Unilateralism or Cooperation?

On February 24, 2022, Russian President Vladimir Putin stunned the world by announcing in a televised address that Russia would conduct a “special military operation” against Ukraine. According to Putin, Russia would not stand idly by while “far-right nationalists and neo-Nazis in Ukraine” committed “genocide” against pro-Russian separatists. Military intervention was also a necessary exercise of self-defense, Putin argued, because Ukraine’s warming relations with the North Atlantic Treaty Organization (NATO) could eventually bring advanced western weaponry to Russia’s doorstep. “Russia cannot feel safe, develop, and exist while facing a permanent threat from the territory of today’s Ukraine,” Putin asserted.Footnote ¹ Moments later, Russian tank columns pierced Ukraine’s borders from multiple directions as missiles rained down on Kyiv.Footnote ²

While Ukraine was resisting Russia on the battlefield, it also mounted a vigorous legal defense. Just two days after Russia launched its invasion, Ukraine instituted proceedings against Russia in the International Court of Justice (ICJ), a tribunal established under the United Nations (UN) Charter to adjudicate disputes between states.Footnote ³ Ukraine urged the Court to adjudge and declare that Russia’s accusations of genocide against separatists in eastern Ukraine were unfounded, having no basis in fact or international law. Ukraine also asked the ICJ to grant provisional measures requiring Russia to suspend its invasion while legal proceedings continued before the Court.Footnote ⁴ Russia declined to appear for oral argument but contested Ukraine’s application, asserting that the ICJ lacked jurisdiction over the case.Footnote ⁵

While Ukraine’s application was pending before the ICJ, the international community rebuked Russia in the strongest possible terms. On March 2, 2022, the UN General Assembly met in emergency session and voted overwhelmingly in favor of a resolution condemning Putin’s televised speech, deploring Russia’s invasion, and demanding that Russia “immediately, completely and unconditionally withdraw all of its military forces from the territory of Ukraine within its internationally recognized borders.”Footnote ⁶ Two weeks later, the ICJ took note of the General Assembly’s resolution in an order granting Ukraine’s request for provisional measures and directed Russia to suspend its military operations in Ukraine.Footnote ⁷ The European Court of Human Rights (ECtHR) also granted interim measures at Ukraine’s request, instructing Russia “to refrain from military attacks against civilians and civilian objects, including residential premises, emergency vehicles and other specially protected civilian objects, such as schools and hospitals.”Footnote ⁸

Russia refused to change course. Shrugging off the interim orders from the ICJ and the ECtHR, Russia continued to pound Ukrainian cities with missiles, artillery, and aerial attacks. In his annual Victory Day speech from Red Square, Putin repeated his grievances against Ukraine and defended Russia’s “pre-emptive strike” as “forced, timely, and the only correct decision. A decision by a sovereign, strong, and independent country.”Footnote ⁹ According to Putin, Russia’s status as an independent sovereign meant that it was entitled to decide for itself whether its attacks against Ukraine were legally justified – without regard to other states’ views or the considered judgments of international courts.

On February 24, 2025, the third anniversary of the invasion, the UN General Assembly met in emergency session and passed two further resolutions. The first resolution reaffirmed the General Assembly’s earlier condemnation of the invasion and reiterated “its demand that the Russian Federation immediately, completely and unconditionally withdraw all of its military forces from the territory of Ukraine.”Footnote ¹⁰ The resolution likewise called for a peaceful resolution to the conflict “consistent with the [UN] Charter, including its principles of sovereign equality and territorial integrity of States.”Footnote ¹¹

Mariana Betsa, Ukraine’s Deputy Foreign Minister, introduced this resolution on behalf of fifty-three state cosponsors. She accused Russia of aiming to replace “the world based on international law” with a “world based on the rule of force.”Footnote ¹² She characterized Ukraine’s struggle as seeking a world in which states are not segregated into “predators and prey.”Footnote ¹³ Denouncing the Great Power politics that preceded the United Nations, she said that Ukraine was “fighting for a world which cannot be divided by another Molotov-Ribbentrop Pact … where a future of a nation cannot be determined [by foreign powers] at another Yalta Conference.”Footnote ¹⁴ In other words, Betsa presented Ukraine as defending the world order envisioned in the UN Charter under which all sovereign states are entitled to equal recognition and respect under the rule of international law.

A second General Assembly resolution adopted the same day originated in a proposal from the United States. One week earlier, the United States had held talks with Russia in Saudi Arabia to discuss resolution of the Russia–Ukraine war, without inviting participation from Ukraine or other European nations.Footnote ¹⁵ At the subsequent General Assembly emergency session, the United States introduced a draft resolution that mourned the loss of life in Ukraine and reiterated the United Nations’ mandate “to maintain international peace and security and to peacefully settle disputes,” but declined to condemn Russia, demand Russian withdrawal, or affirm the core UN principles of sovereign equality and territorial integrity.Footnote ¹⁶ The US draft called for a “swift end to the conflict” and “a lasting peace,” but without specifying anything about how this was to happen or the nature of the peace envisioned.Footnote ¹⁷ Ultimately, European states amended the draft to reaffirm the General Assembly’s commitment to “the sovereignty, independence, unity and territorial integrity of Ukraine,” with a further stipulation that the “swift end to the conflict” must take the form of a “just, lasting and comprehensive peace … in line with the Charter of the United Nations and the principles of sovereign equality and territorial integrity of States.”Footnote ¹⁸

The content of the two February 24, 2025, General Assembly resolutions was hardly groundbreaking; they essentially echoed previous resolutions. What made the resolutions remarkable was that the United States declined to support them, despite having voted for a resolution with near-identical content two years prior.Footnote ¹⁹ The United States stunned the world by joining Russia, Belarus, Hungary, Israel, North Korea, Sudan, and eleven other states in voting against the first resolution. Ninety-three states voted in favor while 65 abstained, marking a sharp overall decline from the 2023 resolution, which garnered 141 votes for, 7 against, and 32 abstentions.Footnote ²⁰ When called to vote on the second resolution, the United States abstained. That resolution passed with ninety-three in favor, eight against, and seventy-three abstentions.Footnote ²¹

The United States’ votes on these resolutions reveal a dramatic shift under the Trump Administration toward an attitude of tolerance, if not outright support, for an international regime of Great Power dominance. The United States’ policy shift also seems to have persuaded many other states to back away from expressly defending the UN Charter’s vision of a world that is not divided into “predators and prey.” By declining to support resolutions affirming the principles of sovereign equality and the territorial integrity of states, the United States breathed new life into a conception of state sovereignty that had fallen into disrepute during the twentieth century. According to that conception, espoused in Putin’s public statements defending Russia’s invasion of Ukraine, sovereigns as “uncommanded commanders” are not subject to any superior authority.Footnote ²² They therefore are free to choose whether to undertake international obligations, as well as to interpret those obligations for themselves. On this understanding, sovereigns have unqualified discretion to decide whether to cooperate with other states on matters of common concern or adopt a recalcitrant or even belligerent approach. In recognition of its salient features, throughout this book we refer to this conception of state sovereignty as unilateralist, absolutist, or voluntarist. A distinguishing feature of this conception is the idea that states are entitled to exercise their sovereign rights and authority however they wish, subject only to those limitations that they voluntarily embrace.

