The Law of Self-Defense and the U.S. and UK Strikes against the Houthis
The Houthis are an Iran-aligned armed group that controls large swathes of territory in northern Yemen. Since 19 November 2023, and to support Hamas during its ongoing armed conflict with Israel, the Houthis have launched attacks against vessels transiting through the Red Sea, including via the use of drones, cruise and ballistic missiles, commando assault units deployed from helicopters, and boats. The crisis escalated significantly on 30 December 2023 when the Houthis attacked an American military aircraft and on 9 January 2024 when they attacked American and British warships.
On 10 January 2024, the UN Security Council adopted Resolution 2722 addressing the attacks in the Red Sea. While the Resolution does not authorize the use of force against the Houthis under Chapter VII of the UN Charter, it,
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.
On 11 and 22 January 2024, the United States and United Kingdom launched joint air and naval strikes against Houthi sites in Yemen, and the United States has carried out several separate strikes since 11 January. The United States and United Kingdom justified their military action as lawful acts of self-defense, a position supported by a number of States. In its letter of notification to the Security Council under Article 51 of the UN Charter, the United States explained,
The strikes were conducted to degrade and disrupt the ongoing pattern of attacks threatening the United States and deter the Houthi militants from conducting further attacks threatening merchant and commercial vessels transiting the Red Sea. These military responses will preserve navigational rights and freedoms, both for naval ships and for commercial vessels, in this important maritime passageway.
The United Kingdom also submitted an Article 51 notification letter to the Security Council, although the British Prime Minister’s statement to the UK Parliament gives more detail. It provides,
Since 19 November, Iran-backed Houthis have launched over 25 illegal and unacceptable attacks on commercial shipping in the Red sea, and on 9 January they mounted a direct attack against British and American warships. They fired on our ships and our sailors—it was the biggest attack on the Royal Navy for decades—and so we acted. We did so in self-defence, consistent with the UN charter, and to uphold freedom of navigation, as Britain has always done.
In this post, I examine whether the United States and United Kingdom invocations of the right of self-defense were lawful, that is, were they subject to an armed attack and was their response necessary and proportionate in the circumstances.
Armed Attack
Under Article 51 of the UN Charter, the exercise of the right of self-defense is contingent upon an “armed attack.” As we have seen, the United States, the United Kingdom, the supporting States, and the Security Council appear to take the view that self-defense can also be engaged to protect commercial shipping as well as navigational rights and freedoms on the high seas. Writing in 1958, Professor Derek Bowett correctly argued that the requirement of an armed attack under Article 51 narrowed the right of self-defense because, prior to 1945, customary law permitted States to engage in defensive action to protect their essential security interests.
Nevertheless, Professor Bowett controversially claimed that this wider right of self-defense survived the inception of the UN Charter and, therefore, customary law continues to permit States to invoke self-defense to protect their essential security interests. Can it be argued that customary law permits the United States and United Kingdom to protect commercial shipping and navigational rights and freedoms on the basis that interferences with these activities implicate their essential security interests?
The reality is that, since the adoption of the UN Charter, States have rarely invoked the right of self-defense to protect their essential security interests. In fact, States routinely cite the occurrence of an “armed attack” as the lawful basis for undertaking self-defense. Moreover, and although discussing self-defense in the context of Article 51, the International Court of Justice (ICJ) seems to have rejected a wider customary right of self-defense when it tersely stated that this right “does not allow the use of force by a State to protect perceived security interests beyond [armed attacks]” (DRC v. Uganda, para. 148).
Significantly, the United States and United Kingdom explained that defensive action was also undertaken to halt and repel armed attacks, which, as said, is a recognized ground for engaging the right of self-defense under Article 51. The first question in assessing such claims concerns the target of the armed attack. The law of self-defense recognizes that States can defend commercial vessels flying under their national flags from armed attacks (Oil Platforms).
The difficulty is that the vast majority of commercial vessels attacked by the Houthis were not flying under the flags of the United States or United Kingdom, with the exception of the Swan Atlantic which was a British-flagged vessel. Flag States have a right to defend their vessels against armed attacks; this includes requesting other States to assist in defensive action (i.e., collective self-defense). But it does not appear that the flag States concerned issued a request for collective self-defense to the United States or United Kingdom. Thus, when assessing the legality of the United States and United Kingdom claims of self-defense, the attacks against non-U.S. and non-UK flagged vessels do not factor into the assessment of whether the armed attack threshold was reached.
However, as noted, the Houthis launched attacks against United States’ and United Kingdom’s warships and military aircraft. Attacks against them can certainly qualify as armed attacks against those States (Definition of Aggression, art. 3(d)). The critical question is whether they did so under the law of self-defense.
