The Lawfulness of military strikes against the Houthis in Yemen and the Red Sea

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For almost two months, since January 11, 2024, the United States (US) and the United Kingdom (UK), with support from Australia, Bahrain, Canada, and the Netherlands, have been conducting airstrikes against Houthi facilities in Yemen and the Red Sea in response to Houthi attacks on commercial and merchant vessels in the Red Sea. With their strikes, they aim to protect navigational rights and freedoms for commercial and naval vessels in the Red Sea. Two weeks ago, also the European Union formally decided to get involved in the conflict and launched a naval mission against the attacks by the Houthis. In contrast to the US-led operation, the EU’s mission serves purely defensive purposes, and its mandate is limited to repelling attacks on the High Seas, thereby excluding any direct involvement on Yemen’s territory (here). On Friday, February 24, also the German Parliament (Bundestag) decided on a mandate for the deployment of the German Bundeswehr as part of EU’s Aspides mission (here), and last week, the German naval frigate Hessen was reported to have repelled an attack by the Houthis for the first time.  

In spite of these current events, little has been written about whether the military strikes by Western states are a lawful exercise of the use of force under international law, i.e. whether they are lawful under the ius ad bellum – with some exceptions here and here. The US and the UK justified their actions under the right to self-defence, referred to UN Security Council Resolution 2722(2024), and claimed to ensure freedom of navigation in the Red Sea (here and here). In a similar vein, also states participating in the EU’s mission claim that their military involvement is compatible with international law. For example, Germany relies inter alia on the right to self-defence, Resolution 2722(2024), and the consent of the neighboring states to undertake the mission in their territorial waters (here). However, the applicability of the right to self-defence in regard to attacks by non-state actors on single commercial and naval ships is far from clear, and it likewise is questionable whether Resolution 2722(2024) can justify the use of force. The present post therefore aims to analyze the lawfulness of these current strikes against the Houthis in more detail.

Taking the US and the UK’s strikes as an example, it will be argued that firstly, UNSC Resolution 2722(2024) cannot justify the military strikes, and that secondly, it remains doubtful whether the right to self-defence is applicable in the present case. Lastly, it will shortly be assessed why the operation in Yemen was not justified as an intervention by invitation based on Yemen’s consent – a question that has not been addressed in other writings.

UNSC Resolution 2722(2024)

The US and UK in their letters to the UN Security Council cited UN Security Council Resolution 2722(2024) with regard to their military strikes in Yemen and the Red Sea. Despite their reference to this resolution, it is argued here that the resolution does not provide a justification for their use of force against the Houthis. This finding, though in line with a short statement in Martin Fink‘s post on this blog a few weeks ago, is not directly self-evident from the wording of the resolution. However, a more thorough examination of the resolution and its wording reveals the reasons for this conclusion.

In paragraph three of the resolution, the UN Security Council:

affirms the exercise of navigational rights and freedoms by merchant and commercial vessels, in accordance with international law, must be respected, and takes note of the right of Member States, in accordance with international law, to defend their vessels from attacks, including those that undermine navigational rights and freedoms”.

When comparing the wording of this resolution to other resolutions issued in the past, one can see that the Security Council did not “act under Chapter VII of the UN Charter”, it did not “decide” on the lawfulness of the strikes, and it also did not “authorize Member states to take all necessary measures”, as it would usually do when it wants to authorize the use of force. Rather, it simply “takes note” of the right of Member states to defend their vessels from attacks, in accordance with international law. The wording of the resolution therefore remains rather unclear. The Security Council did not decide whether states have a right to defend themselves against the Houthis, it merely mentions the right to defend itself in accordance with international law without undertaking a judgment in regard to the present case. This rather ambiguous wording should not be interpreted in a way to create a right under international law to defend vessels from attacks. Rather, the reference to “international law” can be interpreted to mean that if there is a right in international law to exercise self-defence against attacks on vessels – a question that the Security Council’s member states could not agree on – such a right would be applicable here. The resolution in itself therefore does not provide a legal justification for military strikes against the Houthis – as was also highlighted by several other countries in the UN Security Council’s debate. For the legality of the US-led operation, it therefore is decisive whether oher claims can be invoked to justify the use of force.

