Protecting commercial shipping with strikes into Yemen: Do attacks against merchant shipping trigger the right of self-defence?

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On January 11, 2024, the US and UK, supported by a group of other States, commenced attacks against targets in Yemen. These attacks were preceded by a range of efforts to secure the sea lines of communication through the Red Sea against continued attacks by the Houthis from Yemen territory. A maritime coalition taskforce was set up (operation Prosperity Guardian) under the operational umbrella of Combined Maritime Forces (CMF) in Bahrain to protect shipping from incoming drones and missiles and the path to the United Nations Security Council (UNSC) was taken. UNSC resolution 2722 (2024), demands stopping the attacks from the Houthis and affirm member States’ rights ‘in accordance with international law, to defend their vessels from attacks, including those that undermine navigational rights and freedoms.’ The resolution, adopted a day before the attacks, did not impose or authorize any measures taken by the UNSC. Days before the attacks, diplomatic joint statements were issues by States to warn the Houthi to stop.   

Both the UK and US are justifying their actions on the right of self-defence. In the traditional interpretation, this right applies in cases of an armed attack by one State against another State. While the temporal aspect and questions on authorship were in the limelight in recent years – and in the present case also a topical issue – less attention is paid to the question whether an armed attack is limited only to attacks against the territory of another State, or whether a broader view is warranted. This post provides some thoughts on this element of the law of self-defence, in particular whether attacking merchant shipping could trigger the right of self-defence, and if it did so in the present case against the Houthis.

Directed against another State

Customary law accepts that an armed attack must be directed against another State. Discussion exists on a possible broader approach beyond territoriality, by accepting that also other entities of a State, such as the armed forces, diplomatic missions on foreign soil or even nationals abroad can be protected by the right of self-defence. It is firmly accepted that attacks against the armed forces of a State could trigger the right of self-defence. For that, they need not be within the territory of that State, but, in the case of naval forces, can be somewhere out at sea. The view is supported by Article 3(d) of the annex to UN General Assembly decision 3314 (XXIX), containing the definition of act of aggression, which mentions that: ‘d) An attack by the armed forces of a State on the land, sea or air forces, or marine and air fleets of another State’ qualifies as an act of aggression.’ The armed forces of a State are direct representatives of a State and, as Claus Kreβ and Tom Ruys mention ‘extraterritorial manifestations of a State’. In the Oil Platforms Case (2003), dealing with an episode from the Iran-Iraq Tanker War, the International Court of Justice (ICJ) considered that it does 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. In that particular case, based on several elements of the law on self-defence, the Court could not find conclusive evidence that the mining of the USS Samuel B. Roberts could be attributed to Iran, found that in light of the damage done to the warship (damaged but not sunk and no loss of life) reacting in self-defence did not pass the criterion of proportionality and also doubted whether the mine specifically targeted the US warship. Unfortunately, for the purpose of the present subject, although the Court did consider the missile attack on the US reflagged merchant vessel Sea Isle City, it limited its considerations to authorship without spending views on whether a merchant vessel could, in fact, trigger the right of self-defence.

Attacking marine fleets

Next to (military) sea and air forces, 3314 (XXIX) notes that an attack against ‘marine and air fleets of another State’ also qualify as an act of aggression. This part of the sentence distinct naval forces from the merchant fleet. Because it says ‘fleets’ instead of (a single) vessel, there is a threshold that needs considering. But how should the term ‘fleets’ be interpreted? Is this one vessel in a State’s fleet (by which ‘the fleet’ is attacked)? How many ships need damaging before a fleet is considered attacked, or – in the present circumstances – could single consecutive attacks on vessels accumulate to attacking a fleet. Several authors (here, here), referring to the travaux, mention that the wording was expressly chosen to apply only in exceptional circumstances, beyond policing actions against fishing vessels or other lesser incidents. Dinstein, however, opines that one cannot rule out the idea that an attack on a single vessel, such as a passenger liner, might trigger the right of self-defence.

