Eunavfor Aspides and the Ratione Temporis Application of the Right of Self-Defence

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The post is based on my previous one “La missione EUNAVFOR Aspides dell’Unione europea e l’applicazione ratione temporis del diritto di legittima difesa”, published (in Italian) on SIDIBlog on March 19, 2024


On February 19, 2024 the European Union (EU) launched EUNAVFOR Aspides, a military maritime security operation in response to the current crisis in the Red Sea. The Mission is led by Greece, which provides the operation commander (military-strategic level), while Italy holds command of the forces (operational and tactical level). Aspides falls under the EU’s Common Security and Defence Policy (CSDP), which constitutes “an integral part” of the Common Foreign and Security Policy and enables the EU to resort to civilian and military assets “for peacekeeping, conflict prevention and strengthening international security” outside the territory of member states (Treaty on the European Union or TEU, Art. 42).

As is well known, in the wake of the conflict that erupted on October 7, 2023 between Israel and Hamas in the Gaza Strip, an increasing number of incidents off the coast of Yemen were reported, including attacks on civilian and commercial vessels, as well as the seizure and detention of commercial vessels. These attacks – condemned by the Security Council (here and more recently here) – are perpetrated by the Houthis, an armed political and religious group formed in the 1980s and 1990s (see here and here), which control almost the entire western part of Yemen and claim to be part of the Iranian-led “axis of resistance” against Israel, the United States, and the Western bloc in general.

Aspides (from the Greek word aspís, meaning “shield”) is part of international efforts, including the naval military operations Atalanta, also launched under the CSDP, and the US-led Prosperity Guardian and Poseidon Archer, to protect freedom of navigation in the Red Sea and international waters of adjacent maritime zones, including the Arabian Sea and the Persian Gulf. As also declared by the High Representative Borrell (here and here), the Mission “should remain defensive in nature” (CFSP Decision 2024/583 (also “Decision” here), recital 8).

The current scenario in the Red Sea raises some questions on the use of force under international law (see inter alia here, here and here on this blog and here, here, and here). The purpose of this post is to offer some brief reflections on the exercise of the right of self-defence by the personnel employed for the Mission (“EU Forces”) from the standpoint of its application ratione temporis.

Mandate, Objectives, and Use of Force Permitted 

Aspides’ strategic objective is to “ensure a Union naval presence in the Area of Operation with the aim of ensuring freedom of navigation for vessels, in close cooperation with like-minded maritime security providers” (Decision, Art. 1(4)). To this end, EU forces are required to: a) accompany vessels in the Area of Operation (i.e., the area within which the tasks delegated to the Mission are carried out); b) ensure maritime situational awareness in the Area of Operation; and c) protect vessels against multi-domain attacks at sea in a sub-area of the Area of Operation (ibid., para. 5).

The task under (c), in particular, must be performed “in full respect of international law, including the principles of necessity and proportionality” (ibid.). Hence, there is an implicit reference to the right for EU Forces to act in self-defence, in accordance with those two principles of customary international law. This provision is to be read in conjunction with the preamble of the Decision, which states that EU Forces “should act in compliance with applicable international law, including customary international law, including self-defence where conditions are met, to defend against an imminent or ongoing attack on their own, or third-party, vessels” (Decision, recital 8). Moreover, the Mission should act in full compliance with the UN Convention on the Law of the Sea (ibid.).

Therefore, Decision 2024/583 establishes the right of the EU Forces to act in self-defence to repel offensive actions launched by the Houthis in a particular portion of the Area of Operation, in compliance with applicable international law. The aforementioned provisions find their legal basis, at the primary law level, in the EU’s obligation to abide by international law in the conduct of its policies, including those that take place outside its espace juridique, such as the CSDP (Art. 3, para. 5 and 21, para. 1 TEU). Several strategic documents issued by the Council of the EU envisage the obligation for the EU to comply with international law when acting in the security and defence sector. These documents include the European Security Strategy of 2009, the EU Concept for the Use of Force in EU-led Military Operations of 2010 (partially declassified), the Strategic Compass for Security and Defence of 2022, and the revised EU Maritime Security Strategy of 2023 (EUMSS). While some of these mention the EU’s right of self-defence (Concept, 6, para. 2; Strategy, 34), none of them provide indication as to how – or, for what most interests here, at which point in time – the EU intends to exercise this right.

In the absence of more specific normative references, the temporal scope of application of the right of self-defence in the framework of Aspides can be based exclusively on the Decision establishing the Mission and the rules of international (customary) law applicable in this domain.