Ukraine’s appeal to international institutions in response to Russia’s invasion reflects a different conception of state sovereignty. International institutions like the General Assembly, the ICJ, and the ECtHR are predicated on understanding sovereigns as legal equals interacting within an international legal system that imposes certain obligations on all states. Under this conception of international legal order, the principle that states are equally sovereign means that they are not entitled to exercise authority over one another unilaterally, including by asserting the prerogative to act as final arbiters for the international legality of their own actions (nemo iudex in causa sua). Instead, to respect sovereign equality under international law, states sometimes must engage cooperatively with others. That is, they must engage in what we will call mandatory cooperation. This means that states must pursue peaceful dispute resolution and may resort to force only for limited reasons, such as self-defense, and then only in a limited manner. We generally refer to this conception of international legal order as constitutional, although we also sometimes describe it as equitable.

The argument of this book is that when states disagree over matters of mutual or common concern, the constitutional model now undergirds and constitutes international law’s dominant framework for interstate dispute resolution, establishing requirements of mandatory cooperation. Procedurally, mandatory cooperation imposes an obligation to consult and investigate, negotiate in good faith, and seek third-party dispute resolution if negotiation fails to yield an amicable outcome. Substantively, mandatory cooperation looks to equitable principles to limit the exercise of sovereign rights in order to take account of the legitimate interests of other states and foreign peoples. For instance, we shall see that mandatory cooperation limits how upstream states may use a waterway on which foreign nationals in a downstream state depend. Mandatory cooperation also limits states’ authority to apportion migratory fisheries, conduct maritime delimitation, and regulate transit through their territory. Exploring these and other topics, we will show that mandatory cooperation reflects an equitable conception of sovereignty. Conceptualizing sovereignty in this manner not only clarifies international law’s current requirements (lex lata) but also offers legal principles to guide the law’s progressive development in areas where mandatory cooperation has yet to take firm hold (lex ferenda).

To illustrate the continuing tensions between the voluntarist and constitutional conceptions of sovereignty, this chapter further explores how these dueling approaches have been expressed throughout the blood-soaked history of Russia’s efforts to establish sovereignty over Ukraine. We then flesh out what we mean by a “constitutional” conception of sovereignty, and we explain how this conception supports the equitable requirements of mandatory cooperation under contemporary international law. The chapter concludes with a brief roadmap for the remainder of the book.

Into the Valley of Death

With territory extending into the Black Sea from southern Ukraine, the Crimean Peninsula has been a theatre of frequent and brutal wars for centuries. Catherine the Great of Russia annexed Crimea in 1783 after several decades of territorial conflict with the Ottoman Empire.Footnote ²³ Seventy years later, the Crimean War (1853–1856) pitted Russia against the combined might of the Ottoman Empire, France, Britain, and Sardinia, with devastating results. Russia lost 400,000 soldiers in the conflict, and the verdant peninsula was laid waste as “[w]ar devoured every vine, tree, or blade of grass in its path.”Footnote ²⁴ The brute ferocity of the conflict inspired Lord Tennyson’s iconic 1854 poem, The Charge of the Light Brigade, which begins:

To Tennyson, the errant charge of the Light Brigade into withering cannon fire captured the heroism of individual soldiers caught up in a seemingly senseless international conflict marked by savage violence, extreme misery, and frequent mishaps. Reflecting on his service as a Russian captain in Sevastopol during this period, Leo Tolstoy offered a similarly bleak assessment of the prospects for enduring peace, observing that a dispute that had “not been solved by diplomacy will still less be solved by powder and blood.”Footnote ²⁶

The subsequent history of Russian involvement in Crimea confirmed Tolstoy’s fears. In the Treaty of Paris, which ended the Crimean War, Russia agreed to refrain from establishing military bases on the Black Sea in exchange for retaining sovereignty over much of Crimea, but the promised demilitarization did not last. In 1870, Russia unilaterally repudiated the Treaty’s Black Sea clauses and proceeded to rebuild its navy, reigniting regional tensions.Footnote ²⁷ Decades later, in 1954, the Soviet Union incorporated Crimea into what was then the Ukrainian Soviet Socialist Republic. When the Soviet Union collapsed in 1991, Crimea became part of the newly independent Ukraine as an autonomous republic. The 1997 Treaty of Friendship, Cooperation and Partnership between Russia and Ukraine officially recognized Ukraine’s sovereignty over Crimea.Footnote ²⁸ As with the Treaty of Paris, however, this agreement proved to be fragile.

On February 22, 2014, the Euromaidan protests in Ukraine prompted pro-Kremlin President Viktor Yanukovych and many of his ministers to flee Kyiv. An interim government in favor of integration with the European Union replaced the Yanukovych regime and repealed the official status of the Russian language, sparking pro-Russian demonstrations in the south and east of Ukraine, including in Crimea.Footnote ²⁹ On February 27, armed soldiers operating without identifying markings on their fatigues seized control of the Crimean legislature and other government buildings, raising the Russian flag. Although Putin described these “little green men” as self-defense units created by residents of Crimea,Footnote ³⁰ they were later identified as Russian special forces.Footnote ³¹

Already hostile to the pro-EU government in Kyiv, and with yet-unidentified soldiers arriving in greater numbers daily to occupy airfields, ports, and other public venues, the Crimean provincial legislature organized a referendum on independence. The reported vote on March 16, 2014, was 95.5 percent in favor of seceding from Ukraine and joining Russia, with 83 percent turnout, though the process and its results were widely condemned.Footnote ³² On March 17, the provincial legislature declared the Republic of Crimea an independent nation, and on that same day renounced its independence and applied to join the Russian Federation.Footnote ³³ Putin immediately granted the request, purporting to annex Crimea into the Russian Federation.Footnote ³⁴

The next day, the European Union denounced Russia’s aggression and the Crimean referendum, declaring that “the sovereignty, territorial integrity and independence of Ukraine must be respected. The European Union does neither recognize the illegal and illegitimate referendum in Crimea nor its outcome. The European Union does not and will not recognize the annexation of Crimea and Sevastopol to the Russian Federation.”Footnote ³⁵

Russia was subsequently ousted from the Group of Eight, and both the Russian state and key national leaders faced economic sanctions.Footnote ³⁶ The UN General Assembly also rejected the referendum and affirmed “the sovereignty, political independence, unity and territorial integrity of Ukraine within its internationally recognized borders.”Footnote ³⁷

Russia’s aggression against Ukraine extended that same year to the eastern Donetsk and Luhansk provinces, known collectively as the Donbas. In March 2014, ethnic Russians in the Donbas launched protests in response to the Euromaidan protests and the ousting of Yanukovych. Armed separatist groups supported by Russia seized provincial legislatures and government buildings across the Donbas.Footnote ³⁸ Separatists in Donetsk proclaimed the Donetsk People’s Republic on April 7, 2014. Other insurgents in Luhansk declared the Luhansk People’s Republic on April 27.Footnote ³⁹ Ukraine replied with an “Anti-Terrorist Operation” that would eventually retake much of the territory that had been lost in the preceding months.Footnote ⁴⁰ Russian support for the separatists escalated. Russia deployed some 4,000 regular troops across the border, where they defeated Ukrainian forces in the Battle of Ilovaisk.Footnote ⁴¹

Shortly thereafter, the Organization for Security and Cooperation in Europe organized peace talks in Minsk, Belarus, leading to the two sides concluding the Minsk Protocol ceasefire agreement on September 5, 2014.Footnote ⁴² The agreement drew a demarcation line across roughly the eastern third of the Donbas, leaving control of the ceasefire territory (and the Ukraine–Russian border within it) with separatists and Russia.Footnote ⁴³ The agreement soon collapsed when Donbas separatists began a fresh offensive in January 2015.Footnote ⁴⁴ A subsequent and more comprehensive ceasefire agreement (Minsk II) was concluded on February 12, 2015, but sporadic fighting continued in the Donbas.Footnote ⁴⁵

Five years later, the twenty-ninth attempt at a “full ceasefire” came into effect on July 27, 2020.Footnote ⁴⁶ A period of relative calm followed the truce, with Ukraine reporting just twenty-five soldiers killed in the conflict zone in the first three months of 2021, compared with fifty the year before.Footnote ⁴⁷ This brief lull would prove to be the calm before the storm. The following year, Putin’s February 24 speech launched Russia’s full-scale invasion of Ukraine.

