In his thought-provoking essay in Just Security, Chris O’Meara provides an insightful analysis of Ukraine’s recent incursion into Kursk Oblast under the law governing the use of force. O’Meara’s contribution is an important one that sheds light on the legal issues that one needs to consider in assessing the legality of Ukraine’s incursion under this body of law (jus ad bellum). The purpose of my contribution is not to challenge O’Meara’s analysis on the legality of Ukraine’s incursion, a point on which we may broadly agree, but rather to critically engage with the manner in which he interprets and applies the customary international law requirements for the lawful exercise of the right to self-defense: necessity and proportionality. Indeed, it is the latter on which we may disagree most. In the Ukraine case itself, we may both reach the same conclusion on legality, but the test, the very criteria used to determine legality, is where we may have divergent views. That’s especially important because, as O’Meara writes, the key issue may be whether the Kursk operation continues to meet the requirements of jus ad bellum over time.
As O’Meara aptly points out, while the conditions of necessity and proportionality do not appear in the text of Article 51 to the UN Charter, they are nevertheless part and parcel of contemporary jus ad bellum and are critical to assess whether a particular use of force by a State in conducted in manner consistent with its inherent right to self-defense. Furthermore, convincingly, O’Meara submits that the requirements of necessity and proportionality apply throughout the entire duration of an armed conflict, and not only with respect to the initial resort to force. This approach has gained widespread support in the legal literature and has been explicitly endorsed by the International Court of Justice in its recent Advisory Opinion, in which it opined that “an occupation involves, by its very nature, a continued use of force in foreign territory. Such use of force is, however, subject to the rules of international law governing the legality of the use of force or jus ad bellum”.
In the author’s view, it is only by applying necessity and proportionality continuously throughout an armed conflict that the prohibition of the use of force, and one of the main purposes of the United Nations — to save succeeding generations from the scourge of war — can meaningfully be enforced and maintained. Importantly, continuous application of necessity and proportionality may serve another essential function, that is to limit the scope and amount of force that parties to a conflict may resort to, even if such force conforms to the rule of international humanitarian law (the law of armed conflict). In other words, applying the necessity and proportionality ad bellum during an armed conflict may compel belligerents to halt their military campaign, or at a minimum, change course.
When discussing the requirement of necessity, O’Meara properly describes necessity as a measure of last resort, i.e., peaceful measures to force are unavailable or unfeasible and/or, on their own, would be ineffective to repel Russia’s ongoing armed attack. O’Meara submits that while only in exceptional circumstances occupation of part of an aggressor’s territory can ever be deemed a necessary (and proportionate) act of self-defense, the case of Ukraine provides such exceptional circumstances, due to the nature and scale of Russian aggression.
Nevertheless, when assessing the lawfulness of occupying the territory of an aggressor under the jus ad bellum, and the lawfulness of any act of self-defense, it is important to analytically distinguish between necessity and proportionality. Necessity, as alluded by Noam Lubell & Amichai Cohen, asks whether it is necessary to do X to achieve Y. Whereas proportionality asks whether X is proportionate in relation to Y. Thus, one may argue that at least when it comes to the necessity requirement, an invasion and temporary occupation of the aggressor’s territory, as a defensive act, might adhere to the necessity requirement, but not necessarily be proportionate, as will explained below.
Unlike some other authors, O’Meara argues that in order to comply with the requirement of necessity, a State would have to show that (1) there is no other reasonable alternative means other than resort to force to halt and repel the armed attack; (2) there is a rational connection between the force used and the defensive purpose. I fully agree with O’Meara’s analysis on the possible justification offered for the incursion and occupation of Russia’s territory by Ukraine for the purposes of adhering to the necessity requirement. What’s more, as the requirement of necessity applies on an ongoing basis, the legality of the establishment and maintenance of the buffer zone, and indeed the continued occupation of Russia’s territory, as a lawful act of self-defense, should be assessed continuously by Ukraine.
In assessing proportionality, O’Meara argues that Ukraine’s act of self-defense must not be “excessive” when measured against Ukraine achieving a legitimate defensive purpose of halting and repelling Russia’s ongoing aggression. In O’Meara’s view, proportionality allows victims States to defend themselves by halting and repelling the attack of the aggressor, but not more than that. Other leading scholars like Mike Schmitt adhere to O’Meara’s understanding and interpretation of proportionality ad bellum.
