One Rule, Two Standards

When Ukrainian President Volodymyr Zelenskyy arrived in Belgrade in August, on his first visit to Serbia as president, the immediate question was why Ukraine, a country fighting a war against Russia, would seek closer ties with Serbia, a militarily neutral European country that maintains political, economic and security relations with Russia and China. The apparent contradiction, however, obscures a more interesting development.

Over the past several years, Serbia and Ukraine have quietly developed a pragmatic relationship that is considerably more substantial than their different security orientations would suggest. Serbian ammunition has reached Ukraine during the war through European intermediaries, while former U.S. Ambassador to Serbia Christopher Hill has acknowledged Washington’s role in connecting Ukrainian demand with Serbian defense production. The two countries have also maintained substantial bilateral economic exchange, while Serbia has provided up to €60 million in non-lethal, medical and energy assistance since 2022. Belgrade reopened its embassy in Kyiv in October of 2025. Three weeks before Zelenskyy's trip to Belgrade, Vučić made his own visit to Kyiv, his first since the start of the war, and the two governments have maintained political contacts while consistently affirming their respective commitments to one another's territorial integrity and sovereignty. Serbia has also repeatedly supported Ukraine in international forums, including at the United Nations.

There is, however, something deeper drawing the two countries together. Both Serbia and Ukraine find themselves confronting the same fundamental question of international politics: what happens when the territorial sovereignty of a state is challenged? Their circumstances are profoundly different, their histories are different, and their security policies are not identical. Yet both have made territorial integrity a central principle of their foreign policy. For Ukraine, the challenge is Russia’s seizure and attempted incorporation of Ukrainian territory. For Serbia, it is Kosovo’s declaration of independence and the subsequent recognition of Kosovo by many Western powers, though by roughly half of UN member states and no more. Kosovo remains outside the United Nations, and its prospects for membership remain highly constrained by the opposition of Russia and China, both permanent members of the Security Council with veto power. In both cases, Belgrade and Kyiv insist that the territorial configuration of a sovereign state cannot simply be altered because another political actor, domestic or external, considers that alteration justified.

That commonality is more consequential than it might initially appear. Serbia and Ukraine are not geopolitical opposites. Both are European states and both are formally on a path toward European Union membership. The important distinction lies in their security orientation. Ukraine has made its Euro-Atlantic future a central strategic objective, while Serbia remains militarily neutral and continues to maintain relations with Russia and China. During the visit of President Xi Jinping to Belgrade in May of 2024 the bilateral relations were raised to the level of “Community with a shared future in the new era” as the only European country to hold such a status. Serbia, on the other side, is a firm supporter of the One China policy. Concerning Russia, the two countries have deep historical and cultural ties, and contemporary heavy reliance on Russian energy.

Yet on the question of territorial sovereignty, Kyiv and Belgrade find themselves defending positions that have brought them into an unexpected degree of diplomatic proximity. Ukraine does not recognize Kosovo, while Serbia does not recognize Russia’s attempted annexation of Ukrainian territory. Both governments therefore have an interest in preserving the idea that international borders cannot simply be changed through the exercise of political or military power. Serbia’s position is particularly notable because its support for Ukraine’s territorial integrity has existed alongside its refusal to join Western sanctions against Russia.

And that is where the significance of Kosovo extends far beyond Kosovo itself.

THE CONVENIENT EXCEPTION

Whether Serbia is correct about Kosovo, or Ukraine correct about Crimea, is not the question at issue here. The prior question concerns the principle that both governments invoke, and whether that principle retains any determinate content when the states with the capacity to enforce it remain free to determine, case by case, where it applies. That question is answered less in Belgrade or Kyiv than in the general practice of international law.

International law is premised on the idea that rules do not become rules only when they are convenient. The UN Charter does not distinguish between the territorial integrity of states depending on whether the state in question is an ally or an adversary. The principle of sovereign equality is meaningful precisely because it is supposed to transcend differences in power. Yet the history of international relations demonstrates how frequently states have treated legal principles as conditional. Governments invoke sovereignty when it protects their interests, self-determination when it advances another objective, and territorial integrity when it constrains an adversary. The same principle can therefore appear absolute in one conflict and negotiable in another.