Self-defence – three problems

The second claim made by the US and UK in their letters to the UN Security Council was that the right to self-defence justifies their military strikes against the Houthis (here and here). The right to self-defence presupposes that an “armed attack occurs against a Member State” (Art. 51 UN Charter). Here, the question is whether the attacks by the Houthis on ships and vessels in the Red Sea amount to such an armed attack in terms of Art. 51. And in this regard, three problems arise:

The threshold-problem

The first problem is whether the attacks by the Houthis have the required intensity to qualify as an armed attack. As the ICJ in its Nicaragua Case distinguished the “most grave forms of the use of force (those constituting an armed attack) from other less grave forms” (para. 191), a certain threshold must be crossed for an armed attack to occur. This threshold is usually not too high, and for example, the British Prime Minister highlighted that the Houthis conducted the single biggest attack on a British navy warship in decades (here). In light of this, one could argue that the threshold has indeed been crossed. Moreover, in light of recent events in the last weeks, it is submitted here that the Houthi’s attacks have indeed reached a significant gravity. A cargo ship attacked by the Houthis in February was damaged so significantly that it recently sank in the Red Sea, now raising concerns about an environmental catastrophe due to its oil leak, and in the last few days, a Houthi attack even killed three persons. Accordingly, contrary to an assessment made by Stefan Talmon, it is argued here that the required threshold for an armed attack has been reached.

Alternatively, it could be argued that not every individual incident on the ships constitutes an armed attack, but that the accumulation of repeated, minor attacks together meet the threshold (accumulation of events doctrine, see for details here). The US appears to rely on this doctrine at least implicitly, as it claimed that “a series of armed attacks (…) including several attacks against United States Navy ships” occurred (here). However, this theory is not undisputed in international law, so it is questionable whether the theory does apply in this case. Ultimately, the threshold question depends on the facts in the present case, and the burden of proof for this question thereby lies on the states invoking the right to self-defence.

The non-state actor problem

The second problem revolves around the question whether attacks by non-state actors as the Houthis qualify as armed attacks under Article 51 UN Charter. This question has sparked a debate for decades (see details here), and in a post on this blog, Martin Fink recently highlighted that this question, the “question of authorship”, is also a topical issue in this present case. Despite this assertion, however, this question has not been discussed further in regard to the attacks by the Houthis so far, hence a further analysis is warranted here.

Overall, three broader approaches to the question whether attacks by non-state actors can trigger the right to self-defence have emerged. The first approach argues that self-defence is only possible against states or attacks by non-state actors that can be attributed to a state. An argument in favor of this position is that international law and Art. 51 UN-Charter are said to be inter-state norms that only apply to states – or acts that are attributable to a state. Thereby, different ideas exist as to under which conditions the conduct of a non-state actor can be attributed to a state. Some argue that the group must be sent by a state or that the state must be “substantially involved”, thereby referring to the Definition of Aggression, while others require effective control of the state over the group.

In the present case, the conduct by the Houthis cannot be attributed the Yemen under either approach. Despite the ongoing conflict in Yemen since 2014, the international community considers the Presidential Leadership Council to be the internationally recognized government of Yemen, to which the Houthi’s conduct cannot be attributed. Accordingly, under this first approach, no right to self-defence exists.

A second group of states and scholars argues that self-defence against non-state actors is possible under contemporary international law even when no attribution of the non-state actor’s conduct to the state is given. They argue that due to recent state practice (for example responses after 9/11 or responses to ISIS in Syria), self-defence is now also possible against the non-state actor’s attacks. However, within this approach, there is a debate about the specific conditions for self-defence. Some refer to the doctrine of “unwilling or unable” to fight non-state actors, others argue that effective control of the non-state actor group over the territory of the state is required. In the present case, it can be argued that Yemen was unable to fight the Houthis and that they were also exercising effective control over parts of the territory, so the conditions of the different approaches are met. Accordingly, the US and the UK are allowed to exercise self-defence under this approach.

