How should international lawyers understand the modern treaty? For Georg Nolte, a professor at Humboldt University and member of the UN International Law Commission, the Greek adventure of Ulysses and the Sirens provides one answer. Ulysses wanted to hear the Sirens’ bewitching songs while avoiding the fate of other sailors enchanted to follow that music to their deaths. Ulysses's companions filled their ears with beeswax (so they could not hear the Sirens) and tied Ulysses to the mast, agreeing to bind him tighter should he entreat them for release. The Sirens’ songs, of course, led Ulysses to do just that. Yet, the crew kept rowing, only releasing Ulysses after he could no longer hear the Sirens’ call. For Nolte, Ulysses's need for “ties that he cannot remove in a specific situation” (p. 32) exemplifies the treaty's basic purposes—to commit parties to “certain conduct, largely regardless of what will happen and whether they will want to change their minds” (p. 31).Footnote ¹

What makes Nolte's retelling particularly compelling is how he characterizes the “ties” that bind Ulysses. They are not simply the bonds holding Ulysses to the mast, but also “the agreement” with the crew. It is ultimately the crew's practice under that agreement—they “kept on rowing to keep Ulysses clear of the coast of the Sirens” (p. 237)—that saves Ulysses's life. And therein lies the central thrust of Nolte's work—an invitation to appreciate the value that “practice” provides in evaluating modern treaties.

Drawn from Nolte's 2018 specialized course at The Hague Academy of International Law, Treaties and Their Practice – Symptoms of Their Rise or Decline is a concise highly readable paean to treaty practice. Defining practice in broad terms—“all forms of conduct which may be relevant for the interpretation or the state of treaties” (p. 20)—Nolte emphasizes practice as a diagnostic tool. For Nolte, treaty practice is symptomatic—it is a sign of the state of a treaty. Tracking such symptoms over time, international lawyers can gauge parties’ relative commitment to a treaty, asking if it is on the “rise” or in “decline.”

Nolte applies his symptomatic study to five areas: (1) international peace and security; (2) human rights; (3) trade; (4) investment; and (5) the environment (chiefly, climate change). He identifies a common paradigm for some (but not all) of these treaties: “the establishment of basic rules after the Second World War, a blossoming of treaties during the 1990s, and signs of crisis, and perhaps even decline, after the turn of the century” (p. 160). Acknowledging that the post-World War II multilateral treaty system could collapse, Nolte pronounces that outcome “unlikely” (p. 235), rebuffing a “doomsday mood” as premature (pp. 236–37). For Nolte, the ties of treaties still bind and international lawyers, like Ulysses's crew, must keep rowing, interpreting treaties knowing that “history does not proceed in a linear fashion, and that it can make surprising turns, including for the better” (p. 237).

Two years on, few readers will conclude that things are “better,” given a global pandemic, rising authoritarianism, trade wars, systemic racism, and excessive use of police powers. The celebrated World Trade Organization (WTO) Dispute Settlement Understanding (DSU) teeters on the edge of collapse, without a functioning Appellate Body.Footnote ² The United States has exited the UN Educational, Scientific and Cultural Organisation (UNESCO) (again), and remains on track to withdraw from the Paris Agreement, while providing notice to do the same with respect to the World Health Organization.Footnote ³ Meanwhile, the UN Security Council stalemated over a global ceasefire during the pandemic, harkening back to its Cold War operations.Footnote ⁴ Simply put, the treaty world of 2020 looks considerably bleaker than the one Nolte surveyed in 2018.

It would be a mistake, however, to forgo Treaties and Their Practice because some of its snapshots appear dated. After all, Nolte does acknowledge the risk of major crises and continued decline. More importantly, readers looking beyond Nolte's diagnoses will find a larger (and more fundamental) symptom about international lawyers’ assessments of treaties. In short, this is not just a book diagnosing the state of certain treaties, it is a book about how international lawyers make such diagnoses.