The problem is not that international law contains no room for distinction. It plainly does. Nor is it that every territorial dispute must produce the same legal outcome. Rather, the problem is the growing tendency to treat international law as voluntary in application but universal in rhetoric. States rarely reject the legitimacy of international law outright. Instead, they develop arguments explaining why the rules apply differently in the particular circumstances before them. Over time, this can produce an international system in which almost every actor can find a legal justification for its preferred position, while the underlying principles become progressively less constraining.

It is in Kosovo and Ukraine that this tendency has become most visible. The debate surrounding Kosovo and the territories of Ukraine occupied or claimed by Russia—including Crimea and Sevastopol, as well as parts of Donetsk, Luhansk, Kherson and Zaporizhzhia—is often treated as a question of whether the two cases are comparable. That framing is too narrow. International politics is full of cases that differ in their historical circumstances, legal context and political consequences. The more important question is what these cases reveal about how international law operates. Kosovo and Russia’s attempts to alter Ukraine’s territorial status provide two particularly prominent examples of a much broader problem: the tension between international law as a system of universally applicable rules and international law as a collection of principles that states invoke, interpret and sometimes set aside according to their geopolitical interests. The same tension can be seen beyond these cases. Catalonia’s attempted secession from Spain in 2017, for example, involved an appeal to self-determination and the democratic will of a distinct territorial community. Western Sahara, the territory classified by the UN as a non-self-governing territory awaiting decolonization, is majority-controlled by Morocco that does not recognize its right to self-determination.

The circumstances of these cases are fundamentally different, but their juxtaposition illustrates how the language of self-determination can be deployed in very different political and legal contexts.

BEYOND COMPARISON

The distinctions between these cases are real, and drawing them carefully is what any serious legal analysis requires. A system that treated every territorial dispute as identical would not be a legal system at all. But the exercise of distinguishing does not itself resolve the underlying difficulty; it generally serves only to locate that difficulty more precisely. The point is not that Catalonia, Kosovo, Western Sahara and the occupied Ukrainian territories are legally or historically equivalent. They plainly are not. Catalonia was an autonomous region within a democratic EU member state; Western Sahara is a remnant of colonial African past; Kosovo emerged from a conflict involving mass displacements, international administration and NATO intervention; while Russia’s actions in Ukraine have involved the use of military force and attempted annexation of territory belonging to a sovereign state. What connects these cases is not their legal equivalence, but the recurring question of how claims of self-determination interact with the territorial integrity of existing states—and how states determine when one principle should prevail over the other. The Spanish Constitutional Court, for example, rejected the claim that international law provided Catalonia with a unilateral right to secede, emphasizing the distinction between self-determination and a general right of unilateral separation.

The difficulty, however, is not only a matter of how states behave. It is also present in the doctrine itself. The legal tension is particularly visible in the relationship between territorial integrity and self-determination. The principle of self-determination became especially prominent in the context of decolonization, where it was linked to the emergence of new sovereign states from colonial territories. Territorial integrity, by contrast, has generally operated as a protection of the territorial sovereignty of existing states. But international law has never provided a universally accepted formula for resolving every situation in which claims of self-determination collide with the territorial integrity of an existing state. The resulting uncertainty does not mean that all claims to secession are legally equivalent, nor does it establish a general right of unilateral secession. It does, however, leave room for competing interpretations of when and how the territorial status of a state may legitimately be altered.

WHAT THE COURT LEFT OPEN

That uncertainty has been examined directly on one occasion. On 8 October 2008, the United Nations General Assembly asked the International Court of Justice whether the unilateral declaration of independence by the Provisional Institutions of Self-Government of Kosovo was in accordance with international law. The framing of that question would prove consequential, and the opinion delivered in July 2010 is now cited by parties on every side of these disputes, sometimes for propositions the Court did not advance.

