The Legality of ASPIDES Protection Activities in the Framework of the Collective Countermeasures Doctrine

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Any views and opinions presented in this post are solely those of the author and are expressed exclusively in the author’s personal capacity.

Note by the Editors: This post has been amended (three sentences were deleted and several minor edits were made) subsequent to publication at the author’s request, none of which affect the substance of the post.


Several posts on EJIL:Talk! (see Fink, Brassat, Henderson and Carli), and posts/articles published elsewhere (see e.g. Talmon, BuchanMcLaughlin, Svicevic, Raina, Dhar and Pedrozo) have examined the issue of the legal basis for the use of force in the course of current military operations in the Red Sea and Gulf of Aden (GoA), including those of EUNAVFOR ASPIDES. All such analyses, however, are mainly concentrated on whether these activities are legally tenable under Article 51 of the UN Charter or as counter-piracy operations. In doing so, they end up raising doubts on the legality of the actions in question without providing potential solutions, also through the adoption of an alternative line of reasoning.

In this respect, perhaps some further considerations developed “on field” by practitioners could provide some useful insights on the matter and contribute to dispel any lingering doubts on the lawfulness of military activities. At least, this is the purpose of this post; with the caveat that the analysis will be limited to EUNAVFOR ASPIDES and will not encompass military activities, including kinetic activities, carried out by other military operations in the same area.

ASPIDES in a Nutshell

EUNAVFOR ASPIDES is military European Union Common Security and Defence Policy (CSDP) maritime security operation that was launched by the EU Council on 19 February 2024 with Council Decision (CFSP) 2024/632 to safeguard the freedom of navigation in relation to the present Red Sea crisis. ASPIDES’ mandate is contained in Council Decision (CFSP) 2024/583 of 8 February 2024. The Operation’s Mission is that of contributing to maritime security along the main sea lines of communication in the area of Bab-El-Mandeb Strait and the Strait of Hormuz, as well as international waters in the Red Sea, the GoA, the Arabian Sea, the Gulf of Oman and the Persian Gulf (so called “Area of Operation” – AOO). ASPIDES strategic Headquarters (“Operation Headquarters”) is established in Larissa (Greece), while the operational/tactical Headquarters (Force Headquarters – FHQ) is at present afloat, located on board the Italian Navy frigate Virginio Fasan, which is now sailing in the AOO with the role of Flagship.

According to Article 1(5) of CD 2024/583, the Operation is mandated to carry out 3 tasks, that are: (a) accompany vessels; (b) ensure maritime situational awareness; (c) protect vessels against multi-domain attacks at sea in a sub-area of the AOO.  Vessel protection is an executive task and implies the use of coercive powers, including armed force. Protection is provided against any imminent or ongoing attack on any third-party merchant/commercial vessel (CD 2024/583, Preamble, para. 8). Close protection – that is a protection at close distance during the whole transit through the above-mentioned sub-area – is granted, upon request via the MSCHoA, to merchant/commercial vessels of EU interest, irrespective of the flag flown.

According to the mandate (Article 6), ASPIDES cooperates with other military operations in the region, including the Combined Maritime Forces, but cooperation is limited to maritime situational awareness and de-confliction of maritime operations, including information-sharing on imminent/ongoing attacks at sea. ASPIDES is not involved into any kind of armed confrontation against the Houthi Regime in Yemen and does not participate in any manner whatsoever to the bombings on the Yemeni soil.

As the Operation is prevented from conducting military activities on land, its defensive action is limited to “halting and repealing” imminent or ongoing attacks to merchant/commercial vessels, according to the tenets of the classical and most restrictive self-defence theory (see e.g. Kretzmer, p. 260; Lubell & Wood, p. 10).  According to the leading literature on the topic, “imminent armed attack ‘must be an impending attack over which there is a reasonable level of certainty that it will occur in the foreseeable future’ and the threat must be ‘specific and identifiable’. It is uncontroversial that imminence, as it relates to pre-emptive self-defence, must refer to an ‘objectively verifiable, concretely imminent attack’, rather than to unmaterialized and speculative threats, which are the domain of unlawful preventive self-defence” (O’Meara, p. 295, also quoting Lubell, pp. 702-5, 718). Notably, any attack does not need to be “completed” to be tackled, but simply underway. It is not required that its harmful consequences have already been materialised. The traditional example is the case of a missile defence system destroying missiles that have already been launched against a target (see Dinstein, p. 229; Green), also when the response is automated (Grimal, p. 338).

Since ASPIDES assets are bound to “act in full compliance with UNCLOS” (CD 2024/583, Preamble, para. 8), thus in the framework of a peacetime legal regime, force is to be administered according to procedures similar to those in place in maritime law enforcement (see Tondini, p. 260), although such procedures could encompass the use of heavy weapons and missiles as well, when this comes out to be “necessary”, i.e. there are no other options available, and “proportionate” to the threat posed by the single attack (CD 2024/583, Article 1(5)(c)).