The twenty-first century has seen treaty scholarship rise dramatically.Footnote ⁵ Yet, if we follow Nolte's medical metaphor, a common symptom pervades much of that work: myopia. In studying and applying treaties, international lawyers focus largely on texts: writing is used to define the treaty, to elaborate its conclusion, and to direct its entry into force.Footnote ⁶ The text's “ordinary meaning” dominates its interpretation, while compliance, and even effectiveness, are regularly measured in textual terms.Footnote ⁷ And, of course, all this textual emphasis derives from a text—the 1969 Vienna Convention on the Law of Treaties (VCLT)—that functions as the fulcrum for the field.Footnote ⁸

In the face of such myopia, Nolte's emphasis on treaty practice offers a corrective lens. He seeks to “open the minds of readers” (p. 24) to a fulsome view of the state of treaties. To do so, he employs a series of reinforcing dichotomies—contrasting the treaty's rise with its decline; narrow meanings of practice with broad ones; legal applications with political tensions; treaty compliance with treaty effectiveness; and a bird's eye view with that of a worm. Each dichotomy requires readers to reorient their focal points, to broaden their understanding of treaties beyond the myopia dominating conventional visions of the field.

In his opening chapter, Nolte argues that treaty practice is the appropriate symptom for measuring treaties. He acknowledges that practice may mean different things to different audiences. For international lawyers, practice is usually understood in a “narrow sense” as “‘the application of the treaty’ in the sense of the Vienna Convention” (p. 20). Practice, however, may also occur in a “broad sense,” involving “all other conduct ‘in relation to’ the treaty, regardless of whether such conduct is legally relevant, irrelevant, or ambiguous” (id.). Whatever focus the profession gives the VCLT, Nolte argues, “international lawyers should not shy away from assessing broader developments which may affect their field” (p. 23). Hence, he proposes diverse criteria to populate the practice concept, including

• – the number of ratifications,

• – the procedural workings of the institutions set up by the treaty,

• – the respect for specific substantive obligations of the treaty,

• – the fulfilment of the goals of the treaty,

• – the commitment of the parties to the treaty,

• – the conditions for the implementation of the treaty. (P. 25)

Nolte resists treating his list as a hierarchy. Each criterion, standing alone, has flaws (e.g., ratifications say little about party compliance). Rather than relying on any single criterion (or some fixed combination), Nolte prefers a variable combination of factual elements and normative criteria. The result is a purposefully interdisciplinary diagnostic:

In Chapter II, Nolte juxtaposes the rise and decline of international peace treaties. Like the collective agreement to tie up Ulysses, Nolte roots the binding force of treaties (pacta sunt servanda) in “the assumption of an act of collective will of an international community of States, and perhaps even more actors” (p. 34). At the same time, Nolte recalls Hobbes's competing theory that: “Covenants, without the Sword, are but Words, and of no strength” (p. 35). Rather than meet Hobbes's challenge theoretically, however, Nolte invokes practice to explain why treaties “rise” in binding States or “decline” when bonds loosen. As examples, he recounts the durability (but eventual impermanence) of the Peace of Westphalia, the Congress of Vienna, and the League of Nations.Footnote ⁹ Such practice shows these treaties are “neither necessarily ‘a scrap of paper’ when no ‘sword’ is available to enforce them, nor can their legally binding nature be simply relied upon . . .” (p. 54).

In Chapter III, Nolte diagnoses the current state of the UN Charter's collective security system and its rules on the use of force. He pronounces the legacy “mixed” (p. 87). Nolte suggests the collective security system has risen past prior Cold War practices, but acknowledges that the Security Council's more expansive power assertions in Libya in 2011 may have effectively resulted in the system's later stymied approach to Syria. Meanwhile, ambiguity about exceptions to the use of force prohibition (e.g., nonauthorized humanitarian interventions, self-defense against nonstate actors) represent signs of decline: “If it becomes unclear under which circumstances a rule applies, the significance of the rule itself changes. It can then be said to be in ‘decline’—until it turns out that this uncertainty was a phase in the process of the formation of a new rule” (p. 81).