The overwhelming majority of States, as well as the ICJ, have repeatedly defined armed attacks as “grave” uses of force (see e.g., Paramilitary Activities, para. 191), namely, those that are of sufficient “scale and effects” (Paramilitary Activities, para. 195). Professor Stefan Talmon has concluded that the Houthi attacks against the U.S. and UK warships and aircraft were not sufficiently grave to qualify as armed attacks. In doing so, he places much emphasis on the ICJ’s judgment in Oil Platforms.
In that case, Iran allegedly launched missile and mine attacks against several vessels. Of particular importance were the attacks against the United States-flagged Sea Isle City and a United States warship that damaged them and injured several crew members. While the Court did not rule out the possibility that the mining of one vessel could qualify as an armed attack, it held that, “[e]ven taken cumulatively, . . . these incidents do not seem to the Court to constitute an armed attack against the United States” (para. 64).
The key point emerging from the ICJ’s jurisprudence is that the test for an armed attack is highly contextual, that is, it requires a quantitative and qualitative assessment. Thus, whether a use of force is grave depends on the circumstances and requires a close evaluation of the facts. Such facts include, for instance, the target of the attacks, the number of troops involved, the weapons used, the frequency of the attacks, the duration of the attacks, and the degree of harm caused (Buchan and Tsagourias, p. 46).
When assessed in context, I believe that the attacks against the United States’ military aircraft and warships and the United Kingdom’s warships constitute—individually—armed attacks against those States. This conclusion is based on the targets of the attacks (warships and aircraft), their repeated and escalating nature, and the sophisticated nature and range of the weapons employed. As the United States explained in its Article 51 notification letter,
On several occasions, Houthi-launched systems have been directed towards United States Navy ships, necessitating the activation of defensive systems to safeguard the ships and their crews. Houthi militants also shot at United States Navy helicopters on 31 December while the Navy helicopters were aiding a commercial vessel that Houthis were attempting to board.
Similarly, in its Article 51 notification letter to the Security Council, the United Kingdom stated,
Houthi militants launched a further attack, employing more than 20 unmanned aerial vehicles, 3 anti-ship missiles and a combat aircraft.
The British Prime Minister also emphasized the scale of the attacks when he explained that the Houthis “mounted a direct attack against British and American warships. They fired on our ships and our sailors—it was the biggest attack on the Royal Navy for decades—and so we acted” (emphasis added). In short, the attacks against the United States and United Kingdom were of sufficient scale and effects to constitute armed attacks and thus trigger their right of self-defense.
An important issue is whether the right of self-defense can be engaged directly against non-State actors such as the Houthis. Some States have taken the position that non-State actors cannot be the authors of armed attacks, a position shared by the ICJ in the Wall advisory opinion (para. 139). According to this approach, armed attacks by non-State actors must be attributed to States in order for self-defense to be engaged.
This argument is unconvincing for several reasons. First, Article 51 does not stipulate who must be the author of the armed attack. Rather, it explains that States have a right of self-defense where “an armed attack occurs.” Second, many States have repeatedly argued that self-defense can be engaged against non-State actors (Buchan and Tsagourias, pp. 53-58). Third, the Security Council has recognized that States can take self-defense action against non-State actors, as in 2001 when it affirmed the United States’ right of self-defense against Al-Qaeda (e.g., UNSC Resolutions 1368 (2001) and 1373 (2001)). Indeed, in this case, Security Council Resolution 2722 affirms that States have the right to defend themselves against Houthi-launched attacks.
A further issue is raised by claims that Iran backs and supports the Houthis. This raises the question of when States can be regarded as the co-authors of armed attacks launched by non-State actors. This question is important because it determines who can be the lawful target of defensive action.
One view holds that States can be targeted only where the armed attacks can be attributed to them under the effective control test as set out in Article 8 of the Articles on State Responsibility (ASR). However, this approach confuses the law of State responsibility with the jus ad bellum because the latter regime has developed its own attribution criteria for determining whether a State can be considered the author of an armed attack. As the General Assembly explains in its Definition of Aggression, a State can be regarded as the author of an armed attack on the basis of its “substantial involvement” in that activity (Article 3(g)). The ICJ endorsed this test of substantial involvement in its Paramilitary Activities judgment (para. 195).
Whether Iran is substantially involved in the Houthis’ armed attacks depends on the facts, which in this case are complex and contested. That said, it is widely reported that the Houthis benefit from financial, political, and military support from Iran (UK House of Commons Library, p. 8). Yet, the test for substantial involvement is a “high threshold” (de Wet, p. 93) and, in Paramilitary Activities, the ICJ held that a State’s provision of weapons, funds, training, and logistical support to a non-State actor was not sufficient to constitute “substantial involvement” (para. 195). Whether Iran is substantially involved in the Houthis’ armed attacks depends a close analysis of the facts, which are not immediately available. Moreover, it should be pointed out that this question is—at present—largely theoretical given that the United States and United Kingdom have directed their strikes exclusively against the Houthis.
