After two and a half years of full-scale war, Ukrainian forces launched a surprise attack across the Russia-Ukraine border, capturing significant portions of territory in Russia’s Kursk region. Under the law of jus ad bellum, Ukraine’s military operations are consistent with the inherent right of self-defense as recognized in Article 51 of the U.N. Charter because the operations are conducted in response to Russian aggression and constitute a necessary and proportional measure for repelling Russian aggression: no peaceful alternatives are practicable, military operations in Ukraine have stalemated, and military operations in the Russian Federation could alter the course of the war. Furthermore, Ukraine’s border operations are not excessive as compared to the Russian Federation’s ten-year war against Ukraine, and they are defensive since they aim to neutralize ongoing and future threats to Ukraine.
I. Ukraine’s Military Operations in Russia’s Kursk Region
On 6 August 2024, hundreds of Ukrainian troops supported by tanks and armored vehicles launched an unprecedented, cross-border operation into Russian territory. Within a day, they advanced as far as 15 kilometers into Kursk, penetrating two Russian defensive lines and capturing several settlements along the way. Faced with the surprise attack, Russian President Putin called the operation a “large-scale provocation,” convened a meeting of his Security Council, and deployed Russian forces and fighter jets to the affected areas. Nearly five months later, Ukraine maintains a foothold in Kursk, but Russian forces have recovered about 40% of Ukraine’s initial territorial gains. The Russian Foreign Ministry claimed that at least 56 civilians were killed and 266 wounded from 6 August to 23 September and has not yet updated these figures.
II. Ukraine’s operations in Kursk are consistent with the U.N. Charter’s Article 51 right of self-defense and satisfy the requirements of necessity and proportionality
Article 2(4) of the U.N. Charter imposes a general prohibition on the use of force by one state against the territorial integrity or political independence of any other state.[1] However, a state can lawfully use force against another state in two circumstances: when it receives authorization from the U.N. Security Council[2] or when acting in self-defense.[3] Article 51 of the U.N. Charter establishes that U.N. member states have an “inherent right of individual or collective self-defense if an armed attack occurs” against it. The International Court of Justice has specified that the armed attack must be of sufficient “scale and effect” to trigger the right of self-defense.[4] In particular, the Court stated that a government sending its “armed forces across an international border” into the territory of another state meets this threshold.[5]
The Russian Federation’s war against Ukraine breaches the prohibition against the use of force, and it constitutes an armed attack of sufficient scale and effect to trigger Ukraine’s right of self-defense. The U.N. General Assembly recognized, in resolution ES-11/1, that the Russian Federation breached Article 2(4) of the U.N. Charter when it invaded Ukraine on 24 February 2022.[6] In accordance with the Court’s criteria, the Russian Federation deployed its armed forces into Ukraine, openly acknowledging this action as a “special military operation” to “demilitarize and denazify Ukraine.” The U.N. General Assembly confirmed this understanding when it stated that the “military operations of the Russian Federation inside the sovereign territory of Ukraine” constitute aggression and “are on a scale . . . not seen in Europe in decades.[7]” Therefore, the acts of 24 February 2022 triggered Ukraine’s right of self-defense, allowing Ukraine to lawfully use force against the Russian Federation to protect its sovereignty and restore its territorial integrity.
Nevertheless, a state’s ability to engage in self-defense is not absolute. Customary international law sets additional guardrails on what measures a state may take, including the requirements of necessity and proportionality.[8]
a. Necessity
Necessity requires the use of force in self-defense to be the only practicable means to ending or averting the aggressor’s attack. Any peaceful alternative––such as diplomacy or economic sanctions––must be unavailable, unfeasible, or ineffective,[9] but available alternatives need not be exhausted. The operation will be considered necessary if the state acting in self-defense targets the source of the threat.[10] That state may take measures “to remove a continuing threat to future security,” including those that go beyond repelling the aggressor’s original attack.[11] However, the operation cannot modify borders, deplete resources, or inflict retribution on the aggressor. Importantly, the state engaging in self-defense is under no obligation to confine its operations to its own territory.[12] The self-defense “may carry the combat to the source of the aggression” and continue until the threat is neutralized.[13]
Ukraine’s actions are necessary because peaceful alternatives and other military tactics have failed to end the war. Regarding peaceful alternatives, Ukraine has pursued diplomatic channels in the past, but discussions stalled over President Putin’s demand for territorial concessions. Moreover, economic sanctions enacted by the international community, while unprecedented, have failed to create a noticeable change in Russian foreign policy. In any case, international law does not require peaceful alternatives to be fully exhausted, and Ukraine’s reasonable efforts have not brought Ukraine closer to peace.