At the time of writing, the conflict in Ukraine has ground on for nearly three years with no end in sight. Although Ukraine managed to repulse Russia’s initial drive to take Kyiv,Footnote ⁴⁸ Russia has continued its offensive operations in Ukraine’s eastern and southern regions, seeking to extend its control from the Donbas to Crimea and beyond.Footnote ⁴⁹ The war has taken a heavy human toll. By May 2022, the UN High Commissioner for Refugees reported that more than six million people had fled Ukraine and a total of eight million had been displaced.Footnote ⁵⁰ The numbers of dead and wounded are contested, but on all accounts reach into the hundreds of thousands.Footnote ⁵¹ Scores of Ukraine’s cities, or large parts of them, have been reduced to rubble.

Two Models of Sovereignty

We have spent some time recounting Russia’s tumultuous history in Ukraine because it illuminates vividly two models of state sovereignty under international law. Russia’s annexation of Crimea under Catherine the Great and the subsequent prosecution of the Crimean War took place under what Oona Hathaway and Scott Shapiro have called the rules of the “Old World Order.”Footnote ⁵² The Old World Order accepted war as a permissible means for redressing wrongs and accorded sovereigns absolute and unreviewable authority to determine whether there was just cause for war.Footnote ⁵³ Hence, when Catherine first annexed Crimea, and when Nicholas I later battled the Ottomans and their allies in the Crimean War, the sovereigns involved had exclusive and absolute power to go to war, annex foreign lands, or sue for peace. Cooperation with allies was based on good will and a common purpose, while cooperation with the counterparties to a peace treaty was based on assent to the treaty and its terms. Properly understood, then, the legal permissibility of war and conquest under the Old World Order was not its foundational or organizing principle, but rather a consequence of the absolute, exclusive, and unilaterally enforceable sovereign power held by states.

Russia’s purported annexation of Crimea in 2014 suggests a commitment on Russia’s part to the same unilateralist understanding of its sovereign power.Footnote ⁵⁴ Russia intervened solely and unilaterally – amid widespread international condemnation – seizing control of public institutions to stage a faux referendum and annex Crimea to the Russian Federation. As noted above, however, both the UN General Assembly and the European Union have refused to recognize Russia’s attempted annexation. Moreover, the prevailing international legal context was one in which decisions over Crimea and access to her ports had to be decided by treaty between Ukraine and Russia, as they had been, for a time, with the 1997 Treaty of Friendship.Footnote ⁵⁵ It is noteworthy that Russia and her allies in Crimea went ahead with a referendum in 2014 which, even if in reality just a sham, suggests that Russia did not believe it could simply raise its flag over Crimea and expect the world to recognize its annexation. Although decision-makers in Moscow might have subscribed to a unilateralist understanding of sovereignty, they plainly knew that the international community did not. To receive broader international recognition, any settlement of the Crimean crisis would have to be achieved through free negotiation or third-party dispute settlement rather than military conquest or other unilateral coercion.

Much the same can be said of the Russian intervention in the Donbas between 2014 and 2022. Although fighting in the Donbas continued sporadically from 2014 onward, the parties negotiated and agreed to a host of admittedly short-lived ceasefire agreements. In other words, there was available all along an international legal framework to support a pacific end to the conflict. It is revealing that despite the outright wartime violence and the frequent collapse of ceasefires, the parties nonetheless continued to engage in regular negotiations. Such steady participation in peace talks during a time of active fighting reflected the parties’ recognition that the international community now regards negotiation and mediation – not armed conflict – as the preferred means to resolve even entrenched territorial disputes. With international law requiring peaceful dispute settlement, both Russia and Ukraine appreciated that they would suffer reputational harm if they refused to participate in peace negotiations, whatever their views of their sovereign prerogatives.

The 2022 invasion of Ukraine essentially enlarged and extended Russian operations that began in the Donbas and Crimea in 2014. Here, too, Russia’s invasion was undertaken as a unilateral exercise of force, reflecting Putin’s ostensible commitment to an absolutist and exclusive sovereign entitlement to go to war. Nonetheless, Russia has gone to great lengths to attempt to justify its invasion based on settled rules of international law. As noted, Putin claims that Russia is acting in self-defense under Article 51 of the UN Charter,Footnote ⁵⁶ alleging NATO encroachment and danger to Russia from Ukraine. He also justifies Russia’s action as humanitarian intervention on behalf of ethnic Russians in the Donbas. While Russia has reserved to itself the prerogative to interpret and apply international norms to its case (e.g., self-defense, humanitarian intervention), it at least pays lip service to those norms as general principles of international law. Russia thus implicitly acknowledges that there is an international legal order that requires states to justify the use of force on limited terms, and it seeks to justify its actions in accordance with those limited terms – including in the United Nations, where it continues to participate alongside other states. Additionally, Russia continues to engage in sponsored talks with Ukraine, discussing matters such as humanitarian corridors and prisoner exchanges. This too suggests a recognition on Russia’s part that the international community takes international legal order seriously, and that good faith dialogue and negotiation during times of crisis is part of what it means to be a state in good standing within that order. Thus, even as Russia’s invasion reflects a unilateralist assertion of power, flouting the UN Charter’s strict prohibition against aggression,Footnote ⁵⁷ its rhetoric demonstrates that in the public forum, at least, it accepts the prevalent international legal framework on the use of force, as well as the expectation that it must cooperate with other states to resolve the conflict in Ukraine by participating in UN meetings and peace talks.

The tension between Russia’s unilateral actions and its rhetorical commitment to multilateral norms and institutions reflects two competing understandings of sovereignty. Under one account, sovereignty confers exclusive authority on states to determine their own legal rights and obligations and entitles them to decide for themselves whether those rights and obligations have been violated. This voluntarist conception of sovereignty was a central organizing principle of the Old World Order, which authorized territorial conquest and enabled colonial rule. Some contemporary international legal materials – treaty law, customary law, judicial decisions, and scholarship – continue to presuppose a voluntarist account of sovereign equality. In contrast, the second account of sovereignty envisions states as equal participants within an international legal system that imposes limits on their exercise of sovereign rights and powers. According to this constitutionalist account, states are subject to certain positive obligations simply in virtue of their membership and participation in the international legal system, including obligations to resolve their disputes peacefully through good faith negotiation or third-party dispute resolution. Over the past century, the constitutionalist account of sovereignty has emerged as a serious rival to – and, in many respects, has eclipsed – the voluntarist account. Although both accounts endorse sovereign equality as a foundational and organizing principle of international law, they conceptualize the principle differently and draw different implications from it. This is because the two accounts are premised upon fundamentally different visions of international legal order, as we shall now explain.