A third group of states and scholars takes a rather limited approach and considers that Art. 51 UN Charter applies against attacks by non-state actors only if they have established a de-facto regime (see e.g. the argumentation by Germany regarding their strikes against ISIS in Syria in 2014). As the Houthis have established such a regime at least in parts of Yemen’s territory, Art. 51 UN Charter also applies against attacks committed by them.

To conclude, whether the US and UK have a right to self-defence ultimately depends on which approach one takes in regard to this very disputed question, thus rendering the legality far from clear.

The attacks on (naval and commercial) ships problem

Moreover, a third problem exists, which is the most controversial and complex, but also the most interesting problem for the current conflict: the question whether attacks by the Houthis on ships can trigger the right to self-defence (for a discussion of this question see also this post on EJIL:Talk!). In this regard, it must be differentiated between an attack on naval or military vessels on the one hand, and commercial ships on the other hand.

With regard to naval vessels, it can be said that it is widely accepted that military units and military installations abroad are considered as “external manifestations of a state” (see for details e.g. the elaboration here). Therefore, attacks on military ships of a state outside of their territory can constitute an armed attack in terms of Art. 51 UN Charter. This view is supported by Art. 3 letter (d) of the Definition of Aggression, which is used as an indication of what constitutes an armed attack and which lists as an act of aggression the “attack… on the land, sea or air forces of another state”. Moreover, the ICJ in its Oil Platform Case did not “exclude the possibility that the mining of a single military vessel might be sufficient to bring into play the inherent right of self-defence” (para. 72). In the present case, the US and UK have invoked such an attack on their naval vessels explicitly (here and here), so that they are in principle entitled to the right to self-defence against the attacks on their naval vessels.

However, the US and UK’s naval vessels that were attacked on the Red Sea were actually defending the commercial ships in the Red Sea – one therefore can observe a circle where the naval vessels arrive to defend the commercial ships and only at that moment the naval ships get attacked. In light of this circular argumentation, the claim that there was an attack on naval ships seems to be more of a pretext for the attacks on commercial vessels. Moreover, this line of argumentation poses the question whether the Houthi’s attacks on naval vessels could have been “aimed at the specific vessel” or rather “simply programmed to hit some target in the […] waters”, as argued by the ICJ in its Oil Platform judgment (para. 64) (see also Talmon for a similar elaboration, albeit he refers to a slightly different passage of the Court’s judgment (to the phrase “aimed specifically at” in para. 64), where the Court referred to the minelaying – instead of a missile fired at a vessel). Ultimately, the question whether the right to self-defence is triggered by the attacks on US’ and UK’s naval vessels, therefore, remains very unclear from this point of view.

Contrary to military ships, commercial and merchant vessels as the second category of vessels are not considered quasi-territorial extensions of their home states. Therefore, it is disputed among scholars and states whether an attack on these objects nevertheless can be seen as an attack on a state. In the UNSC meeting in January, states expressed diverging opinions on this question: while the US argued that it was “long established that States have a right to defend merchant and commercial vessels from attacks”, Russia and other states held that no such a right exists.

The dispute ultimately revolves around different interpretations of the Definition of Aggression, which lists as an act of aggression an attack on “marine and air fleets of another state”. This wording suggests that military force can be used to defend fleets from attacks, including commercial vessels. However, ambiguity arises regarding the threshold for what constitutes an attack on fleets. Some argue that the reference to fleets implies that a state’s entire merchant fleet and not just single vessels must be under attack to qualify as an “armed armed”, a position for example taken by Iran in the Oil Platforms case. Others argue that an attack upon commercial vessels qualifies as an “armed attack” in terms of Art. 51 UN Charter regardless of the number of commercial vessels that are under attack.