Regarding other military tactics available to Ukraine, attempts to repel Russian forces from Ukrainian territory have been unsuccessful. A relative stalemate has persisted for over two years (since Ukraine’s first major counteroffensive in 2022), with few territorial gains on either side. As Russian forces continue to commit widespread violations of humanitarian and human rights law, and Ukraine’s survival continues to be at stake, Ukraine must shift its strategy.[14]
Ukraine’s operations in the Russian Federation could neutralize immediate threats to its security and create enough negotiating leverage to bring an end to the Russo-Ukrainian war. Ukraine is generally targeting infrastructure and weapon systems that facilitate attacks against its territory, as well as establishing a buffer zone to ensure that no further attacks reach Ukraine from this region. These operations are necessarily defensive because they remove the immediate threat posed by the Russian military to Ukrainian cities and people. In fact, it is a much more efficient and effective strategy than shooting down every missile as it crosses the border. Russian military infrastructure is not protected by international law merely because it is located outside of the frontlines: as noted above, Ukraine is under no obligation to confine its operations to its own territory.[15] Ukraine’s long-term goal is to use Russian territory to negotiate the withdrawal of Russian forces from Ukraine; since President Putin has not stopped the war in response to diplomatic and economic efforts, military operations in the Russian Federation may create new incentives for negotiation.
b. Proportionality
Proportionality prohibits the use of force in self-defense that goes beyond what is needed to end the aggressor’s attack; this is commonly referred to as “the prohibition against excess.”[16] Importantly, proportionality does not require the self-defense operation to be commensurate to the harm suffered by the aggressor’s particular attack or confined to the location where it occurred.[17] Rather, the operation must be either quantitatively or teleologically proportional: quantitative proportionality balances the harms between the defensive action and the action that prompted the self-defense, while teleological proportionality balances the defensive action against “the legitimate ends of using that force.”[18] The latter approach allows the state acting in self-defense to consider previous, ongoing, and possible future attacks when determining the lawfulness of its operations.[19] Ukraine’s operations are proportional since Ukraine is acting with a defensive purpose that aims to stop Russian attacks against it, and its methods are neither quantitatively nor teleologically excessive.
First, Ukraine’s operations are defensive because they were initiated in Russian territory to: destroy military infrastructure and long-range weapon systems targeting Ukraine; establish a buffer zone from which Russian forces cannot attack Ukraine; coerce the Russian Federation into redeploying its forces to the affected areas, thereby decreasing the number of troops present in Ukraine; and, ultimately, drive President Putin to negotiations that will lead to the complete withdrawal of Russian forces from Ukraine and an end to the Russo-Ukrainian war.[20] These objectives not only seek to eliminate immediate threats emanating from Russian territory, but also strategically coerce the cessation of future attacks against Ukraine by using Russian territory as a bargaining chip during peace negotiations.[21]
Second, Ukraine’s operations satisfy the balance of harms of both quantitative and teleological proportionality. Quantitatively, Ukraine targeting lawful military objectives and temporarily occupying Russian territory in the Russia-Ukraine border area is proportional to the Russian Federation’s war against Ukraine, which has claimed about one million casualties, endured for over ten years, and witnessed the regular commission of severe violations of humanitarian and human rights law, including the killing and torture of civilians, the targeting of civilian infrastructure, the deportation of children, and more. Against this, the Russian Federation has not made any detailed accusations of Ukraine violating international humanitarian law. However, Ukraine’s operations have reportedly led to civilian deaths and caused damage to civilian infrastructure.
Teleologically, Ukraine’s limited operations on the Russia-Ukraine border are not excessive in relation to Ukraine’s goal for using the force: to compel President Putin to withdraw Russian forces from Ukrainian territory and negotiate a peace deal, thereby ending the ten-year Russo-Ukrainian war. Developments on the ground reveal Ukraine’s inability to end the war through status quo operations within Ukraine: minimal territorial gains in Ukraine from either side since September 2022; a decrease in military, economic, and political support from the United States; increasingly low morale among Ukrainian forces and the Ukrainian population generally; and a lack of operational surprise on Ukraine’s frontlines. If successful, Ukraine’s localized military operations in the Russian Federation may alter the course of the war.
Interestingly, Ukraine invited the United Nations and the International Committee of the Red Cross to monitor its compliance with international humanitarian law. At the time of writing, neither organization has announced a plan to do so.
III. Conclusion
Ukraine’s military operations in Kursk are consistent with its right of self-defense, as recognized in Article 51 of the U.N. Charter. To ensure that Ukraine’s operations remain consistent with its international legal obligations, Ukraine must continuously re-evaluate the necessity and proportionality of its operations, such that when Russian forces stop attacking and withdraw from Ukraine, Ukrainian forces also cease their operations against the Russian Federation.
Author: Sofia Polishchuk is a Juris Doctor Candidate at the George Washington University Law School, specializing in national security and foreign relations law. She works in the political section of the Permanent Mission of Ukraine to the United Nations. She also serves as the Director of the Children of Ukraine Foundation, a 501(c)(3) public charity providing humanitarian assistance to Ukrainian children. Sofia has completed post-graduate training at Harvard University, Oxford University, and the Hague Academy of Public International Law.
Editor: Albert Anderson