The surprise large scale incursion by Ukrainian forces into Russia’s Kursk Oblast, which began on August 6, 2024, has stunned the world and raised fair-minded questions regarding both the strategic and the political wisdom of this bold response to ongoing Russian aggression. Two further questions should also be asked of this latest development in this tragic and bloody war. The first is whether Ukraine’s invasion of Russian territory is a lawful act of self-defense under international law. The second is why it matters, namely, what is the significance of the lawfulness of the incursion for Ukraine itself and for the rest of the world?
In terms of the publicly known facts at this time, Ukraine’s incursion into Russia has resulted in its largest battlefield gains since its counteroffensive in late 2022. It comprises the invasion and occupation of Russian territory. Just over two weeks into the operation, Ukrainian forces have reportedly advanced 35km into the Kursk region, taking 1,150 square kilometers under control, including numerous Russian settlements, and setting up a military administration office. Hundreds of Russian soldiers have been captured and the incursion is advancing into neighboring Russian regions as the Ukrainian forces push on. Russia meanwhile has referred to the invasion of its territory as a “mass terrorist attack.”
On the issue of lawfulness, the question at hand is whether invading foreign territory in response to a war of aggression is lawful under international law. The short answer is yes, potentially at least, but subject to several important caveats. The full answer to this question lies in States adhering to a number of bodies of international law, including international humanitarian law (IHL) that governs a state’s conduct of hostilities. IHL speaks to how Ukraine carries out its operation. Fundamentally, the answer as to legality also relies on another body of law, grounded in the UN Charter and customary international law, that regulates when States may use military force outside of their own borders, including in self-defense.
The Right of Self-Defense
In considering Ukraine’s military actions in Russian territory, the starting position is that States are generally prohibited from using force against other States by Article 2(4) of the UN Charter, with the right of self-defense acting as a limited exception to that prohibition. In response to Russia’s full-scale invasion of Ukraine, which began in February 2022, Ukraine has an undeniable prima facie right to defend itself. This right of self-defense is enshrined in Article 51 of the UN Charter and there is no doubt that the scale and effects of Russia’s actions meet the required threshold (being an “armed attack”) that allows Ukraine to exercise this sovereign right in response. Yet, the key issue relates to how Ukraine is exercising this right such that it remains lawful and not in breach of the Article 2(4) prohibition. International law applies to both sides of the war. In defending itself, Ukraine must comply with its legal obligations (regardless of Russian non-compliance), and international law does not provide a carte blanche to States that are victims of violence, even the most egregious violence. States are not permitted to do whatever they like to protect their territory and citizens. A State’s right of self-defense is not absolute and is not a superior prerogative that automatically trumps other legal rights, even those of an aggressor like Russia. Rather, the right is conditioned, being restricted by legal rules that ensure that victim States do not overact beyond what is required to defend themselves.
Necessity and Proportionality
It is universally recognized that a State’s defensive force must be both “necessary” and “proportionate.” Although not referenced in the UN Charter itself, the International Court of Justice has confirmed that necessity and proportionality are requirements of customary international law and must be strictly adhered to in order for acts of self-defense to be lawful. The best view of the law is that these requirements apply on an ongoing basis (pp. 166-170) to the entirety of any defensive military operation, alongside and in addition to other bodies of international law, most notably IHL. Accordingly, for international observers (whether they be strategists, policymakers, lawyers, States, international courts, and/or international organizations), an important question is whether the recent Ukrainian advances into Russian territory satisfy these cardinal rules of international law that govern a State’s recourse to armed force.
There is little doubt that Russia’s ongoing aggression satisfies the question of the necessity of Ukraine’s ongoing self-defense, at least in some form. Necessity requires that military force be a measure of last resort (pp. 38-42) (i.e., peaceful alternatives to force are unavailable or unfeasible and/or, on their own, would be ineffective to repel Russia’s ongoing armed attack), and few would dispute that force is the only reasonable choice of means available to Ukraine in the face of the continuing Russian aggression. A significant incursion across the Russian border represents a major shift in Ukraine’s defensive war, however, and the factor of occupation inside Russia adds a significant twist to the legal analysis of whether this incursion constitutes a lawful act of self-defense.