The Different Ratione Temporis Interpretations of The Norm and The (Broad) Scope of The Right of Self-Defence in Decision 2024/583

Decision 2024/583 explicitly provides, for the first time in the context of CSDP missions, for EU Forces to exercise the right of self-defence. Acts relating to other naval operations that are currently ongoing (Atalanta and Irini), as well as missions of a military nature (e.g., EUMAM Ukraine and Althea), contain only general references to the obligation to comply with applicable international law in the conduct of the mission. This confirms the purely defensive nature of Aspides. On the other hand, the right of self-defence enjoyed by EU forces is seemingly broad from a temporal perspective, since it may be exercised also against an imminent armed attack.

The long-standing and wide-ranging debate, which has arisen especially since the beginning of this century, about the possibility of acting in self-defence before an armed attack materially takes place is well-known. Article 51 of the UN Charter is inconclusive as to the specific moment from which such a qualified type of attack can be considered existent and the affected state can legitimately respond, stipulating that the relevant right shall be exercised if an armed attack “occurs” against a UN Member.

Over time, the theory in favour of expanding ratione temporis the notion of self-defence has gained ground (anticipatory self-defence). This possibility is directly related to the ascertainment of the principle of necessity of self-defence, according to which the use of force must constitute the only option available to respond to, or possibly avoid, the armed attack (as enshrined in the famous “Webster’s formula”). The “expansionist” interpretation of the right of self-defence is also due to the necessity to face threats coming from non-state actors (terrorist groups in primis), having at their disposal increasingly destructive weapons, often made available by “allied” states. This is undoubtedly true also with regard to the Houthis, whose military arsenal – consisting of cruise and anti-ship ballistic missiles, kamikaze drones and, according to alleged statements made by the group’s leader Abdulmalik al-Houthi and his affiliates, underwater weapons and hypersonic missileshas increased exponentially over the years, due largely to Iran’s support.

For the sake of simplicity and keeping in mind that the difference between the various terminologies is not uniformly perceived in the practice of states and in literature, the notion of anticipatory self-defence has received at least three different interpretations, which are summarized below.

The most expansive reading of the norm under Article 51 of the UN Charter (embodied in the well-known Bush doctrine) considers an “armed attack” to include an unmaterialized and merely potential threat (preventive self-defence), but it has been strongly criticized (the Italian scholar Conforti has defined the Bush doctrine a “crude expression of force”; see also, among others, Kolb) and its legality under international law is to be excluded.

On the other hand, the use of force in self-defence to respond to imminent attacks or threats (pre-emptive self-defence) has received some consensus within the international community. However, a generally agreed interpretation of the concept of imminence, as well as of the criteria for defining it, are missing to date. Therefore, such an acknowledgment still does not, in all likelihood, correspond to an ad hoc customary rule, given the lack of both the element of diuturnitas and especially that of opinio iuris (Ruys, 526). Moreover, neither the International Court of Justice in the main cases in which it has ruled on the application of self-defence (Nicaragua and Armed Activities in Congo), nor the International Law Commission (ILC) with regard to Article 21 of the 2001 Articles on State responsibility concerning self-defence, nor the NATO 2022 Strategic Concept, nor, finally, any EU act, have provided any confirmation to this effect.

What perhaps international law permits today is a third interpretation of Article 51, less extensive than the other two mentioned above, namely the interceptive self-defence doctrine (see, e.g., Ruys, 527). This consists of the right to exercise self-defence in the face of an attack that has already been initiated but has not yet hit the target and, more generally, “has not yet fully developed in its consequences” (Dinstein, 231-234). Thus, it is still around the concept of “imminence” that the question revolves; however, unlike pre-emptive self-defence, in which the threat is imminent but has not yet materialized, interceptive self-defence responds to an armed attack “which is already in progress, even if it is still incipient” (ibid.).

In the case of Aspides, as noted above, it is not known how the EU interprets the notion of “imminent attack” used in Decision 2024/583, i.e., whether in the sense of a threat that is indeed imminent but has not yet materialized (pre-emptive), or in the more restrictive sense of a threat that has already partially materialized (interceptive). Acts that could provide guidance in this regard, such as the Operation Plan and the rules of engagement, are not accessible.

In the absence of explicit references, the text of the Decision seems to lean toward the first possibility, that is, pre-emptive self-defence. According to authoritative scholarship, the factors to be taken into account to qualify an attack as imminent include, among others, the nature of the threat, the probability and the expected scope of the attack, and the injury, loss, or damage to persons and property likely to result from it the absence of mitigation action, while the lack of specific evidence as to where the attack will take place or its precise nature does not preclude it from being imminent where the threat is objectively verifiable and concrete (Bethlehem, 775-776). If this interpretation is accepted, EU vessels could use force if there is clear knowledge of the intention by the Houthis – although not yet enacted – to launching missiles, drones or deploying submarine bombs, against the EU or other commercial vessels transiting certain maritime zones. Likewise, the lack of knowledge of the target of such attacks and their exact nature – plausible assumptions in light of the wide range of weaponry used to date by this armed group and the unpredictability of their attacks – should not preclude the right of EU Forces to act in self-defence. If the EU were to endorse the doctrine of pre-emptive self-defence, one could not underestimate its significance for developing both the objective and subjective elements of a customary rule, possibly regional in character.