In the present case, one does not have to delve into this dispute further, because the US and UK in any case did not claim that an armed attack upon their commercial vessels has occurred, but only referred to attacks on commercial vessels as such. A reference to their commercial vessels, however, would have been required, as an attack on commercial vessels can generally constitute an attack only on the vessel’s flag state, as stated by the ICJ in the Oil Platform case, where it argued that an attack is only an attack on the state if the ship is flying under the flag of that state (para. 64). As such attacks were not claimed by the US and the UK, they cannot invoke the right to individual right to self-defence (in line with this see Talmon).

The only option then would be for them to rely on the right to collective self-defence. But neither the US nor the UK has in their letters to the UN Security Council referred to the right to collective self-defence or to any request of the flag states to assist them in their right to self-defence, so that the right to collective self-defence also does not apply. And in any case, the strikes would also have to meet the criteria of necessity and proportionality – a point that also raises some doubts here.


In this post, it has been argued why UNSC Res 2722(2024) does not provide a legal justification for the US’ and UK’s military strikes in Yemen and the Red Sea, and why the applicability of the right to self-defence remains disputed for various reasons. Given these controversies, one can have serious doubts as to whether the justification provided by the US and the UK in their letters to the UN Security Council renders their strikes lawful under the ius ad bellum. In light of this, their current strikes rather give the impression the protection of navigational rights and freedoms for commercial vessels in the Red Sea is being pursued for political reasons at all costs and by all means, leaving aside the legal limits provided for military actions in the UN Charter. Overall, it would have thus been preferable – at least from a legal point of view – if the US and UK limited their actions to defensive strikes against attacks on their own ships, or to strikes that would be undertaken with the consent of the respective territorial states – as been claimed by Germany for its operation.

Interestingly, however, such a reference to the consent of Yemen’s government for their military strikes has neither been made by the US nor the UK in their letters to the UN Security Council. Whether Yemen’s government has consented – at least implicitly – to the military strikes on their territory thereby remains questionable. Yemen’s Presidential Leadership Council said to be “in the process (…) to ask the US to expand and coordinate operations and strikes” (here). While one could debate whether this can be seen as a form of implicit consent to the US-led military operation in Yemen, such consent was, in any case, only given after the strikes had started (here). Moreover, Yemen’s (possible) consent was not referenced by the US and UK in their justification letters. Accordingly, the US and UK did not justify their strikes in Yemen and the Red Sea as an intervention by invitation under Yemen’s consent, thus limiting themselves to the aforementioned claims of self-defence and an authorization by the UN Security Council – which remain highly contentious.

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Ash Stanley-Ryan says

March 19, 2024

Dear Leonie,

This is a very interesting piece! I am working on something similar, specifically focused on a few of the coalition partners in Operation Poseidon Anchor. As you will be aware, outside of the article 51 letters the coalition and its constituent members have not just used the claim of individual self-defence, but have occasionally made reference to the law of collective self-defence (this is the stated position of New Zealand), and have increasingly moved away from legal justifications as the operations continue—a shift which risks undermining the legality of their acts, if we assume the strikes are lawful self-defence (I agree with you that this is questionable, in part since it relies on identifying whether the Bethleham principles reflect accepted customary law). I would incidentally be interested to know whether you believe necessity and proportionality in self-defence are static (i.e. tied to the attack which triggers the right), or ambulatory/tied to the factual situation at hand.

It might also interest you to know re: collective self-defence that on January 10, an informal note was produced by the New Zealand Foreign Ministry titled “United States request for New Zealand support for action against the Houthi”. They have declined to release the note's contents, but to weeks later an NZDF contingent was deployed to provide "precision targeting" support. That deployment was justified as an act of collective self-defence. This does, of course, also raise the question of whether a request for collective self-defence need be made overtly, or if this is just convenient for lawyers.

Leonardo Franchi says

March 20, 2024

Thank you for your interesting post.

what are your views about the attribution of the Houthis' actions to Yemen under art. 9 of the Articles of State Responsibility for Internationally wrongful acts.

Kind regards