So, and if indeed an act of aggression is considered similar to an armed attack, the indicative list of what can be considered an act of aggression would open to the right of self-defence in relation to a civilian vessel. Tom Ruys elaborates that the 3314 (XXIX) aims to define acts under Article 39 of the Charter, to which self-defence may be overlapping, but is not necessarily the same thing. Discussing a possible the legal basis for attacking Houthis in Yemen, James Kraska progresses smoothly from the act of aggression to self-defence in relation to attacks on warships, ‘Against such a force, national self-defense may be taken to counter the imminent and ongoing threat of future attacks.’ What then prevents applying the same train of thought to attacks against a merchant fleet?

Five concerns on taking that path

Several reasons would caution to take that path. First, a State’s merchant fleet is clearly not a direct representative of a State and is in its (peacetime) role of a wholly different character than the naval forces of a State. If it even represents a State, it obviously does so in a lesser manner than diplomatic missions on foreign soil on which discussion exists whether the right of self-defence applies to attacking diplomatic missions. More difficult to assess in this context would be attacking state vessels or commercial vessels that are used by the government for non-commercial purposes, that are somewhere between naval and civilian vessels. Second, while through the maritime flag-principle, a State has exclusive jurisdiction over vessels flying its flag and also holds responsibility over them, contrary to the S.S. Lotus-case view, current law of the sea does not proscribe to the idea that merchant vessels (nor warships) are ‘floating territories’. Third, it may be questionable whether common interests of the freedom of the sea qualify as something that can be defended under Article 51 of the UN Charter in the same vein as territory can be defended. We seem to accept a view that the right to freedom of navigation can be asserted, which may include the use of force. But how vital must hampering this freedom and the commerce its seeks to protect be for the State’s to trigger the right of self-defence? In connection to this (fourth) if self-defence is triggered by attacking a merchant vessel, is a certain link between the reacting State and the attacked vessel, such as flag, needed? Resolution 3314 implies that it not just any merchant fleet. Fifth, the traditional view is that the attack must come from another State. If one accepts that a non-state actor can also be the author of an armed attack, do we accept that an attack against a non-state vessel from a non-state actor qualifies as triggering the right of self-defence? And in the present case, against vessels from States with different flags. How much water can this wine bear before its stops being wine?    

Defence against Houthis

Viewing the present case, it seems that the involved States have steered away from the question whether an attack against merchant vessels could also be considered within the context of self-defence. With regard to the attacks on Yemen, the UK’s legal position notes the following: 

  1. The Houthis have been carrying out dozens of serious attacks on shipping in the Red Sea for a sustained period. British flagged vessels, as well as the vessels of many other States, have been the subject of those attacks. On 9 January, this culminated in an attack against HMS Diamond, involving multiple drones. The government assesses that attacks will continue unless action is taken to deter them.
  2. …[…]…
  3. The UK is permitted under international law to use force in such circumstances where acting in self-defence is the only feasible means to deal with an actual or imminent armed attack and where the force used is necessary and proportionate.

Although the UK points at the attacks on British-flagged merchant shipping, it is careful to underline the attack against the British warship HMS Diamond. In the third paragraph, the UK unfortunately does not specify whether the right of self-defence is triggered by the attack against the warship or whether it considered the situation as whole, including the attacks on civilian shipping, as its basis. In its letter to the UNSC (S/2024/25), the UK seems somewhat more to the point mentioning that the decision to react must be seen ‘in view of the armed attack against HMS Diamond and the ongoing risk to British ships,’ thereby dividing into an armed attack against a warship and – the more general – ‘risks’ for commercial shipping.    

Similarly, the US statements are not explicit. In his statement, US president Joe Biden, first pictures the situation as a whole and then underlines the attacks on US warships:

‘These strikes are in direct response to unprecedented Houthi attacks against international maritime vessels in the Red Sea—including the use of anti-ship ballistic missiles for the first time in history. These attacks have endangered U.S. personnel, civilian mariners, and our partners, jeopardized trade, and threatened freedom of navigation. More than 50 nations have been affected in 27 attacks on international commercial shipping. Crews from more than 20 countries have been threatened or taken hostage in acts of piracy.  More than 2,000 ships have been forced to divert thousands of miles to avoid the Red Sea—which can cause weeks of delays in product shipping times. And on January 9, Houthis launched their largest attack to date—directly targeting American ships.