Voluntarism

Hugo Grotius, the great Dutch international lawyer and diplomat, laid the groundwork for the voluntarist theory of sovereign equality in the early seventeenth century. For Grotius, states on the international stage are akin to individuals in the state of nature; in both contexts, there is no common public authority to offer protection and vindicate rights. However, Grotius did not think that the state of nature was a condition bereft of moral rights and duties. In the state of nature, Grotius claimed, individuals possessed natural rights and powers under the law of nature, including rights to life and bodily integrity.Footnote ⁵⁸ The law of nature likewise recognized in individuals a capacity to possess things rightfully, eventually as property, and to enter binding agreements.Footnote ⁵⁹ In addition to these substantive rights, individuals in the state of nature enjoyed enforcement powers to seek redress for wrongs and act in self-defense.Footnote ⁶⁰ Importantly, individuals under Grotius’s law of nature possessed these kinds of natural rights and powers equally.Footnote ⁶¹ They could also transfer these rights and powers to states through “collective agreement.”Footnote ⁶²

By analogizing states in international affairs to individuals in the state of nature, Grotius was well placed to model the law of nations on the law of nature.Footnote ⁶³ According to Grotius’s account, states enjoy sovereign equality in the limited sense that they have equal rights to territorial integrity and equal capacity to enter treaties with other states. They also possess equal enforcement rights exercisable through war to pursue redress for wrongs committed against them or their subjects (including unpaid debts and treaty breaches), self-defense, and punishment of transgressors of the law of nations (sometimes called “punitive war”).Footnote ⁶⁴ Crucially, under Grotius’s theory states alone were the arbiters of whether they had been wronged, imperiled, or whether the law of nations had been violated; that is, states were entitled to decide unilaterally whether there was just cause to go to war. Furthermore, when states went to war and a victor annexed territory or took spoils, Grotius’s solution to the legality of conquest or taking of booty was that might makes right; battlefield success determined and secured title.Footnote ⁶⁵ The main justification Grotius offered was that title to territory and goods taken by the victor had to be settled conclusively so as not “to lay a Foundation for a new War” to recover lost lands and chattels, potentially ad infinitum.Footnote ⁶⁶ In the absence of courts or other institutions capable of governing international affairs, Grotius filled the void with the equal sovereign powers of states to assess unilaterally the fitness of their causes, go to war as they deemed justified, and acquire good title to any territory they conquered or prizes they seized. Sovereign equality on Grotius’s model was thus fully consistent with an absolutist, voluntarist, and unilateralist understanding of state sovereignty under international law.

For centuries after Grotius, international publicists continued to analogize states in international relations to individuals in the state of nature.Footnote ⁶⁷ Reasoning from this premise, Emmerich de Vattel offered a classic statement of sovereign equality:

Significantly, Vattel disputed Grotius’s assertion that might makes right – “Strength or weakness … counts for nothing” – insisting that “a small Republic is no less a sovereign state than the most powerful Kingdom.”Footnote ⁶⁹ On Vattel’s view, therefore, no state was entitled to impose legal obligations on another state unilaterally.Footnote ⁷⁰ Nonetheless, Vattel agreed that because there were no international institutions in a position of authority to establish legal order among states, when disputes arose concerning the interpretation and application of international legal norms, states remained the ultimate arbiters of the legality of their actions.Footnote ⁷¹ This voluntarist conception of sovereign equality, which Vattel bequeathed to subsequent generations of international lawyers, gives pride of place to the thought that sovereigns exercise power without being subject to the authority of any other sovereign. As the Permanent Court of International Justice (PCIJ) put it in the 1923 Status of Eastern Carelia case, “[i]t is well-established in international law that no State can, without its consent, be compelled to submit its disputes with other States either to mediation or arbitration, or any other kind of pacific settlement.”Footnote ⁷² On this view, sovereigns are supreme within their jurisdiction and independent in their relations to other sovereigns. The voluntarist view captures some of modern international law’s prohibition on unilateralism in that no sovereign state is entitled to dictate terms to another, for such an entitlement would compromise the formal principle of sovereign equality. However, for voluntarists the independence of states also implies that “States have the right to be the judge in their own case” and that without their assent “they are subject to no court or tribunal for the interpretation of the rights conferred upon them by international law.”Footnote ⁷³ Thus, voluntarists view state consent as the sine qua non – the necessary and sufficient normative condition – of all international obligations.Footnote ⁷⁴

Voluntarists often invoke The Lotus, a 1927 judgment of the PCIJ, as their doctrinal touchstone.Footnote ⁷⁵ In a passage of the judgment that has been endlessly debated, a narrow majority of the Court opined that all “rules of law binding upon States … emanate from their own free will as expressed in conventions or by usages generally accepted as expressing principles of law … Restrictions upon the independence of States cannot therefore be presumed.”Footnote ⁷⁶ According to the Court, states enjoy “a wide measure of discretion, which is only limited in certain cases by prohibitive rules; as regards other cases, every State remains free to adopt the principles which it regards as best and most suitable.”Footnote ⁷⁷ From these broad pronouncements, voluntarists have extrapolated a general principle that “whatever is not explicitly prohibited by international law is permitted.”Footnote ⁷⁸ Thus, Prosper Weil celebrates The Lotus for endorsing a “voluntarist conception of international law” under which states “are bound by a rule of law only because, and to the extent that, they have consented to it. Freedom to act remains the basic principle.”Footnote ⁷⁹

There are significant conceptual problems with this account of international legal order.Footnote ⁸⁰ Recall that an attraction of the theory is that it purports to subscribe to international law’s prohibition on unilateralism under which no state can impose terms on another. However, because states are subject only to legal rules or norms to which they consent, it follows that consent is the only means of establishing a rule or norm under international law. Recalcitrant and nonconsenting states therefore would find themselves subject to few, if any, international legal obligations. They would find themselves effectively outside international legal order, and with a virtually unlimited liberty-right of action – the absolute discretion of the uncommanded commander. Yet, by ceding to nonconsenting states a potentially unlimited liberty-right of action, voluntarism gives these states the liberty-right to decide for themselves the terms on which they will interact with others, regardless of whether the counterparty consents. If a recalcitrant state refuses to agree to fair and equal terms, voluntarism lacks the resources to censure that state from a legal point of view. Under voluntarism, the intractable state will have breached no legal rule to which it was subject.

Our point is not to equate voluntarism with unilateralism, but rather to explain why voluntarism, taken seriously, may permit or collapse into unilateralism within the conception of international legal order it endorses. During the colonial era, voluntarism emboldened states from the Global North to exploit and dominate other political communities for their own gain. In the absence of “prohibitive rules” affirmatively outlawing territorial conquest and colonialism, states used war as an instrument of national policy and built overseas empires on the backs of foreign peoples.Footnote ⁸¹ While international law no longer permits territorial conquest and colonialism, the voluntarist thinking that enabled those practices continues to drive the idea that states are subject only to legal requirements they have affirmatively endorsed. Voluntarism also lends credence to the idea that states are entitled to judge for themselves whether their conduct is consistent with their obligations under international law. Rather than resolve disputes over the meaning or application of international law through dialogue, negotiation, or third-party dispute resolution, voluntarism suggests that if states have not actively opted in to international cooperation, they are free to go it alone. It is this aspect of voluntarism – the notion that states are entitled to judge the legality of their own actions – that Putin has brandished as both sword and shield to rationalize Russia’s aggression against Ukraine and deflect international criticism.

Whatever merit the voluntarist approach may have had in the age of Grotius and Vattel, it no longer accurately describes international legal order in the twenty-first century. Voluntarism stands in tension with international law’s contemporary understanding of sovereign equality because sovereign equality, we explain now, denotes equal sovereign authority and a formal right to independence from foreign intervention and domination. As formal equals under international law, states are subject to reciprocal duties that safeguard their equal independence. These duties apply to all states, regardless of whether they have affirmatively accepted them via treaty or customary international law.