Necessity
The principle of necessity is a requirement of customary law (Oil Platforms, paras 76-77) and stipulates that defensive action must be the only means available to thwart the armed attack. Some commentators argue that the American and British invocations of the right of self-defense were unnecessary because their response was carried out several days after the armed attacks against their warships and aircraft had been committed (Talmon).
This line of reasoning is difficult to sustain because it assumes that the armed attacks against the United States and United Kingdom have “ceased” and are “over” (Talmon). On the contrary, the Houthis’ attacks are ongoing; they are part of a wider campaign by the Houthis to target Israeli-linked vessels and are becoming more frequent, intense, and damaging. This is a critical point because, where an armed attack is continuing, there is an “almost irrebuttable presumption” that defensive action is necessary (Akande and Lieflander, p. 564). In other words, where an armed attack is underway victim States cannot be reasonably expected to pursue peaceful means to stop the attacks, such as having recourse to diplomatic or legal mechanisms. Interestingly, the British Prime Minister explained that “[we] have attempted to resolve this through diplomacy . . . [including] numerous international calls for the attacks to stop.” Such efforts may be important to garner political legitimacy for the UK strikes, but they are not required by the law of self-defense.
In any event, the United States and United Kingdom cannot be expected to attempt to use peaceful means to stop the attacks because they do not have any realistic prospect of success. For example, it is futile for these States to try to negotiate with the Houthis given that this group is committed to supporting and defending Hamas (which is a proscribed terrorist organization by many States), it is designated as a terrorist organization by certain States, and continues to carry out attacks in the Red Sea. Similarly, it is futile for these States to try to work with the Yemeni government given that it is embroiled in a bitter civil war with the Houthis and is thus unable to immediately put an end to their armed attacks against the United States and United Kingdom.
Proportionality
The principle of proportionality is established in customary law (Oil Platforms, paras 76-77) and limits defensive action to that which is needed to halt and repel the armed attack. In my view, this “halting and repelling” approach allows victim States to take defensive action where there is a “sound basis for believing that further attacks will be mounted” (Schmitt, p. 65-65).
Adopting this approach, given that the United States and United Kingdom were subject to an armed attack and that these attacks are continuing, they can take defensive measures against the Houthis to the extent that they are necessary to prevent reasonably expected future attacks. As the United Kingdom explains, “[Houthi] attacks will continue unless action is taken to deter them.”
Some commentators maintain that the principle of proportionality prohibits defensive action where the damage it causes is excessive when compared with the damage inflicted by the armed attack (Cannizzaro, p. 782). With regard to the action undertaken by the United States and United Kingdom, Talmon claims that the air and naval strikes against the Houthis were disproportionate because their scale and ferocity was “excessive” when compared with the damage inflicted on the warships and aircraft.
This interpretation of the proportionality principle is misguided. In the ad bellum context, the principle of proportionality only requires victim States to do no more than achieve the specific objective of defending themselves (for the United Kingdom’s position, see UN Doc. A/PV.1063, para. 20). Whether the harm caused by the self-defense action is lawful does not depend on the application of the jus ad bellum but instead on other regimes of international law, such as international humanitarian law and international human rights law.
In the context of the present discussion, the sole question is whether the defensive action carried out by the United States and United Kingdom was designed to repel the armed attacks committed against their warships and military aircraft and prevent reasonably foreseeable future attacks. I have already answered this question in the positive when discussing the necessity principle and, understood in this way, the principles of necessity and proportionality under the law of self-defense converge and are “truly two sides of the same coin” (Ruys, p. 123).
Conclusion
The action undertaken by the United States and United Kingdom was a necessary and proportionate response to the armed attacks committed by the Houthis and therefore meets the conditions precedent for establishing the right of self-defense under international law. The United States and United Kingdom can engage in defensive action against the Houthis because it is necessary to repel an ongoing armed attack and prevent reasonably expected future attacks. States such as Iran can be considered co-authors of these armed attacks where they provide substantial assistance to the Houthis, although it is far from clear as to whether this high bar has been met. Moreover, the prudency of undertaking defensive action against Iran must be closely scrutinized because it would lead to a significant escalation of the crisis currently engulfing the Middle East.
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Dr Russell Buchan is Professor of International Law at the University of Reading, UK.
Photo credit: U.S. Navy