The International Court of Justice’s 2010 Advisory Opinion on Kosovo illustrates the distinction. The Court did not determine that Kosovo possessed a positive right under international law to become independent. Instead, it addressed the narrower question of whether the adoption of Kosovo’s declaration of independence on 17 February 2008 violated international law. The Court determined that it did not, as the authors of the declaration had not acted as the Provisional Institutions of Self-Government established under the UN-administered constitutional framework, but rather as persons acting in their capacity as representatives of the people of Kosovo outside that framework. The Court consequently concluded that the declaration did not violate Security Council Resolution 1244, the Constitutional Framework, or any applicable rule of international law as it was not adopted by the provisional institutions set up by those institutions.

The significance of the opinion therefore lies partly in what it did not decide. The Court remained silent on whether Kosovo had a right to become independent. It did not establish a general legal right of unilateral secession, nor did it pronounce that Kosovo’s independence was itself required or authorized by international law. Its ruling was narrower: the adoption of the declaration of independence itself did not violate applicable international law. That distinction matters because subsequent political debates have sometimes treated the opinion as though it resolved the broader question of Kosovo’s legal entitlement to independence. It did not.

Serbia`s situation and Russia’s actions in Ukraine are especially revealing in this respect because both have become symbols of competing interpretations of territorial sovereignty. For Serbia, Kosovo represents the proposition that the territorial integrity of a sovereign state should not be overridden by political decisions that establish a new state without the consent of the state from which the territory is being separated. For Ukraine, the Russian occupation and attempted annexation of Ukrainian territory represent the proposition that territory cannot be acquired through military force and subsequently transformed into sovereign territory through political processes conducted under occupation. Crimea is the earliest and perhaps most prominent example of this process, but the issue extends well beyond Crimea to the Russian-occupied and purportedly annexed territories of Donetsk, Luhansk, Kherson and Zaporizhzhia. The two disputes raise different legal and historical questions, but both confront the international system with the same foundational problem: can the territorial status of a state legitimately be changed, and who has the authority to make that determination?

The difficulty is that international law provides principles for answering this question but not a universally accepted mechanism that resolves every collision between those principles. The prohibition on the use of force, respect for territorial integrity, sovereign equality and self-determination all form part of the contemporary international legal order, but their interaction has generated some of the most contested questions in international law. The problem becomes especially acute when states invoke one principle to justify an outcome while treating the same or competing principle differently in another case.

RULES THAT BEND

What states have made of that silence is the more revealing question. Where the law declines to supply a determinate answer, governments supply their own, and those answers have tended to correspond closely to the interests of the governments supplying them. Any single position may well be defensible when examined on its own terms. The correspondence becomes apparent only when the positions are set alongside one another.

This is where the Kosovo–Ukraine debate becomes relevant to the broader credibility of international law. Russia has sought to justify its actions in Ukraine through arguments involving self-determination, historical claims and the protection of populations, even as Moscow insists upon the territorial integrity of Russia and rejects separatist claims that threaten its own borders. Western governments have made the defense of Ukraine’s territorial integrity a central principle of their response to Russia, while many of those same governments supported Kosovo’s independence in circumstances that Serbia continues to regard as a violation of its territorial integrity. Serbia, in turn, invokes the UN Charter and international law in opposing Kosovo’s independence while maintaining a political relationship with Russia, whose actions in Ukraine represent a profound challenge to the same principle of territorial integrity. Ukraine rejects Russia’s territorial claims while maintaining its long-standing position of non-recognition toward Kosovo. Russia champions Serbia’s territorial sovereignty as a matter of declared policy, while Ukraine arrives at a comparable position through its own long-standing non-recognition of Kosovo. Two governments at war with one another thus converge on the same position toward Belgrade. Each government in this dispute defends territorial integrity in one context while qualifying it in another, and the pattern is difficult to reconcile with the claim that any of them is applying a consistent rule.

Sounds confusing? It surely is.

None of these positions is necessarily reducible to simple hypocrisy. States can distinguish between cases, and sometimes those distinctions are legally and historically justified. The problem emerges when the explanation for the distinction becomes inseparable from the political interests of the state making it. International law is inevitably interpreted by states operating within a political system. It cannot be entirely insulated from power. But its legitimacy depends on the belief that legal principles possess some meaning beyond the immediate interests of those invoking them.

