterrence as the aim.


While a level of academic agreement has coalesced around the meaning of the principle of necessity in the jus ad bellum, upon what basis the proportionality of any defensive military action is to be determined is somewhat less clear (see the discussion in Henderson at 316-322). One way of making a proportionality assessment would be through weighing the harm caused by the defensive military action to that caused by the preceding attack. In this respect, at the time the military strikes commenced on 11 January 2024, at least, no casualties and very little damage to the vessels concerned had been reported because of the Houthi attacks. Rather, the concern appeared to be more focused on the effects of the Houthi attacks on the disruption to the free flow of commerce than any actual physical harm or potential risk of harm to the ships and mariners onboard. In this light, the proportionality of the launching of many rounds of missile strikes upon the territory of a state and which have led to relatively extensive infrastructure damage can be questioned.

A perhaps more settled interpretation of the jus ad bellum principle of proportionality would, however, seem to require that any defensive military action is restricted to that which is required to achieve the specific defensive objective. As such, seen in the context of the clearly continuing attacks by the Houthis, in addition to the acting states having already engaged in more limited ‘unit’ or ‘on the spot’ defensive action which had clearly proved unsuccessful in halting the attacks, going to the source of the attacks might arguably be seen as proportionate, notwithstanding the potentially catastrophic broader regional implications of doing so. However, and as with the assessment regarding necessity, this is qualified by the lack of clarity regarding which attack(s) the strikes on Yemeni territory were taken in response to. While by 11 January an array of flagged vessels had been attacked, there was limited evidence to suggest that the attacks which had affected UK and US vessels, even taken cumulatively, justified anything further than a unit-level response.


While it is of course to be welcomed that the acting states attempted to engage with the jus ad bellum in justifying the military strikes, doing so appears to have raised as many questions as answers, both from the perspective of the armed attack criterion, as discussed in Leonie’s piece, but also in regards to the necessity and proportionality criterion, as discussed above. This raises the question as to the extent to which the jus ad bellum is sufficiently determinate to be able to adequately regulate such actions, as well as provide the necessary clarity to facilitate sufficiently objective post-facto assessments to be made. It is, in addition, open to the question of how it deals with situations involving such a degree of factual uncertainty.

As a final consideration, Leonie made the point in her piece that the legal justification of self-defence was seemingly being used as a front to enable action to be taken to create stability in the region in the protection of the free flow of commerce. In this respect, the coherence and integrity of the jus ad bellum would have been maintained if the self-defence justification had been restricted to any attacks on their own ships or if the strikes had been undertaken with the consent of the PLC. However, one might also ask why the UN Security Council seemingly provided its – what might be seen as controversial – blessing to the invocation of self-defence for these purposes, but did not authorise states to use ‘all necessary means’ to achieve these arguably non-defensive aims, or at least, and from the perspective of justifying the necessity of the defensive actions, why some attempt was not seemingly made for it to do so. Indeed, while it is difficult to view the disruption to international commerce perpetrated in this way as falling within the concept of an ‘armed attack’, it might perceivably be categorised as a ‘threat to the peace’, thereby opening the door to Chapter VII measures at the Council’s disposal. The Council seemingly providing its blessing for the invocation of the right of self-defence to protect such broader interests may have a precedential value that may not, as yet, be fully appreciated.

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DH says

April 1, 2024

Dear Christian,
Thank you for this very interesting piece, and for indeed addressing an issue that is sometimes overlooked in discussion and debates in the field of the law on the use of force.
Although it might seem as mere semantics, I would argue, contrary to your assessment that : "going to the source of the attacks might arguably be seen as proportionate, notwithstanding the potentially catastrophic broader regional implications of doing so" seems to conflate between necessity and proportionality ad bellum. To my mind, in light of the circumstances as described aptly in your post, it might have been necessary for the defending States to go after the source of the attack, therefore arguably compatible with the necessity requirement. However, going after the source of the attack, coupled with the potential serious implication emanating from the response, might lead to the conclusion that it was nevertheless incompatible with the proportionality requirement, depending of course on how one interprets the test of proportionality. If one adopts the view that the test for proportionality is a harms-benefit one, this attack may be regarded as disproportionate. However, if one adopts the view that the test is a means-end, then the analysis becomes much more nuanced and questionable.
Again, many thanks for this very interesting and excellent post.

Chris H says

April 3, 2024

Thanks DH for your comment.

Part of the problem is that it is not entirely settled what necessity and, to an even lesser extent, proportionality require in the ad bellum context.

I had taken necessity in this sense to mean that there was no other reasonable option available than the recourse to military force in self-defence. And there are, as discussed, some question marks on this issue, particularly why consent of the de jure recognised government was not seemingly sought for the strikes on Yemeni territory (although I didn't mean to suggest that there was a hard obligation to do so, only that it is arguably of probative value in assessing necessity).

While the harms-benefit assessment of proportionality is one that has been discussed in the context of the Gaza conflict, I'm not sure it is one that has, as yet, become the dominant interpretation of this criterion, with means-end arguably still holding sway in this respect. Again, there are, as discussed in the post, questions regarding whether the military strikes were proportionate on this basis, given the lack of clarity over which attacks they were in response to and the aims of the strikes.

But, yes, if we were to view the situation from the harms-benefit perspective, the escalation in the response and the real possibility for greater regional conflict would be relevant, and again possibly leading to the strikes falling foul of this criterion.