If, on the other hand, the EU adheres to the doctrine of interceptive self-defence, the attacks to be qualified as “imminent” would be only those that are in progress but have not yet reached the target and produced consequences. In this case, the notion of an “ongoing” attack, also contained in the preamble of the Decision, should be understood in a “collective” sense as referring to a set of offensive actions, which have already hit at least part of their intended targets. This is what apparently took place in the first case of use of force in self-defence occurred during the Mission, in which the German frigate Hessen shot down two Iranian-made kamikaze drones (known as unmanned aerial vehicles – UAVs), which were in the air at the time of their destruction. A similar situation likely occurred in other incidents shortly thereafter, in which naval vessels placed at the EU’s disposal by France (here and here), Italy and Greece shot down several drones and ballistic missiles launched from Houthi-controlled territories. All EU press releases regarding the events state that “[a]ny response will always come as a consequence of an attack and be necessary, proportionate and limited to international sea or air-space.” The language used is vague, but it almost seems to deny the possibility that EU Forces could act before Houthi attacks occur.

Finally, according to a third interpretation, the EU would embrace both doctrines under Aspides. Playing in favour of this reading is the fact that, as mentioned, the preamble to the Decision admits the right of self-defence to respond both to “imminent” and “ongoing” attacks. Where an “ongoing” attack would mean a single attack that has begun but has not yet hit the target (e.g., a missile or drone that is still in flight), the first type of attack (“imminent”) would be subsumed under the notion of pre-emptive self-defence, while the second (“ongoing”) under that of interceptive.


The broad scope of the right of self-defence ratione temporis as provided in the Decision can be seen as an effect of the EU’s tendency to pay increasing attention to threats coming from non-state actors (Compass, 11 ff.). While, in the present case, such an interpretation is justified by the seriousness of Houthis’ actions, it is unclear to what extent the EU considers an “armed attack” to be imminent and, consequently, how “in advance” it is entitled to respond.

This has some relevant legal implications.

First, adherence to either of the two aforementioned doctrines (pre-emptive or interceptive) would be important for the purposes of contributing to the development of the constituent elements of a customary rule, especially in the case of interceptive self-defence. On the other hand, adoption of either of the two theses, especially the more controversial one of pre-emptive self-defence, could raise doubts about the legality of the conduct of EU forces under international law. Even the “cumulative” hypothesis of considering both interpretations as valid would arguably entail an excessively broad reading of the rule in question.

Another issue is the complex legal interoperability among EU Forces participating in Aspides. First, the possibility of contributing states acting in self-defence differently from the common line established by the EU cannot be ruled out. In this regard, the stipulation that self-defence is exercisable by the “[f]orces deployed for the operation” and by the Mission as such (Decision, Recital 8 and Art. 1(5)(c)) leaves unaffected, in the author’s view, whether it is the EU as an autonomous legal entity, or the individual states sending their own assets and personnel, that hold this right. In the former case, the claim would receive support that international organizations, like states, are entitled to use force in self-defence (see references on this in Palchetti, p. 242, footnote no. 1).

Furthermore, if EU forces exceed the temporal bounds of self-defence, the question emerges as to who – the EU or the single participating state(s) or possibly both – should be held internationally responsible. To this end, attribution of wrongful conduct can be problematic, considering that vessels participating in the Mission are nothing but state organs placed at the disposal of the EU. In the case where the EU is found not to have exercised any “effective control” over a certain conduct (within the meaning of Art. 7 of the 2011 ILC Articles on the Responsibility of International Organizations), that same conduct could be attributed to the state of nationality of the vessel, according to the customary rules on state responsibility. 

Finally, at least three other legal issues related to the topic discussed here are also worthy of attention. First, any use of force by the states participating in Aspides is to be considered lawful only to the extent that the offensive actions put in place by the Houthis can qualify as armed attacks, as established by Article 51 of the UN Charter and customary international law. Second, the lawfulness of (collective) self-defence in Aspides can be put into question, where force is used to respond to an armed attack that is not directed at the vessels of the states participating in the Mission, but at those (including non-military ones) of other states – a possibility that Decision 2024/583 explicitly permits (recital 8) and that has seemingly occurred so far –, in the absence of a request or consent by those targeted states (on this point, with regard to other missions in the same area, see here and here). Third, problems concerning the ratione loci application of self-defence also arise, given that, according to the Decision (recital 6), such a right shall be exercised on “maritime areas”, arguably including the high seas and other maritime spaces.


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