From the perspective of the law governing self-defense, although there is no obligation for a victim State to limit its defensive response to within its own borders (p. 285), the invasion and occupation of an adversary’s territory is rarely going to be necessary or proportionate. This is because self-defense is designed to allow for victim States to protect their own territory, and international law prohibits States from using force to seize the territory of others. Indeed, the very idea of protecting territorial integrity is at the core of Ukraine’s resistance against Russian aggression. Only in exceptional circumstances, therefore, will occupation of part of an aggressor’s territory ever be deemed a necessary and proportionate act of self-defense.
The nature and scale of Russian aggression against Ukraine over several years and the enduring occupation of Ukrainian territory arguably calls for an exceptional response on the part of Ukraine. On the issue of necessity, Ukraine has pointed to the fact that it has been limited in its ability to carry out strikes into Russian territory in order to target the source of long-range attacks against targets in Ukraine. As such, the invasion of Russian territory may justifiably be said to comprise the necessary means to achieve this defensive aim and to establish a buffer zone against such attacks, which enables it “to save the lives of our people and protect the territory of Ukraine from Russian strikes,” in the words of the spokesperson of Ukraine’s Ministry of Foreign Affairs Heorhiy Tykhyi. Indeed, such an exception may be more likely to apply in warring States with contiguous territory. Ukrainian President Volodymyr Zelenskyy has since emphasized the defensive need to create such a “buffer zone,” with Defense Minister Rustem Umerov telling a U.S. delegation that “our goals there are to clear the border from Russian military threats and make enemy shelling and attacks on our towns and villages impossible.”
As to the requirement of proportionality, Ukrainian acts of self-defense must not be “excessive” (pp. 97-125) when measured against Ukraine achieving a legitimate defensive purpose of halting and repelling Russia’s ongoing aggression. Proportionality allows victim States to defend themselves effectively, in this case by halting and repelling the attack of the aggressor, but no more than that. Accordingly, Ukraine must exercise caution in its response, or such action risks being deemed excessive and, therefore, disproportionate and unlawful. One compelling view (p. 117) is that 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. The threat to Ukraine posed by ongoing Russian aggression might readily be understood to be so great that it cannot adequately be resisted without temporary occupation of Russian territory.
In support of this argument, as well as the aforementioned point that the incursion allows Ukraine to neutralize the source of long-range strikes against its territory, we should note that the war has ground on for over two years (longer if we go back to the initiation of military operations by Russia against Ukraine in February 2014) without any clear signs as to its end. Russia has given little indication that it is willing to halt its aggression and reinstate the territorial integrity of Ukraine, instead increasingly solidifying its hold on the territories it has unlawfully occupied. The incursion seeks to change this state of affairs. Ukraine has publicly stated that an aim of its attack on the Kursk region is to force Moscow to start fair peace talks to end the war. Furthermore, over and above the morale boost to the Ukrainian forces and citizens, Ukraine’s allies might also be bolstered by this bold and unexpected assault and encouraged to increase military support to Ukraine. The Ukrainian advance also draws Russian attention from the front line in eastern Ukraine and aims to prevent Moscow from sending additional reinforcements to the front in the Donbas. Some military analysts point to Russia already being forced to redeploy units from that region to counter the Ukrainian offensive in the Kursk Oblast. All this is to the benefit of Ukraine’s self-defense and support to the argument that its actions are proportionate to this legitimate aim.
Who decides? Although proposals exist for a Special Tribunal on the Crime of Aggression for Ukraine, the question of the legality of Ukraine’s incursion is unlikely to be reviewed and resolved by an international court or tribunal any time soon (if ever). In my view, the Ukrainian incursion thus far is potentially necessary and proportionate under international law governing the use of force. We must wait to see whether the international community will share this view. In terms of State reactions so far, President Joe Biden’s response has been limited to noting the “real dilemma” for President Vladimir Putin, and the U.S. Embassy in Moscow has recently stated, “we were not engaged in any aspect of the planning or preparation of this operation.” That said, some level of U.S. and British backing might be indicated by U.K. and U.S. tanks reportedly having been used during the incursion. The EU meanwhile has been more stalwart in expressing its support for the Ukrainian counteroffensive, going so far as to assert that according to international law, Ukraine “has the legal right to defend itself, including striking an aggressor on its territory.” The reaction of these and other States and international organisations (like the UN) as the incursion unfolds is crucial to ultimately shaping the international community’s understanding of the necessity and proportionality and, as a result, the legality of the Ukrainian military operation.