The Problem of Finding a Viable Legal Basis to Military Activities in the Red Sea and Gulf of Aden

As observed by Martin Fink, assessing the legality of any military actions in response to attacks carried out by Houthis against merchant/commercial vessels in a jus ad bellum perspective under Article 51 of the UN Charter presents some major legal problems. The first is that it is rather difficult to consider these attacks against merchant vessels as an “armed attack” tout court, because: a) they are not targeting “marine fleets”, following the original notion of fleet (see the 1974 UNGA Decision on the definition of aggression, Annex, Article 3(d)); b) they entail a level of force/violence that is not enough severe in scale and effects (see the ICJ in Nicaragua, paras 191 and 210). In addition, while an attack against a single warship may hypothetically be conceived as an UN-Charter-Article-51-type of “armed attack” against its flag-State (ICJ, Case Concerning Oil Platforms, para. 72), it is highly questionable whether an attack against a private vessel may per se amount to the same kind of attack, although there are authors who admit this possibility (see e.g. Lott & Kawagishi, p. 135). Nonetheless, even if one considered Houthi attacks as armed attacks, the only party allowed to respond would be the victim vessels’ flag-States (Oil Platforms, para. 64), or potentially other States acting in collective self-defence (Raina). However, to be invoked, collective self-defence would require at minimum a request by the victim flag-States, and an agreement with the State acting in their defence. At the moment, such conditions are not present. As to the EU, for instance, Article 42(7) of TEU and Article 222 of TFEU are inapplicable, as they both refer to attacks on/assistance in a Member State’s territory. Ultimately, even if one applied the “accumulation of events theory” (Ruys, p. 168; Oil Platforms, para. 64), accumulation should be calculated only with regard to assaults to ships belonging to the same flag-State.

Problems also arise if one attempts to apply the legal framework of counter-piracy activities. Indeed, under Article 101 of UNCLOS, piracy concerns “any illegal acts of violence or detention, or any act of depredation, committed for private ends by the crew or the passengers of a private ship or a private aircraft against another ship or aircraft, or against persons or property on board such ship or aircraft [emphasis added]”. On the opposite, Houthis’ attacks are first and foremost politically-motivated. Besides, a Houthi missile attack clearly does not fit in this definition. Conversely, with regard to aerial drone attacks, in principle they could possibly be considered within the definition of piracy, as aerial drones are regarded as “aircraft” (EU Regulations 2019/945 and 2019/947). However, the private ends would still be missing. In addition, Houthi drones could hardly be held as “private aircraft”.

Notably, military activities are conducted in the absence of a specific UN Security Council Resolution authorizing the use of coercive powers. Although in paragraph 3 of Resolution 2722 (2024) the Security Council “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” [emphasis added], this statement does not represent a carte blanche granting the authority to any States or international organization to act in defence of any merchant vessels (Talmon; BuchanSvicevic).

Finally, framing the protection of merchant/commercial vessels, including foreign vessels, within the principle of self-defence as established in domestic law would be somewhat unconvincing (see also Christian Henderson’s analysis on “unit/on the spot” self-defence). First of all because applying domestic law (most likely domestic criminal law rules on self-defence) vis-à-vis third parties in international waters (thus outside any national espace juridique) seems highly questionable; secondly, in view of the protecting warships’ lack of jurisdiction towards foreign private vessels. 

Protection of Merchant Vessels as a Collective Countermeasure

A viable solution to the legal hurdles seen in the previous paragraph could be that of framing the international response to Houthi attacks against merchant/commercial vessels at sea in the framework of a collective countermeasure (see e.g. ARSIWA, Article 54 & Commentary, p. 137; ICJ in Activities in the Area, para. 180; Jackson & Paddeu; Dawidowicz; Lim & Mitchell; Gestri). Indeed, Houthi attacks against merchant/commercial vessels transiting thorough the Bab-El-Mandeb Strait hamper the freedom of navigation and communication, governing international waters, that is often described as one of the classical examples of obligation erga omnes (Fasia, p. 529). The breach of this obligation is undertaken by means of forcible actions not reaching the level of an armed attack against a large and indefinite number of flag-States. As a consequence, the international community, including the EU (Commentary to ARIO, pp. 72 and 92-96; O’Connell; White) – and not only the single flag-States of the merchant/commercial vessels attacked – would be entitled to react with a collective countermeasure that could entail a use of force up to the same degree and effect of that used in violation of the obligation, even if they were not directly injured by Houthi attacks. The collective countermeasures doctrine is getting the momentum due to the international response to the Russian invasion of Ukraine (see e.g. Clancy, pp. 535 ff.; Hathaway et al., Benvenisti & Cohen; Bartolini).

The application of this theory, however, does not come without its problems (see Guilfoyle, pp. 70-71). Traditionally, in fact, countermeasures are to be taken against States (ARSIWA, Article 49), whereas the Houthi Regime is not recognised by the international community; be preceded by an offer to negotiate (ARSIWA, Article 52); be proportionate and readily reversible (Commentary to ARSIWA, p. 131); and, more importantly, should not involve forcible measures (ARSIWA, Article 50; Commentary to ARSIWA, p. 132).