Constitutionalism

During the same period when the PCIJ was endorsing voluntarism in The Lotus, an alternative account of sovereignty was emerging based on the idea that states are members of an international legal order that constrains how they may exercise their rights and powers. In an influential 1920 monograph titled The Equality of States in International Law, Professor Edwin DeWitt Dickinson derived sovereign equality from the formal international legal personality of states.Footnote ⁸² Dickinson’s conception of sovereign equality was formal, rather than substantive, in that it focused on states’ legal status and standing rather than, say, the allocation of military power or material resources. This formal account of sovereign equality had important substantive implications. Like Grotius and Vattel, Dickinson accepted that states were entitled to participate as equals in establishing international law by concluding treaties and contributing to the formation of customary international law. But Dickinson disagreed that each state was entitled to decide for itself whether its conduct satisfied international law. “International persons are equal before the law when they are equally protected in the enjoyment of their rights and equally compelled to fulfill their obligations,” Dickinson observed.Footnote ⁸³ To respect the principle of sovereign equality, in other words, states must exercise their rights and powers in a way that respects the rights and powers of other states. And when disputes arise, states are entitled to be treated by their peers as formal equals under the rule of law. Accordingly, no state is entitled to hold itself out as the final arbiter of the international legality of its own actions.Footnote ⁸⁴

Implicit in this formal account of sovereign equality is the idea that states interact with one another within an international legal system, not a state of nature. Dickinson observed that “equality before the law is essential to any legal system,” and this principle held special significance for international legal order as “the necessary consequence of the denial [to states] of universal empire, and of the claim of separate states to live together in an international society controlled by law.”Footnote ⁸⁵ The alternatives to equality before the law for separate states, Dickinson warned, were “universal empire or universal anarchy.”Footnote ⁸⁶ Whereas Grotius and Vattel believed that states could judge the legality of their own actions for themselves, Dickinson anticipated that the legitimacy of international legal order would depend on international institutions playing an active role in interpreting international law and resolving international disputes.

The idea that states could look to international institutions for assistance in resolving their disputes had become plausible by the 1920s, when Dickinson published his monograph. A generation earlier, during the Hague Peace Conference of 1899, the international community had established the first Permanent Court of Arbitration (PCA) to handle interstate disputes. Argentina’s delegate to the Conference celebrated this development as ensuring “that the political equality of states has ceased to be a fiction and that it abides, established as an obvious reality.”Footnote ⁸⁷ The PCA would be followed by the creation of the League of Nations in 1920, and the establishment of the PCIJ, which held its inaugural sitting in 1922. Hence, by the time the PCIJ decided The Lotus five years later, the majority’s vision of international relations as a state of nature wherein states could generally do as they please was rapidly falling out of favor. The following year witnessed the signing of the Kellogg-Briand Pact, also known as the Pact of Paris, which outlawed “recourse to war for the solution of international controversies” and committed states to settle their disputes exclusively “by pacific means.”Footnote ⁸⁸ With the obligation to resolve disputes peacefully and the establishment of international courts and tribunals like the PCA and the PCIJ, states became equal members of an international legal system in which they were no longer free to rely solely on their own independent judgment to determine what international law required of them. By the end of World War II, Dickinson’s account of sovereign equality had become the prevailing wisdom among international publicists.Footnote ⁸⁹

Some members of the Lotus Court anticipated and embraced this paradigm shift. The dissenting opinion of Judge Bernard Loder offers an especially clear account. Judge Loder held that states, as equal participants in the international legal system, relate to each other on terms of “mutual independence.”Footnote ⁹⁰ For states to be sovereign, they must be independent, and for them all to be independent, they must be independent mutually or equally. At the heart of Loder’s opinion, then, is the constitutional insight that universal independence in a community of formally equal states implies mutual or equal independence. If independence is a necessary condition of sovereignty, then sovereign equality must secure states’ independence on equal terms. One sense in which all sovereigns are equal is that none are entitled to dictate terms to others unilaterally, and conversely, none can be made to suffer such terms. Under international law, sovereign states are independent within their respective spheres of exclusive jurisdiction: none depends on the permission of another to exercise domestic or international public functions.Footnote ⁹¹ Thus, as Judge Charles Andre Weiss observed in a separate dissenting opinion, international law serves “to harmonize and reconcile the different sovereignties over which it exercises sway.”Footnote ⁹²

On this constitutional reading of sovereign equality, international law both empowers states as international legal actors and constrains them in their dealings with other states and nonstate actors. There is now a considerable literature on the constitutionalization of international law, much of which emphasizes how international law shares with national law certain core commitments to human rights, self-determination, and the rule of law.Footnote ⁹³ The constitutional perspective offers a congenial framework within which international law empowers states by recognizing them as the de jure governors of their people at the domestic level and as the de jure representatives of their people in international affairs. International recognition empowers states to establish political and legal order nationally, as well as to enter binding treaties and otherwise represent their people internationally. Crucially, the constitutional perspective foregrounds international legal norms that are constitutive of international legal order as a legal system. In The Lotus, Judge Weiss observed that among “these rules there is one which is paramount and which does not even require to be embodied in a treaty: that is the rule sanctioning the sovereignty of States,” without which “no international law would be possible.”Footnote ⁹⁴ This “paramount” rule of state sovereignty is defined, in part, by the principle of sovereign equality, as affirmed in the UN Charter and other authoritative instruments.Footnote ⁹⁵ Thus, the principle of sovereign equality has become a constitutional norm of international law, subjecting states to reciprocal limits that reflect their equal status and secure their mutual independence vis-à-vis one another.

Two Threats to Sovereign Equality

The mutual independence that states enjoy under international law constitutes a form of freedom or liberty. On our view, the liberty of independence has two constitutive elements: a principle of noninterference and a principle of nondomination.Footnote ⁹⁶ The principle of noninterference prohibits states from wrongfully interfering with each other through any unilateral act that infringes another state’s legal interests or authority. International law’s principle of nonintervention can be seen as a salient legal expression of the normative principle of noninterference. The most general formulation of this legal principle in the UN Declaration on Non-Interference provides that “[n]o state has the right to intervene, directly or indirectly, for any reason whatever, in the internal or external affairs of any other State.”Footnote ⁹⁷ Consistent with the UN Charter,Footnote ⁹⁸ the Declaration specifically decries “armed intervention” as a prohibited form of conduct, as well as “the use of force to deprive peoples of their national identity” and efforts “directed towards the violent overthrow of the regime of another State.”Footnote ⁹⁹ Military aggression, such as Russia’s invasion of Ukraine, is an especially striking example of wrongful interference, since one state violates the territorial integrity of another through the unilateral use of force.

The Declaration makes clear, however, that the principle of nonintervention is broader than the principle of noninterference. Specifically, the Declaration avows that threats of unilateral acts – and not just unilateral acts themselves – are legally wrongful. The Declaration stipulates that “attempted threats against the personality of the State or against its political, economic and cultural elements are condemned.”Footnote ¹⁰⁰ States may not “use or encourage the use of economic, political or any other type of measures to coerce another State in order to obtain from it the subordination of the exercise of its sovereign rights or to secure from it advantages of any kind.”Footnote ¹⁰¹ Because the principle of noninterference only explains how actual and wrongful unilateral acts compromise freedom, it cannot explain how unrealized threats of those acts compromise freedom.Footnote ¹⁰² For an account of the wrongfulness of threats and their infringement of liberty, we need the principle of nondomination.

For republicans, domination consists in subjection to the arbitrary will of another and is the chief cause of unfreedom.Footnote ¹⁰³ In the classic example of the enslaved person and the benevolent master, republicanism invites us to imagine a master who wishes only the best for the person enslaved. She treats him like family, supports his hopes and aspirations, encourages his projects, and so on. In short, the master does not interfere with, but rather supports, the enslaved person and his choices. Nonetheless, the individual enslaved is still unfree, republicans assert, because the master is entitled to crack the whip at any time, for any reason (including malicious or spiteful reasons), and to do so with impunity. Republicans point out that this asymmetrical power structure ordinarily leads to bowing, scraping, and a constant sense of unease (if not terror) on the part of the dominated party.Footnote ¹⁰⁴ But the distinct evil of domination – the evil and unfreedom suffered by the enslaved person of the benevolent master – is simply subjection to the looming threat of arbitrary force against which there is no recourse.