In my view however, the test applied by O’Meara and other scholars when assessing proportionality ad bellum, in fact pertains to the necessity requirement, rather than proportionality. Consider Schmitt’s formulation, which can be read as part of necessity not proportionality:
“Proportionality, which limits the scale and scope of force used in self-defense to that necessary to end the armed attack, is the more nuanced requirement.
…
Restated, the legal question is whether the force being employed in self-defense exceeds what is required in the circumstances to defeat the enemy.” (emphasis added).
With the words “necessary” and “what is required,” we can see that this statement actually registers in the frame of the necessity requirement, and indeed is already subsumed by the necessity test. O’Meara makes the same move in applying so-called proportionality to the case of occupation, when he writes: “temporary occupation of territory might be considered proportionate defensive action where it is the only means possible to help prevent the continuation of armed attacks” (emphasis added). O’Meara also refers to whether the force is ”excessive,” but that too is all about exceeding what is necessary.
Proportionality, however, should not be made redundant with necessity and must perform an independent function.
At its most basic form, proportionality seeks to strike a balance between two or more competing rights and/or interests. For instance, in international humanitarian law, the rule on proportionality seeks to strike a balance between the expected incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, and the concrete and direct military advantage anticipated from an attack. Thus, in order for an act of self-defense to be proportionate, it is submitted that it is not sufficient to argue that for a State to comply with the proportionality requirement, it may halt an repel an armed attack, but not more than that, as there are various ways in which a State may halt and repel an armed attack – some would be proportionate, and some would be disproportionate, even if they can be deemed to be necessary in order to halt and repel the armed attack. One could think of a scenario in which the only reasonable or feasible option to halt and repel an armed attack is to invade, occupy, and arguably force a regime change of the aggressor; while these actions may adhere to the necessity requirement, they will nevertheless, in some cases, be disproportionate, whether one adopts the tit-for-tat, harm-benefit, or teleological approach for proportionality in the meaning of jus ad bellum. In the author’s view, the better position is that proportionality ad bellum requires the defending State to take measures that are proportional to the benefit of repelling the armed attack and other imminent armed attacks, and thus preserving a delicate balance between the expected harms caused by its defensive response and the anticipated value emanating from such defensive response. It should be highlighted, that such position should not be interpreted as requiring an equivalence of force between the armed attack and the defensive response, as the tit-for-tat approach suggests. O’Meara’s recent book supports the view that such balancing of values is part of the proportionality equation as well. My point is that is the desirable and full understanding of proportionality, and is even better understood by not conflating proportionality elements with what is, in truth, necessity.
In the final section of his Just Security essay, O’Meara submits that Ukraine’s adherence to the law governing the use of force is highly significant, both for Ukraine and for the rules-based international order. Complying with international law, O’Meara proceeds, enhances the legitimacy of Ukraine’s actions, bolsters the righteousness of their cause, helps Ukraine to achieve its strategic aims, and garner support and assistance from its allies.
That said, adhering to the law governing the use of force may be highly significance from a legal standpoint alone. If one adopts the view that the requirements of necessity and proportionality apply throughout the armed conflict, that will mean a military campaign that started as a lawful act of self-defense, may well turn into an unlawful use of force, and arguably even armed attack, which in turn would trigger the right of self-defense. It is debatable whether there can be self-defense against self-defense. In the Ministries Case, the Nuremberg Military Tribunal concluded that “he who initiates aggressive wars loses the right to claim self-defense against those who seek to enforce the Treaty [the Kellogg-Briand Pact]. This was merely the embodiment in international law of a long-established principle of criminal law: ‘…there can be no self-defense against self-defense.’”
However, such a fixed aggressor-defender relations, as pointed out by Eliav Lieblich, does not withstand scrutiny and ought to be rejected if one takes seriously the continuous application of necessity and proportionality ad bellum. And as Haque notes, the proposition that there can be no self-defense against self-defense holds “at least so long as the defensive response is necessary, proportionate, and otherwise lawful.” In practice, this raises a host of legal and practical questions: if Ukraine’s response is deemed to be disproportionate or unnecessary ad bellum (to be clear, the author is of the view that currently, Ukraine’s incursion adheres of the requirements of necessity and proportionality), would that entitle Russia to exercise its inherent right of self-defense, thus making Ukraine and Russia both victims and aggressors? Regardless, surely such alleged right of self-defense will not have any bearing or implication whatsoever on the previous acts of aggression committed by Russia in 2014 and 2024, nor on Ukraine’s inherent right of self-defense against these continuous acts of aggression.