That said, even if Ukraine’s temporary invasion of Russian territory might be regarded as a necessary and proportionate act of self-defense at this point in time, prolonged occupation beyond a defensive necessity (or any annexation of territory) is never justified (p. 164). Ukrainian occupation of Russian territory is only potentially proportionate while the threat to Ukraine persists. Once Russia’s aggression has been successfully halted and repelled, any ongoing occupation will be unnecessary and must cease. Fortunately, Ukraine’s intentions appear to accord with these requirements of international law. Ukraine’s stated objective in invading the Kursk region is not to “take over”’ this Russian Oblast. Indeed the Foreign Ministry has confirmed that ‘the sooner the Russian Federation agrees to restore a just peace … the sooner the raids of the Ukrainian Defense Forces on the territory of the Russian Federation will stop.” This statement helps establish a prima facie defensive purpose to the invasion, while affirming that the occupation is temporary. Such a position stands in stark contrast to Russia’s enduring and unlawful occupation of Ukrainian territory.
Legality also hinges on other factors. Although the occupation of foreign territory is a distinctive and highly significant feature, the proportionality of the Ukrainian incursion will depend in large part on the overall effects of the operation on Russia. Most significant (pp. 139-146) is the resulting harm to Russian civilians and to civilian infrastructure. Proportionality does not allow for a tit for tat response on the part of Ukraine, one which would mirror the devastating attacks by the Russian State on Ukraine’s territory and civilians and to mete out that same devastation. Ukraine must contain its response to what is necessary to achieve a defensive purpose and not exceed that purpose. Excessive civilian harm risks being regarded as punishment, revenge, or retribution, which, by definition, goes beyond self-defense and into the realm of an unlawful use of force. These considerations apply in addition to the IHL protections for civilians and civilian objects, and only time will tell if they are met.
Lawfulness and its Significance
The question of whether Ukraine is adhering to international law governing the use of force is highly significant both for Ukraine as it fights for its survival and for the status and integrity of international law. The latter concern is magnified when we consider the current dismal geopolitical picture in which conflict is raging in different corners of the globe and the post World War II rules-based international order is under increasing threat.
Over and above the self-evident argument that all States should abide by the law that applies to them, the legality of the Ukrainian advance has particular significance. In this war, Russia is the clear aggressor. By waging an aggressive war against its neighbor, Russia has egregiously violated the UN Charter and challenged the very international peace and security that, as a permanent member of the UN Security Council, it is tasked to protect. Additionally, the Russian invasion has been marked throughout by allegations of widespread and systematic violations of IHL. Arrest warrants have been issued by the International Criminal Court for alleged Russian war crimes, including against President Putin himself. The dominant narrative, therefore, is one of Russian illegality that poses a direct threat to the rules-based international order.
In contrast, Ukraine fights as the victim of aggression. The UN Charter is on their side as guardian of their sovereignty and their territorial integrity. By acting in lawful self-defense, Ukraine’s conduct is protected by a mantle of legality and legitimacy. In the face of Russian violations, compliance with international law enhances the legitimacy of Ukrainian actions and bolsters the righteousness of their cause. Abiding by the international law ultimately benefits Ukraine and helps them to achieve their strategic aims, including garnering support and assistance from allies and partners around the world that profess to care about the rule of law.
Ukraine’s adherence to the law also benefits the world more generally. The rules of the UN Charter and customary international law that regulate when States may use force were designed to underpin international peace and security and to prevent future generations from the “scourge” of the large interstate wars of the past. In defending itself, Ukraine also acts on behalf of the international community in upholding these cardinal rules and helping to prevent their decline. This matter is something in which all States and their citizens have an abiding interest.