When the Declaration on Non-Intervention prohibits “threats” against a state’s “personality” or against its “political, economic, and cultural elements,” the Declaration puts into positive legal form the principle of nondomination. A state’s personality is the law-created artificial entity to which the words and actions of the state’s representatives are attributed when they act in the name of the state.Footnote ¹⁰⁵ Consequently, certain threats against the state’s personality may constitute existential threats. The threat of military aggression and occupation is this kind of threat. And threats against a state’s “political, economic, and cultural elements” are likewise threats of wrongful unilateralism. A powerful state may threaten valued public goods in order to impose terms on a weaker state – for example, by threatening to cut off the weaker state’s access to fresh water or natural gas if the weaker state does not capitulate to its demands. Notably, Russia used these tactics in 2022, seeking to deter European countries from opposing its aggression against Ukraine.Footnote ¹⁰⁶ Such threats are arbitrary in the sense that they aim to secure through a unilateral assertion of will the “subordination” of another state or the gaining of “advantages” to which the dominating state has no entitlement.

These features of the Declaration on Non-Intervention reflect a paradigm shift that reshaped international legal order during the middle decades of the twentieth century, one driven in large part by global pressures for decolonization. Throughout the colonial era, international publicists offered pious tributes to the principle of sovereign equality as all the while European nations declined to recognize other peoples around the world as equal sovereigns. As Edward Keene has observed, this approach led to a bifurcated international legal order:

By the early twentieth century, this division of the world into fully sovereign states, on the one hand, and “semi-sovereign” states and colonial possessions, on the other, had become conventional wisdom among Western legal scholars, cementing as a formal legal hierarchy the practical subordination of non-Western peoples.Footnote ¹⁰⁸

The decolonization movement sought to dismantle these features of the colonial era – not only by extending formally equal statehood to former colonies but also by outlawing foreign intervention and domination wherever and however they might manifest in international relations.Footnote ¹⁰⁹ In the celebrated 1960 Declaration on the Granting of Independence to Colonial Countries and Peoples (Declaration on Colonial Independence), the UN General Assembly solemnly declared that “[t]he subjection of peoples to alien subjugation, domination, and exploitation constitutes a denial of fundamental human rights, is contrary to the Charter of the United Nations and is an impediment to the promotion of world peace and co-operation.”Footnote ¹¹⁰ Hence, states must “observe faithfully and strictly” their obligations under the UN Charter to respect the principles of self-determination and nonintervention “on the basis of equality, non-interference in the internal affairs of all States, and respect for the sovereign rights of all peoples and their territorial integrity.”Footnote ¹¹¹ A decade later, the General Assembly reaffirmed these principles in its unanimous Declaration on Principles of International Law concerning Friendly Relations and Cooperation among States in accordance with the Charter of the United Nations (Friendly Relations Declaration):

1. (a) To promote friendly relations and co-operation among States; and

2. (b) To bring a speedy end to colonialism, having due regard to the freely expressed will of the peoples concerned; and bearing in mind that subjection of peoples to alien subjugation, domination and exploitation constitutes a violation of the principle, as well as a denial of fundamental human rights, and is contrary to the Charter.Footnote ¹¹²

The Declaration on Colonial Independence and the Friendly Relations Declaration underscore that international intervention and domination violate two bedrock norms of contemporary international law. First, intervention and domination are incompatible with the principle of sovereign equality because they subordinate a state’s sovereign authority to another’s unilateral will. A state that is subject to foreign coercion does not enjoy the mutual independence of sovereign equality. Second, intervention and domination violate the right to self-determination – a people’s exclusive privilege to choose the laws and institutions by which it is governed.Footnote ¹¹³ Whenever a state intervenes unilaterally in another state’s exclusive territorial jurisdiction, or threatens to do so, it denies a foreign people their legal privilege to decide for themselves how they will arrange their internal affairs. It consigns them to live under “alien subjugation, domination and exploitation.”Footnote ¹¹⁴ By prohibiting foreign intervention and domination, contemporary international law prohibits powerful states from reviving territorial conquest and other features of colonial-era unilateralism. Thus, sovereign equality and self-determination complement one another as constitutional foundations for a postcolonial international legal order.

Mandatory Cooperation

By preserving domains within which states are mutually independent, international law aims to safeguard sovereign equality and facilitate national self-determination.Footnote ¹¹⁵ During the colonial era, the close connection between independence and sovereign equality led some voluntarists to conflate the two concepts, suggesting that they were synonymous.Footnote ¹¹⁶ However, in some contexts, mutual independence is not enough to support a constitutional approach to sovereign equality. The ideal of mutual independence presupposes that state jurisdiction is settled and mutually exclusive, but this premise does not always hold true. Sometimes, state jurisdiction is unsettled, concurrent, or shared jointly. At other times, states hold rights and powers that are in tension with one another, raising the possibility that the exercise of a right or power by one state would adversely impact another state’s legal entitlements. In these cases, a state cannot act unilaterally without violating the principle of sovereign equality because any steps it takes on its own with respect to such matters would effectively subordinate another state’s legal rights or authority to its policy choices and decisions.

Consider the following scenarios:

• Unsettled Jurisdiction – Two neighboring states disagree in good faith over the location of their common border. An important function of international borders is to define the territories within which states are entitled to claim mutual independence. Nonetheless, as long as the location of a border remains unsettled, it may be unclear which state has jurisdiction over the disputed territory. Both states may view the other’s territorial claim as wrongful interference with their sovereignty. Without an international legal framework in place to address the dispute, each state may plausibly claim to suffer domination and interference from the other’s unilateral assertion of sovereign power.

• Concurrent Jurisdiction – A large oil field straddles the maritime boundary between two states. Each state in the exercise of its territorial jurisdiction is entitled to determine how much oil it will extract from the field. However, due to the unitary nature of the field, any oil extracted on one side of the boundary will reduce the volume available on the other side. In this scenario, the sovereign interests of the neighboring states are inextricably connected. Neither can drill for oil without adversely affecting the other’s sovereign interests, and either could dominate the other simply by threatening to drill unilaterally.

• Jointly Shared Jurisdiction – An international river flows through multiple states on route to the sea. The states concerned conclude a treaty designating the river as a shared resource subject to their joint stewardship and collective administration. Having placed the river under their joint jurisdiction in this manner, any unilateral regulation, use, diversion, or adulteration of the river by a single state would undermine the other states’ equal rights and authority under the collective regime.

• Rights Conflicts – An organized crime group in State A counterfeits the national currency of State B. International law obligates State A to exercise due diligence to suppress counterfeiting.Footnote ¹¹⁷ However, State A is unable to eradicate the counterfeiting operation without foreign assistance due to the organized crime group’s power. If State B offered to dispatch police to State A to disrupt the counterfeiting operation, State A might object, invoking its territorial sovereignty as a basis for withholding consent to foreign intervention. However, by exercising its sovereign right to exclude in this way, State A would undermine State B’s sovereign authority over its own currency.