Nolte's UN Charter review suggests two important implications for evaluating treaties in light of their practice. First, like Thomas Franck, Nolte emphasizes how practice can change the state of a treaty without altering its text.Footnote ¹⁰ Formally, the UN Charter was the same from 1945 to 1985, yet decolonization fundamentally reconstituted the organization, changing the meaning and operation of procedural processes and substantive commitments. Second, whether such changes represent a “rise” or a “decline” “may be perceived differently by different States and other actors” (p. 88). Some, for example, see the 1999 Kosovo intervention as a rise in the “use of force” paradigm, reflecting a commitment to human rights. Others see it as a symptom of decline, deemphasizing strict observance of the Charter, and facilitating things like Crimea's annexation. This portrayal of practice moves readers far from formal Charter interpretations. Much like the Sirens are simultaneously a story of sound for Ulysses and silence for his crew, we see how a treaty can be understood as both rising and declining at the same time, breaking down the dichotomy into a more pluralist perspective.

Nolte focuses on human rights treaty practice in Chapter IV. Continuing the rise/decline heuristic, this chapter is notable for juxtaposing practice in narrow and broad senses. The narrow, “lawyer's story” of human rights suggests a rise; human rights treaties entered into force quickly with treaty bodies that identify violations and make influential nonbinding recommendations. Although treaty violations and limited domestic enforcement are potential negative signs, Nolte suggests we see a rise where human rights machinery provided “the focal point for a multitude of debates” (p. 96). From a broader, social science perspective, the symptoms are mixed, including evidence of decline. Empirical research suggests that human rights treaties have improved state practices. Others critique their application as a hegemonic “Western” foreign policy that faltered in the twenty-first century. For Nolte, the solution is not to pick sides, even if empirical work is “methodologically closer to what practice-oriented lawyers” (p. 104) do. International lawyers need to “make sense of the broader political and historical developments, and of the empirical research” simultaneously (p. 105).

In discussing the narrower version of human rights treaty practice, Nolte highlights claims that universal treaty bodies have exceeded their mandates and also criticisms of regional human rights courts’ jurisprudence. More broadly, human rights treaties have generated increased “political contestation,” especially with more authoritarian—but popularly supported—governments. Again, however, Nolte declines to prioritize, calling on lawyers to look at individual cases and the big picture: “while changes in the conduct of States ‘cannot be explained simply by populist movements in politics only,’ it is also true that such changes ‘cannot be explained as a response to the quality of the rulings or the judges’” (pp. 116–17). The result? These treaties are in a period of “‘vibrant dynamism’ . . . with the direction of the dynamism not always being very clear” (p. 117).

These two dichotomies—rise/decline, narrow/broad—pervade the chapters on WTO agreements (Chapter V) and bilateral investment treaties (Chapter VI) alongside a third—law and politics. For the WTO, Nolte surmises a state of stagnation with ominous signs on the horizon (signs that hardened into the reality of the Appellate Body crisis). That diagnosis derives more from political practices outside these agreements than the legal practice, which, considered alone, suggests a rise. The WTO DSU's enforcement provisions constituted a carefully calibrated (and much-needed) “sword” lacked by its GATT predecessor. Except for the United States, members appear satisfied with its outputs (e.g., 80–90 percent acceptance of Appellate Body decisions). Nolte attributes this to Appellate Body success keeping legal jurisprudence separate from political tensions relating to trade relations (and avoiding overly expansive interpretations like those disrupting human rights treaties). It is, however, political reactions to the potential overreach of the WTO agreements that ultimately stymied further rise. Despite decades of negotiations, states failed to conclude the anticipated post-Uruguay round agreements, while rising numbers of megaregional trade deals suggest states are no longer as committed to a global trade regime as in the 1990s.