The first three conditions can be easily met. Although the Houthi Government is not an internationally recognised government, it can be held a de-facto State authority, as it exercises effective control over a large portion of the country’s territory and population, having also established rather solid State-like administrative structures. Moreover, the Houthi Government has been subject to an individual sanctions regime by the UN Security Council since 2015 and in 2022 was added as a single entity to the UN Yemen sanctions list. This gives a clear indication of the Houthis’ de-facto capacity to bear passive international legal obligations (see mutatis mutandis, the ICJ in the Advisory Opinion on Kosovo’s independence, paras 116-117). Alternatively, it can be argued that exactly in the same way as self-defence for the purpose of Article 51 of the UN Charter can today be invoked against non-State actors located within the boundaries of a State, especially when the legitimate territorial government is “unwilling or unable” to tackle them (see, ex plurimis, Starski), a countermeasure can be addressed towards the same type of actors in the same circumstances, when their actions rise to such a degree of power that may bear meaningful effects at the international level vis-à-vis third States.

Coming to negotiations, even if a proper negotiation with the Houthi Government has not been conducted by the EU before the launch of the Operation, the latter comes at the end of a long political process, during which Ansarallah has been requested several times by the whole international community to stop the attacks (see e.g. UNSC Res. 2722 (2024), par. 2) but it firmly decided not to follow the requests received. ASPIDES actions are also definitely proportional to the harm caused by the violation of the aforesaid erga omnes obligation, as its actions are limited to halting and repealing the attacks, that is a way to render the violation itself ineffective. The actions in question are also obviously reversible. Having their missile and drones downed, or vessels/aircraft stopped and repealed, when found assaulting, or about to assault, merchant vessels would not bring irreversible effects for the Houthi Government.

Legality of Forcible Countermeasures Carried Out by ASPIDES

The analysis on the use of force in countermeasures deserves a little more attention. Here, “forcible countermeasures” are to be understood in the sense that they entail the use of coercion and means/method to administer such coercion, but force is kept under the level used in response to an armed attack and is not related to the prohibition established in Article 2(4) of the UN Charter. Indeed, the countermeasures undertaken by ASPIDES are not meant to be punitive (so they cannot be seen as a retaliation) but are simply aimed at inducing the Houthi Regime to stop attacks at sea by rendering such attacks ineffective. Indeed, “halting and repealing” imminent or ongoing attacks against merchant vessels, inter alia, does not meet the contextual requirements of the prohibition of the use of force as established in Article 2(4) of the UN Charter, since ASPIDES’ forcible actions do not hamper Yemen’s territorial integrity or sovereignty, neither are they against the UN purposes, as actually they are aligned with such purposes (ICJ, Nuclear Weapons, par. 48). The rationale is similar to that characterizing third States’ military interventions by host State’s invitation, where the use of force is deemed lawful for the lack of the same contextual requirements (ILA, p. 18; Byrne, p. 99). Here, in addition, the level of force is not enough severe in scale and effects to affect international relations (see Pobjie, pp. 179-180), as any defensive action is undertaken against each specific attack, at tactical level, under a ‘tit for tat’ approach (Kretzmer, p. 260; O’Meara, p. 94), without even extending to the Yemeni soil.

Generally, within the international legal community there is some support to the legality of a low-level use of force in response to a similar level of use of force exercised in an unlawful manner, although “the proper legal basis for such forcible responses remains controversial” (Ruys, p. 177). Remarkably, the legality of forcible countermeasures “short of war” was purported, inter alia, by Judge Simma in his famous separate opinion in the Oil Platforms case (paras 12-13) and by the Institut De Droit International in its 2007 Resolution on Self-Defence (par. 5). There are studies that consider attacks by warships against drones as forcible countermeasures and posit their legality in cases when drones are not even considered as a security, but a mere safety, threat (Jackson, p. 250). Others invoke the countermeasures regime as a legal basis for lawful actions in response to cyber-attacks. In all evidence, accepting the legality of these armed reactions “resolves the problem of leaving states defenceless in the face of a small-scale attack against them, ‘plugging’ the ‘gap’ between article 2(4) and article 51” (Green, p. 379). In this respect, the US pragmatically equates “use of force” and “armed attack”, so that any kind of attack suffered end up being lawfully contrasted by a proportional counter-attack (Schmitt, p. 720).

Conclusion: Inspiring and Coherent

 The brief analysis developed in the previous paragraphs has attempted to offer a viable and sound interpretation of the legal basis for the conduct of protection activities carried out by Operation ASPIDES. This interpretation is alternative to the ones elaborated so far by other scholars in their assessments of the ongoing military interventions in the Red Sea and Gulf of Aden. The latter interpretations are in fact mainly centred on self-defence under the UN Charter, but do not seem to offer final answers to the numerous questions raised by the application of this principle to the case at hand, whereas our view is meant to bypass all such legal challenges, also trying to inspire the legal debate on the topic. By contrast, our reading may also raise criticisms as it departs from the traditional view on the unlawfulness of forcible countermeasures and paves the way to the legality of collective countermeasures. However, it has the merit of being coherent with the Operation’s mandate. Aiming at coherence in international law may be considered somewhat “chivalric” (d’Aspremont), but perhaps sometimes it is even effective.

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