At first glance, these scenarios might seem like exceptional cases at the periphery of international legal order. Yet nothing could be further from the truth. As we show in subsequent chapters, states frequently encounter circumstances where their sovereign rights and powers are unsettled, concurrent, shared jointly, or in tension with one another. States rely on international law “to harmonize and reconcile” their “different sovereignties” in these cases of sovereign interdependence, just as in settings where their sovereignties are mutually independent.Footnote ¹¹⁸ International law performs this service, for example, when international disputes arise over transboundary rivers, oceanic fisheries, and maritime delimitation. Other relevant disputes include controversies about transboundary pollution, global cartels, transnational terrorism, piracy, cybercrime, and atrocity crimes. The jurisprudence of international courts and tribunals is replete with cases in which states have tangled over unsettled jurisdiction, concurrent jurisdiction, jointly shared jurisdiction, or conflicting legal entitlements. An inventory of these cases reads like a “greatest hits” collection of international case law, including the Alabama Arbitration,Footnote ¹¹⁹ the Trail Smelter Case,Footnote ¹²⁰ the River Oder Case,Footnote ¹²¹ the Lake Lanoux Arbitration,Footnote ¹²² the Corfu Channel Case,Footnote ¹²³ the Fisheries Jurisdiction Cases,Footnote ¹²⁴ the North Sea Continental Shelf Cases,Footnote ¹²⁵ and the Bosnian Genocide Case.Footnote ¹²⁶ Far from being outlier exceptions to the general rule of mutually independent sovereignty, these cases involving unsettled jurisdiction, concurrent jurisdiction, jointly shared jurisdiction, and conflicting rights are ubiquitous in international legal practice. Thus, addressing the challenges these cases pose to sovereign equality has become a major preoccupation of international law.

The voluntarist and constitutionalist conceptions of sovereign equality respond to these challenges in different ways. Under the voluntarist approach, states are free to avoid the norms of mandatory cooperation arising from nonexclusive forms of jurisdiction and rights conflicts, even at the expense of dominating their neighbors’ sovereign interests. In practice, this gives great powers like China, the European Union, Russia, and the United States license to exercise their sovereign rights and powers as they see fit, subject only to international law’s limited set of prohibitive rules. By allowing more powerful states to dictate the terms of their relationships with foreign nations in this manner, voluntarism exposes foreign peoples to “alien subjugation, domination and exploitation,” arguably in violation of the right to self-determination.Footnote ¹²⁷ In this respect, the voluntarist account of sovereign equality is a recipe for great power domination.

The constitutionalist approach, in contrast, holds that when sovereign rights and powers clash or the equitable use of a sovereign entitlement is otherwise in question, no state may claim the prerogative to resolve the matter on its own because doing so would dominate other states by threatening to subject them to its unilateral will. Even if a state acted in perfectly good faith, imposing a solution that favored its neighbors, its unilateral decision-making would undermine sovereign equality by arrogating to itself a prerogative and standing that is not held by others. The constitutional approach to sovereign equality therefore requires that states refrain from dictating the terms of their interactions with other states unilaterally. Instead, they must resolve international disputes cooperatively through dialogue, negotiation, or third-party dispute resolution. A simple refusal to negotiate with other states on a matter that calls for multilateral cooperation would undermine sovereign equality because it would frustrate the authority of other states to negotiate fair terms of cooperation jointly for all concerned.Footnote ¹²⁸ Hence, states must cooperate with one another to resolve disagreements about, among other things, the location of shared land bordersFootnote ¹²⁹ and the coordinated exploitation of border-straddling oil fields.Footnote ¹³⁰ The tensions inherent in unsettled jurisdiction, concurrent jurisdiction, jointly shared jurisdiction, and conflicting rights can be resolved only through international cooperation. Constitutionalism thus embraces mandatory cooperation as the institutional framework for safeguarding sovereign equality when states operate outside their exclusive jurisdiction or when the equity of an otherwise ordinary use of sovereign power is in question.

Substantive Obligations – Mandatory cooperation requires states to pursue equitable solutions to international controversies with an eye to balancing their own legal rights and authority, on the one hand, with the legal rights and authority of other individual states and the international community collectively, on the other. International treaties and judicial decisions refer to this as an equitable requirement that states accord “due regard” to other states’ sovereign rights and authority.Footnote ¹³¹ Due regard does not demand “total or maximum regard” for foreign interests, but rather “reasonable, appropriate, optimal regard.”Footnote ¹³² States must pursue “an objectively positive interaction with another [state]’s interests and with those of the international community”Footnote ¹³³ for the purpose “of reconciling them in as equitable a manner as possible.”Footnote ¹³⁴ Mandatory cooperation thus aims to harmonize conflicting sovereign rights and powers through a proportionality analysis that equitably balances contending national interests.Footnote ¹³⁵

The due regard standard is often linked to other legal concepts, such as “abuse of right,” signaling that states must avoid exercising their rights in a manner that causes arbitrary or disproportionate injury to others’ rights.Footnote ¹³⁶ An abuse of right is acting within the scope of a right but in a way that produces manifest inequity.Footnote ¹³⁷ Accordingly, in contexts where states’ rights and powers are unsettled or enmeshed, the constitutional approach suggests that these rights and powers must be porous to equitable principles and peaceful dispute resolution to prevent an abuse of right. Thus, mandatory cooperation ensures that ordinary exercises of sovereign power and assertions of sovereign rights are equitable rather than abusive.

Procedural Obligations – Mandatory cooperation also imposes procedural obligations. Within mandatory cooperation’s domain, states must investigate,Footnote ¹³⁸ consult,Footnote ¹³⁹ and negotiate with one another in good faith.Footnote ¹⁴⁰ The duty of good faith negotiation demands initiative and sustained commitment.Footnote ¹⁴¹ Good faith negotiation “entail[s] more than the plain opposition of legal views or interests between two parties, or the existence of a series of accusations and rebuttals, or even the exchange of claims and directly opposed counter-claims.”Footnote ¹⁴² Instead, negotiations must be “meaningful” in the sense that each side demonstrates a willingness to pursue agreement, not merely to go through the motions.Footnote ¹⁴³ Good faith negotiation also requires openness to “compromise, even if that mean[s] the relinquishment of strongly held positions earlier taken. It implies a willingness for the purpose of negotiation to abandon earlier positions and to meet the other side part way.”Footnote ¹⁴⁴ Hence, the idea that one side would “insist upon the complete capitulation of the other side” is “inconsistent with … ‘negotiation.’”Footnote ¹⁴⁵ States must undertake a “persevering quest for an acceptable compromise,”Footnote ¹⁴⁶ demonstrating that they are “internally concerned with balancing the [relevant] rights and interests,” rather than simply going through the motions.Footnote ¹⁴⁷

In the end, however, the requirement of good faith negotiation does not obligate states to actually conclude an agreement resolving their disputes.Footnote ¹⁴⁸ Mandatory cooperation imposes obligations of conduct, not result.Footnote ¹⁴⁹ Sometimes states disagree in perfectly good faith about the terms of an equitable solution. To require states to accept an agreement that they genuinely believe is inequitable would subject them to the unilateral will of the other side, which is inconsistent with sovereign equality and joint stewardship. Accordingly, mandatory cooperation does not require states to conclude agreements, provided they persevere in pursuing an equitable resolution in good faith.Footnote ¹⁵⁰

International courts and tribunals have been reluctant to find bad faith absent “clear and convincing evidence” that a state seeks “to prevent any reasonable agreement.”Footnote ¹⁵¹ Bad faith might be established if a state arbitrarily breaks off or delays negotiations, declines to follow established procedures for negotiation, or refuses to consider counter-proposals or relevant equitable considerations.Footnote ¹⁵² Under any of these circumstances, a state’s intransigent behavior would reflect unilateralism through covert means, which in turn would infringe sovereign equality.