In the US-letter to the UNSC (S/2024/56), the US seems to have gone through some effort to strengthen the link between the attacks and targeting US naval ships by elaborating on this part of the attacks, next to the threats to commercial shipping. 

By explicitly underlining the attacks on warships, it seems to have fulfilled the criterion that the attacks are directed against the armed forces of a State, leaving it unnecessary to argue in detail whether the attacks against merchant shipping are ground for triggering the right of self-defence. What the conflating of the attacks on civilian vessels and warships in one flowing argument appears to do, is presenting a case where the circumstances as a whole seem grave enough, thus shaping arguments for a necessary and proportional response.


Whether attacks on civilian vessels triggers the right of self-defence, remains subject to debate. The  present case, in which in which self-defence is used as a justification to defend from attacks against merchant shipping, is inconclusive as to whether States have accepted that attacks on merchant vessels can trigger the right of self-defence. In combination with the attack on civilian vessels, both the UK and US underline the attacks against their warships, bringing their argument more firmly within their right of self-defence. Its legal justifications have not brought clarity on whether an attack against civilian shipping could be accepted as triggering the right of self-defence. From all this, a careful conclusion can perhaps be distilled that it shows that reacting against attacks of civilian shipping currently does not sit comfortably within the law of self-defence. 

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Andreas Paulus says

January 26, 2024

Thank you for the very informative post. However, the Security Council itself - or at least 10 States voting in favour - seems to regard, by res. 2722 (2024), self-defense as justified (though without mandating it under Chapter VII).
Para. 2 reads: The SC "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" The combination of the assertion of certain rights by unclear references to international law as their basis rather than acting under Ch. VII, is well-known from the IS-resolution. But it is difficult to claim this para. does not defend the position that general attacks on civilian ships are to be treated as attacks on the fleet. Any thoughts on this?

Din Darzon says

January 27, 2024

Thank you for this thought provoking piece.
Just a quick comment, if I may - To my mind, taking into account the object and purpose of Article 51 to the UN Charter, I find it hard to conceive why an armed attack on the territory of another State, which targets civilian infrastructure, should be any different from an armed attack against a civilian vessel, for the purpose of triggering the right of self-defense. However, in order to avoid a misuse of the right of self defense in such cases, I think the more pertinent issues would be the observance of the customary principles of necessity and proportionality, which could operate in order to limit the scope of the right of self defense against armed attack on civilian vessels. In cases of armed attack against civilian vessels, I think, that at least in most cases, the scope of the right of self-defense would be rather limited, in particular the principe of proportionality, as it would require the State to use force in self-defense in order to halt a repel an ongoing (and potentially future imminent) armed attack against these vessels
Again, many thanks for this very interesting piece.

Rob McLaughlin says

January 29, 2024

Martin, thank you for this really interesting post - this is a really difficult question and you have done it justice.

My query is as follows: Although I agree there is a difference between thresholds for, labelling of, and consequences flowing from, uses of force against a warship on one hand and merchant vessels on the other (requiring, in the latter situation, more focus upon factors such as scale, damage, the depth of link to the flag state, and many other factors, as you note), I am not sure the divide between warships and other vessels on state non-commercial service is as equally stark? I am not sure the perspective of 'merchant vessels' - the more demanding threshold - when assessing use of force against these ships is the correct analytical start point. I think it is perhaps more legally consistent to start from the 'other end' with these vessels as they are (in most manifestations) as equally sovereign immune as warships. Thus, a single attack on a non-warship but sovereign immune vessel could, I think, be treated as more akin to an attack on a warship than on a merchant vessel.

In the former case, a use of force against a single vessel is highly likely to cross the armed attack threshold; in the latter, as you say, there are real hurdles (not insurmountable, depending upon the context, but real and quite differentiating none the less) to distilling an armed attack from a use of force against a single merchant vessel.