In the investment context, evidence of decline is more striking. The rise to three thousand bilateral investment treaties (BITs) was impressive and arbitration of 855 BIT disputes “seems to be a sign of their good health” (p. 147). Yet, states decried the results of investor-state arbitrations for being too investor friendly, disregarding legitimate policy choices, and imposing harsher remedies (e.g., monetary damages) than at the WTO. Nolte suggests lawyers should have anticipated the “application of investment treaties in practice would . . . overburden the determination of States to adhere to them politically” (p. 154). In other words, unlike the WTO, BIT practice failed to effectively segregate law from politics. At the same time, “broader economic and political developments” undermined the “dominance of Western conceptions for the process of globalization” (p. 155), shifting state attitudes. The results are stark. States like India and South Africa denounced BITs; others (e.g., Bolivia, Ecuador, Venezuela) withdrew from the Convention on the Settlement of Investment Disputes Between States and Nationals of Other States. However, previously pro-investor actors (e.g., the EU) now favor more balance between investor protections and regulatory autonomy. In a move reminiscent of the Calvo Clause, South Africa enacted legislation prioritizing national law over foreign investments. The resulting symptoms suggest a “crisis for investment treaties” (p. 153), although Nolte counsels it may turn out to be a “bump in the road” or these treaties’ maturation into less politically important instruments.

In Nolte's final case study (Chapter VII), he examines environmental treaties’ symptoms. He acknowledges historical parallels to his other case studies, but declines to offer a similar overarching prognosis. Nolte finds these treaties are too diverse (regulating everything from European bats to climate change) with a much larger regulatory role for general principles and soft law. Instead, he suggests a treaty-by-treaty diagnosis. Certain treaties clearly rose, such as the Montreal Protocol on Substances that Deplete the Ozone Layer (which achieved its fundamental purpose). Climate change, however, is more complicated. The Kyoto Protocol rose throughout the 1990s, only to decline when it failed to garner sufficient ratifications while parties had trouble complying with its detailed thresholds. In contrast, Nolte finds it too soon to judge the Paris Agreement (even with U.S. intentions to withdraw). Instead, he juxtaposes two additional aspects of treaty practice—compliance and effectiveness. The Paris Agreement's commitments are largely procedural, letting parties set their own nationally determined contributions (NDCs) rather than imposing measurable, substantive obligations like the Kyoto Protocol. Nolte suggests that this creates a paradox for treaty assessments:

As such, international lawyers must critically evaluate both treaty compliance and effectiveness when they examine treaty practice. The results may diverge. While some see the Paris Agreement as an “empty formality,” others simultaneously expect its emphasis on procedural rules may effectively “lead to the solution of substantive problems” (id.).

Nolte's case studies illuminate how treaty practice can expand our evaluation of treaties far beyond the conventional, textual myopia. The dichotomies he invokes—rise/decline, narrow/broad, legal/political, compliance/effectiveness—provide additional lenses for evaluating the health of a treaty. That said, Nolte does not reject traditional perspectives entirely. Just as eye doctors correct for myopia without taking away patients’ ability to see things closely, Treaties and Their Practice acknowledges the VCLT's value. Chapter VIII thus shifts from the bird's eye view of previous chapters to a “worm's eye view” of international lawyers interpreting treaties in practice. Again, he does so holistically, without favoring one particular view: “Neither a bird's eye perspective nor a worm's eye perspective are inherently more valuable or preferable. The truth lies in their proper combination” (p. 174).

The power of Nolte's holistic approach is undercut, however, by his suggestion (without much justification) that treaty interpretation assumes a capacity to find “one correct meaning” (p. 176) or a “right” interpretation (p. 226). Recognizing theoretical challenges to such capacity, he nonetheless suggests that it gives international lawyers “a common goal” (p. 176). For Nolte, international lawyers must apply a treaty as “loyally as possible and with a long-term perspective in mind” (p. 236). This idea that international lawyers share a common purpose, however, has come under close scrutiny in recent years.Footnote ¹¹ It would have thus been useful to see Nolte defend it, especially given how much his dichotomies suggest relative or pluralist outcomes. These are in considerable tension with the idea of a single, objective vision of a treaty to which all actors must show fealty.