This is particularly important for the United States and its European allies. The defense of Ukrainian territorial integrity is not simply a defense of one country’s borders. It is an assertion that the fundamental rules governing the international system cannot be rewritten by force. That argument is strongest when it is understood as universal. Its credibility becomes weaker when states appear to treat the same principles as conditional upon political circumstances. For Washington and its European allies, the challenge is therefore not merely to distinguish Kosovo from Crimea on the facts, which can be done, but to explain clearly to its public and the global community what justifies the distinction in their approaches in these two cases.

WHAT BELGRADE AND KYIV SHARE

These considerations return the argument to Belgrade. The visit itself was not, on its face, an occasion of legal significance. Two presidents met, and the commentary that followed concerned largely what each government had gained from the other. What makes the meeting worth examining is not its diplomatic content but the condition that made it possible.

This is ultimately what makes the emerging Serbia–Ukraine relationship so interesting. It is not simply another example of transactional diplomacy in an increasingly fragmented Europe. It is an unexpected partnership built, at least in part, around a shared concern about the erosion of territorial sovereignty. Serbia and Ukraine are not becoming allies in the conventional sense, but neither are they geopolitical opposites. The relationship is particularly revealing because this convergence has emerged despite an important difference in security orientation rather than because the two countries occupy fundamentally different geopolitical camps. Serbia is an EU candidate country, remains militarily neutral and maintains relations with Russia and China, while Ukraine has committed itself to a Euro-Atlantic future. Yet Serbia has supported Ukraine’s territorial integrity, provided substantial non-military assistance, maintained diplomatic representation in Kyiv and expanded bilateral economic cooperation. Their relationship demonstrates how states that share a broad European trajectory, while differing in their approach to the Euro-Atlantic security order, can nevertheless discover a meaningful convergence around territorial integrity.

The irony is that neither Serbia nor Ukraine needs the other to abandon its broader geopolitical position for this convergence to matter. Ukraine can continue to view Russia as an aggressor and Serbia can continue to maintain its relations with Moscow and Beijing. Serbia can continue to reject Kosovo’s independence and Ukraine can continue to refuse recognition of Kosovo. Their common interest lies at a more fundamental level: both have a stake in preserving a system in which territorial sovereignty cannot simply be overridden whenever a sufficiently powerful actor believes that doing so is politically justified.

The question raised by Zelenskyy’s visit to Belgrade is therefore much larger than whether Serbia and Ukraine can cooperate, whether Belgrade will move closer to Kyiv, or whether the two countries can compartmentalize their disagreements over Russia and Kosovo. The more consequential question is whether the principle that brings them together—territorial integrity - can survive as a genuinely universal rule.

International law was never designed to eliminate geopolitics. Great powers will continue to interpret rules through the prism of their interests, and weaker states will continue to invoke those rules as protection against the powerful. In the words of Thucydides, “the strong do what they can and the weak suffer what they must”.

But there is a fundamental difference between acknowledging that reality and accepting it as the organizing principle of international law. If states apply international law only when it coincides with their geopolitical interests, then the system may retain its institutions, treaties and vocabulary while losing the quality that makes law meaningful: the expectation that rules constrain conduct even when they are inconvenient.

That may ultimately be the most important message of Zelenskyy’s visit to Belgrade. Ukraine and Serbia did not expect to find themselves on the same side of a geopolitical divide. They are not on the same side of every geopolitical question. Yet both have discovered that the defense of territorial sovereignty creates an unexpected point of convergence. Their relationship consequently offers a paradox that is larger than either Kosovo or Crimea: the more states invoke international law selectively, the more important universal application becomes, and the harder it becomes to achieve.

The real test of the international legal order is therefore not whether states can find legal arguments for their preferred outcomes. They almost always can. It is whether the principles they invoke against their adversaries are principles they are prepared to accept as constraints upon themselves.

One principle may be bringing Belgrade and Kyiv closer together. The future of the international order may depend on whether that principle can be made universal.