If negotiations stall, states must pursue mediation, arbitration, adjudication, or some other form of third-party dispute resolution. Support for this principle can be found in Article 33 of the UN Charter, which charges states with resolving their disputes through peaceful means.Footnote ¹⁵³ The General Assembly’s Friendly Relations Declaration likewise describes the pursuit of peaceful third-party dispute resolution as an obligation incumbent upon all states.Footnote ¹⁵⁴ Third-party dispute resolution promotes sovereign equality under the constitutional approach by ensuring that states do not appoint themselves judge and party to the same cause. In cases where states are unable to agree on a negotiated solution or a forum for arbitration, they must maintain a continuous dialogue and refrain from taking steps that would prejudice the outcome of negotiations.

These procedural duties are explained by international law’s commitment to sovereign equality: in settings where states are not mutually independent, they can harmonize and reconcile their respective rights and powers only through negotiations, mediation, democratic procedures (e.g., fair voting), or by submitting to third-party dispute resolution. Mandatory cooperation ensures that states have an opportunity to participate and have their preferences taken into account in decisions that impact their legal interests.Footnote ¹⁵⁵ It also attends to the collective right to self-determination by ensuring that states resolve their disputes in a manner that reflects the collective will of the peoples concerned.Footnote ¹⁵⁶

Mandatory cooperation suggests that international legal order is not concerned merely with establishing minimal terms of co-existence among independent states.Footnote ¹⁵⁷ Instead, international law requires states to embrace international cooperation, pooling their sovereign authority to develop common norms and institutions, in settings where their respective sovereign rights and authority are unsettled or otherwise entangled. These features of the constitutional approach to sovereign equality are well-suited to our current era of global interconnectivity and interdependence, when activities occurring in one country routinely reverberate across national borders to affect people, markets, ecosystems, and legal and political systems around the world.Footnote ¹⁵⁸

Roadmap

In subsequent chapters, we explore how mandatory cooperation has been incorporated into a wide range of fields of international law. We shall see that mandatory cooperation finds expression in numerous international treaties and norms of customary international law. It also features prominently in dozens of decisions from national and international courts, including judgments of the ICJ and arbitral tribunals convened under the PCA. Our methodology throughout the book is mostly descriptive, inasmuch as it begins with the existing practice and self-understanding of international law. But our method is also interpretive, since we use the organizing principle of sovereign equality to explain the extant legal norms of mandatory cooperation. Collectively, the case studies we develop in this book chronicle how the constitutional approach to sovereign equality is gradually eclipsing (but has not yet fully eclipsed) the older voluntarist approach.

The remainder of this book is divided into three parts. The first two parts use case studies to characterize and assess international law’s deployment of mandatory cooperation in contexts where the practice is reasonably well entrenched, while the last part will explore contexts where mandatory cooperation is less well developed.

In Part I – Conflicting Sovereignties – we consider cases of overlapping rights, shared sovereign powers, or unsettled jurisdiction to explain why sovereign equality requires states to resolve disputes through international cooperation. In Chapter 2, we begin by examining how international law governing rivers has established a requirement that upstream and downstream sovereigns must consult and negotiate in good faith to determine mutually satisfactory solutions for the shared use of rivers. Chapter 3 shows how the international law of the sea moved away from state unilateralism in favor of the equitable model of sovereignty by requiring states to resolve disputes over international fisheries and maritime boundaries through cooperation in accordance with equitable principles. Chapter 4 considers how duties of international cooperation safeguard sovereign equality by reconciling the territorial sovereignty of coastal states with landlocked states’ rights to access the oceans through negotiation or binding arbitration. Chapter 5 demonstrates that states have accepted obligations of mandatory cooperation with respect to a variety of other transboundary harms, including piracy, terrorism, and at least some cyberattacks. Chapter 6 shows that international environmental law embraces an equitable approach whereby states must cooperate to suppress transboundary pollution. In both Chapter 5 and Chapter 6, we argue that states where transboundary harms originate bear legal obligations to cooperate with injured states to suppress threats from nonstate actors by taking effective measures to prevent or suppress those threats.

In Part II – Collective Stewardship – we show that cooperation is required when international law commits authority to groups of states for the collective stewardship of the international community as a whole or a matter of common concern. In Chapter 7, we examine climate change as a transnational and existential threat to humanity generally, and to certain smaller and vulnerable states most dramatically. Since it poses an existential threat to low-lying coastal states and raises the frequency and intensity of extreme weather events, we make the case that climate change should be understood to trigger duties of international cooperation. Chapter 8 considers international territorial administration through multilateral governance structures, devoting special attention to Antarctica as a global res communis for peaceful exploration and scientific study. In Chapter 9, we explain how mandatory cooperation applies to the international community’s collective responsibility to safeguard international peace, security, and human rights. With the establishment of the United Nations, the Grotian paradigm of states deciding when to use force for themselves shifted to a collective stewardship model whereby states must cooperate with one another to avoid armed conflict, punish atrocity crimes, and combat global poverty, including through collective consultation, negotiation, and peaceful dispute resolution.

Part III – Future Directions – begins with Chapter 10, where we examine whether states bear duties of international cooperation with respect to forced migration, including the mass displacement caused by Myanmar’s campaign of ethnic cleansing against its Rohingya Muslim minority. We make the case that the international community has accepted the protection of forced migrants as a common concern of humanity under the community’s joint stewardship. Although states have not come close to satisfying this collective responsibility, the system of national pledges established under the 2018 Global Compact for Safe, Orderly and Regular MigrationFootnote ¹⁵⁹ and the Global Compact on RefugeesFootnote ¹⁶⁰ represent substantial (albeit nonbinding) measures in the direction of fulfilling states’ nascent duties of mandatory cooperation. Chapter 11 uses antitrust law as a case study to explore the phenomenon of extraterritorial regulation. Over the past several decades, many states have extended their domestic competition laws extraterritorially, subjecting foreign actors outside their borders to the concurrent regulation of multiple states. We argue in Chapter 11 that the principle of sovereign equality requires states to resolve disputes involving fields of concurrent prescriptive jurisdiction through mandatory cooperation. Chapter 12 offers a provocative and jurisprudentially ambitious argument: that sovereign equality requires states to submit to international adjudication or arbitration even in ordinary legal disputes that do not involve overlapping sovereign rights or powers. Drawing on the ICJ’s groundbreaking judgment in the Case Concerning Military and Paramilitary Activities in and Against Nicaragua,Footnote ¹⁶¹ we argue that even if international law permits states to opt out of the jurisdiction of a particular court, in practice the ICJ interprets that entitlement restrictively, and the principle of sovereign equality prohibits states from abandoning third-party dispute resolution entirely. Finally, Chapter 13 concludes by recapping the book’s key themes, considering potential obstacles to mandatory cooperation, and identifying other matters of international concern, such as pandemics, that are good candidates for mandatory cooperation under the equitable conception of sovereign equality.

While this chapter began with a story of international cooperation in retreat – the ongoing calamity in Ukraine, where the failure of cooperation is being paid in blood – we shall see that in many other contexts states’ obligations of mandatory cooperation are taken seriously and have attained the status of customary international law, even where their initial source is treaty-based (e.g., international watercourses, maritime delimitation, the duty to resolve disputes using peaceful means). Even in an era when multilateralism faces serious setbacks under assault from nationalist movements, fueling pessimism about the future of international cooperation, the fact remains that international law continues to thrive across many fields. International treaties and the judgments of international courts and tribunals continue to affirm that states must cooperate to address a host of common challenges. Mandatory cooperation is the legal framework that enables states to make policy together as equals, as joint stewards of both their people and humanity.