Russia's (unlawful) aggression against Ukraine, and the international armed conflict that it has triggered, raises many novel questions concerning the interpretation and application of international law. In this post, I want to focus on the law of self-defence as contained in Article 51 of the UN Charter. In particular, I want to zero in on two questions that I believe have been given insufficient attention in the international legal commentary to date notwithstanding the fact that a vast amount of literature has been written on how the law of self-defence applies to the Russia-Ukraine conflict. First, I want to explore how the law of self-defence interacts with the principle of due diligence. Second, I want to examine how the law of self-defence operates within the context of the law of neutrality.
Due Diligence
There is disagreement among States as to whether the principle of due diligence is a political norm of good behaviour in international relations or, instead, a binding rule of customary international law. I believe the necessary State practice and opinio juris is present to support the latter position, and I will proceed on this basis. It should be observed that the obligation of due diligence is more felicitously characterized as the obligation to prevent transboundary harm because due diligence is actually a standard of conduct that conditions the application of many rules of international law, such as those found in international environmental law, international human rights law, and international humanitarian law among others. That said, I will refer to the customary obligation to prevent transboundary harm as the obligation (or principle) of due diligence because this is increasingly the language used by States (as indicated by the recent flurry of national and regional statements on the application of international law to cyberspace).
It is important to recall that the due diligence obligation is activated where a State is aware (either actually or constructively) that its territory (including the cyber infrastructure located on its territory) is being used to interfere with the international legal rights of other States. This means that State A commits a breach of the obligation of due diligence where it knowingly allows State B to use its territory to develop its military capabilities in order to commit an unlawful use of force against State C within the meaning of Article 2(4) of the UN Charter (I will not consider State A's complicity here, as this is a notoriously high threshold to meet). In a similar albeit different context, in the Alabama Commission case the Confederates commissioned a number of warships to be built by private companies in the territory of the United Kingdom and, subsequently, used them against Union forces during the American Civil War. The dispute ended up in arbitration, and the panel found that the United Kingdom had failed to take reasonable measures to prevent its territory from being used to harm the international legal rights of the United States of America (the panel determined that the United Kingdom was required to exercise due diligence to prevent such harm under the Treaty of Washington and (importantly) customary law – see Jensen & Watts, p. 8).
The hypothetical scenario just sketched out is very different, however, from the one in Russia-Ukraine. Here, Russia's aggression amounts to an armed attack against Ukraine, and Ukraine can invoke the right of self-defence to halt and repel it under Article 51 of the UN Charter. It is widely known that allied States are allowing their territory to be used by Ukraine to enhance its military capabilities and bolster its capacity to carry out self-defence action against Russia. For instance, allied States are allowing their territory to be used to produce military equipment for Ukraine and run extensive military training operations for the Ukrainian armed forces. In my view, provided Ukraine's conduct against Russia complies with the conditions precedent for the lawful exercise of right of self-defence—namely, that the measures taken are necessary and proportionate to ward off the armed attack—its allies do not commit a breach of the principle of due diligence by allowing their territory to be used by Ukraine to strengthen its military capabilities. This is so because allied States are not allowing Ukraine to use their territory to cause harm to the international legal rights of Russia. To be clear, Russia is the aggressor, Ukraine has the right of self-defence, and thus Ukraine's actions against Russia (again, provided they meet the principles of necessity and proportionality) do not unlawfully interfere with Russia's international legal rights.
It is important to emphasize that, according to the argument just developed, the principle of due diligence does not apply because the conditions for its application have not been met. Another argument could be made that supporting States do not commit a breach of the due diligence obligation because Ukraine has been subject to an armed attack and they can therefore rely on Ukraine's right of collective self-defence. This raises the question of whether collective self-defence can be invoked to justify a breach of the due diligence obligation. This engages a similar question to the one discussed below, that is, can collective self-defence be relied on to justify a breach of the law of neutrality. As we shall see, in my view the right of self-defence can be invoked to justify a breach of these types of rules (i.e., due diligence and neutrality). But the importance (and value) of the argument made in this section is that the criteria for the application of the due diligence obligation have not been met and, because no breach of international law has been committed, no defence needs to be raised by supporting States.
Neutrality
The international armed conflict between Russia and Ukraine has also raised various questions concerning the interpretation and application of the law of neutrality. As readers of this blog know, an important question is whether the law of neutrality has survived the inception of the UN Charter and in particular its prohibition on the use of force. I do not want to wade into this tricky debate in this post. Instead, for sake of argument, I will assume that the law of neutrality continues to apply in the UN era and thus applies in the context of the Russia-Ukraine international armed conflict notwithstanding the fact that Russia is an aggressor.
The question arises as to whether the support being offered by States allied to Ukraine breaches the law of neutrality. I do not need to remind readers of the significant amounts of political, economic, and military support given by (largely) western allies to Ukraine. Whether all forms of support to Ukraine constitute a breach these States' obligations under the law of neutrality is contested and requires a careful examination of the law of neutrality and what it prohibits and what it does not. It is not really necessary to engage in this assessment because, at a minimum, it is clearly the case that certain types of support to Ukraine would prima facie breach the law of neutrality, specifically, the provision of massive amounts of lethal war materials (such as weapons).
In my view, these prima facie breaches of the law of neutrality can be justified by way of the right of self-defence under international law and, in particular, by way of the right of collective self-defence. Remember that Article 51 of the UN Charter affirms the inherent right of individual and collective self-defence where an armed attack occurs. Thus, Russia's invasion of Ukraine amounts to an armed attack and Ukraine can invoke its right of individual and collective self-defence. As a matter of principle, where a victim State engages its right of collective self-defence, allied States can assist it in halting and repelling the armed attack and, where that support constitutes a prima facie breach of international law, the right of collective self-defence can be raised as a legal justification.