It is harder to dispute Nolte's ensuing point about the role international lawyers’ practice can have in influencing the state of a treaty. It can contribute to a treaty's rise “by applying it in a way that is generally well received and which is followed by others” or decline “by applying it in a way which provokes resistance or which makes parties, or others, lose their identification with the treaty” (p. 178). Yet, such practice is not outcome determinative. Broader geopolitical factors may sometimes prove the driving force in a treaty's fate (Nolte notes, for example, “better” EU treaty interpretations were unlikely to forestall Brexit).

Chapter IX is the volume's longest, examining the VCLT's treatment of “subsequent agreements and subsequent practice.” Drawing on his years of work as International Law Commission special rapporteur, Nolte elaborates a framework for treaty interpretation that elevates and aligns subsequent agreements (whether binding or not) and subsequent practice alongside a treaty's text, context, and object and purpose. Where subsequent agreement or practice reflects the common will of all parties, VCLT Article 31 treats it as an authentic means of interpretation. Subsequent agreements or practice by less than all parties may still have value resolving ambiguities pursuant to VCLT Article 32. Treaties and Their Practice thus revisits old battle lines in interpreting the VCLT's interpretative provisions, including the roles of silent states and nonstate actors, the dividing line between interpretation and amendments, and the availability of evolutive interpretations. At the same time, Nolte does not lose sight of broader horizons, including other types of practice beyond treaty applications relevant to treaty interpretation.

The volume ends by combining the bird's eye and worm's eye views with an emphasis on state responsibility (legally and politically) as a vehicle for assessing the state of a treaty. States (and their lawyers) decide whether to comply and contribute to the treaty's interpretation and application or chose not to comply, triggering international legal responsibility and signaling a weakening treaty status. Other actors matter to Nolte as well—“be they courts . . . or other persons” (p. 236). More discussion of such other actors’ practice would have been useful, particularly in evaluating how their authority to interpret—and their agenda in applying—treaties differ from states (the authority and agenda of international courts is not, for example, that of a state party, even assuming both operate in good faith). That said, Nolte does recognize actors’ perceptions of treaties can change quickly and that we “may be facing another major crisis” (id.). He concludes with a call for more attention to treaties, asking if it is time to seek “stronger swords” or consider substitutes—“informal, specific, and short-term arrangements in an electronically accelerated and interconnected world” (pp. 236–37).

Treaties and Their Practice is a significant and useful intervention in a field where treaty lawyers have long lived with myopia. It offers a welcome, nuanced, and broad-ranging evaluation on how international lawyers can—and should—evaluate treaties. Its ambit stretches well beyond the VCLT's confines, without diminishing the importance of that canonical text. Nolte does not, however, overclaim; his case studies are not meant to be representative of all treaties, let alone a diagnosis of international law generally (p. 24).Footnote ¹² As such, he leaves room for diagnostic tools beyond treaty practice. Indeed, his closing suggestion that we evaluate alternatives to treaty-making hints at one such approach—comparing states’ use of treaties and political commitments.Footnote ¹³

Alternatively, studying treaty functions would add another perspective for evaluating them. Nolte's story of Ulysses, for example, gives value to “constrictive” agreements, restricting Ulysses's (and states’) freedom of action. But Ulysses's story did not end with the Sirens. He had other adventures that did not involve proscribing his own behavior, such as he and his crews’ innovative escapes to freedom (e.g., from the cyclops Polyphemus) or difficult decisions on limiting losses (e.g., in steering toward the monster Scylla to avoid Charybdis). In short, Treaties and Their Practice may mark the beginning of a longer odyssey for treaty lawyers. Ulysses's odyssey ultimately brought him back home. Where this one takes us remains to be seen.