Certain conditions must be met in order for States to be able to rely on the right of collective self-defence. For example, it is well accepted that a victim State must issue a request for support and that the support furnished must stay within the four corners of the request. The principles of necessity and proportionality must also be met when supporting States exercise collective self-defence on behalf of the victim State.
A trickier issue is whether supporting States can rely on the right of collective self-defence under Article 51 of the UN Charter to justify prima facie breaches of the law of neutrality. The difficulty flows from the belief among certain commentators that the right of self-defence is a narrow exception to the principle of non-use of force under Article 2(4) of the UN Charter (see e.g., Fletcher & Ohlin, p. 66). On this reading, the right of individual and collective self-defence can be engaged only to justify the use of defensive forcible measures. The implication of this argument is that the right of (individual and collective) self-defence cannot be relied on to justify a breach of other rules of international law, such as those contained in the law of neutrality.
This is somewhat of a bête noire for me. I have previously argued that self-defence (individual and collective) is a general right that can be engaged to justify the breach of rules of international law other than the principle of non-use of force. I do not want to rehash the various reasons for this assessment, other than to repeat that I consider this interpretation to be well-established in State practice (both pre and post the UN Charter); indicated by the text and structure of the Charter; and supported by the very logic of self-defence as an “inherent right.†This approach means, for example, that a victim State can assert the right of self-defence to justify a cyber operation aimed at repelling an armed attack even though it does not rise to the level of a use of force but instead breaches the principles of sovereignty and/or non-intervention (and therefore requires legal justification).
However, this approach does not mean that self-defence can be invoked to justify a breach of all rules of international law. Clearly, self-defence cannot be engaged to justify a breach of the prohibition on the annexation of territory by force (para. 254), or an “intransgressible†rule of international humanitarian law (para. 79). This is because they are “obligations of total restraint†(para. 30) insofar as they have been formulated by States in such a way as to indicate that they can never be breached even by way of self-defence (Articles on State Responsibility (ASR), art. 21, para 4). But this is a high threshold to meet, and the law of neutrality does not reveal any indication that States intended for it to act as an obligation of total restraint and which thus forecloses the potential for self-defence to be invoked to justify a breach of it (and, incidentally, the same can be said with regard to the principle of due diligence).
To return to my key point, the right of self-defence can be engaged to justify a breach of a wide array of rules of international law, including the law of neutrality. Thus, if Ukraine continues to request defensive support, the support remains within the limits of this request, and the defensive measures taken are in conformity with the law of self-defence and especially the principles of necessity and proportionality, allied States can justify prima facie breaches of the law of neutrality on the basis of collective self-defence. Indeed, a number of States have justified their provision of military support on the basis of Ukraine's right of individual and collective self-defence (for a discussion of this practice, see here). Before the UN Security Council, for example, France maintained that it “is providing, and will continue to provide, the Ukrainian people with all the support they need to exercise their right to self-defence … . That includes military support†(S/PV.9256, p. 13). Admittedly, it is not immediately clear whether France is arguing that the law of neutrality simply does not apply because Russia is the aggressor and Ukraine is acting in self-defence (the so-called doctrine of qualified neutrality, discussed above), or whether any prima facie breaches of the law of neutrality that arise can be justified on the basis of the right of collective self-defence.
Finally, it should be pointed out that construing self-defence as a general right is particularly important given that, in my view, the doctrine of collective countermeasures is not supported by the necessary State practice and opinio juris to conclude that it is established in customary law (for my analysis see here, chapter 9 – although there are different approaches). For clarity, collective countermeasures are measures taken by indirectly injured States (as defined by ASR, art. 48) to induce wrongdoing States into complying with their erga omnes or erga omnes partes obligations. Collective countermeasures are relevant to the Russia-Ukraine conflict because Russia has committed an act of aggression against Ukraine, which is unambiguously (para. 34) a breach of an erga omnes obligation. But on my assessment, the doctrine of collective countermeasures is not established in customary law and cannot be therefore invoked by supporting States to justify (or more likely excuse) a breach of the law of neutrality.
Conclusion
A lot of work has been done over many centuries to help clarify the nature, content, and scope of the right of self-defence in international law. But the interpretation and application of the right of self-defence continues to throw up novel questions. By using the conflict between Russia and Ukraine as a case study, this post has focused on how the right of self-defence interacts with the principle of due diligence and the law of neutrality. It shows that the obligation of due diligence is not breached where a State's territory is used by a State to commit lawful acts against another State (e.g., because it is acting in self-defence).
Moreover, this post has argued that (collective) self-defence is a general right under international law which can—provided the conditions for the lawful exercise of self-defence are met—be invoked to justify prima facie breaches of a range of rules of international law, such as those contained in the law of neutrality and which may apply to the Russia-Ukraine international armed conflict.
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Dr Russell Buchan is Professor of International Law at the University of Reading, UK.
The views expressed are those of the author, and do not necessarily reflect the official position of the United States Military Academy, Department of the Army, or Department of Defense.
Articles of War is a forum for professionals to share opinions and cultivate ideas. Articles of War does not screen articles to fit a particular editorial agenda, nor endorse or advocate material that is published. Authorship does not indicate affiliation with Articles of War, the Lieber Institute, or the United States Military Academy West Point.
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Photo credit: President of Ukraine
