Legal Theory and International Law
Updated: Sep 8
Introduction
John Austin thought international law was not law. In The Province of Jurisprudence Determined, he assigned the rules governing relations between states to the category of positive morality, binding in the way that etiquette or reputation binds, because no sovereign stood above states to command them and no organized sanction followed their breach (Austin, 1832). Nearly two centuries later, his challenge still shapes the meeting point of legal theory and international law: what makes an obligation legal where there is no legislature, no compulsory jurisdiction, and no enforcement agent independent of the states being governed.
A practitioner can work for years without confronting that question directly. It nevertheless sits beneath the doctrine. Whether a customary rule binds a state that never consented to it, whether a treaty can be void for conflicting with a higher norm, whether a tribunal may find a rule where states have said nothing at all — each answer depends on a prior account of where international legal obligation comes from.
Those accounts are what the theories of international law supply, and the disagreements among them are visible in pleadings, in judgments, and in the drafting decisions of the International Law Commission. The field has also acquired its own institutional weight over the past four decades, with dedicated journals, book series, doctoral programs and learned-society sections carrying the name international legal theory (d'Aspremont, 2025). What follows sets out the principal traditions, the objections each has attracted, and the doctrinal consequences that turn on choosing among them.
1. What Legal Theory and International Law Ask of Each Other
1.1 An unstable line between theory and practice
International legal theory is usually identified negatively. It covers the questions treated as standing apart from the routine business of identifying, applying, interpreting, and contesting legal rules, which means the category depends on a distinction between theory and practice rather than on any subject matter of its own (d'Aspremont, 2025).
The distinction does not hold up well. An argument that a rule of custom exists already presupposes a position on what makes practice into law, on whose practice counts, and on how silence is read. Counsel who tells a court that restrictions on sovereignty cannot be presumed is advancing a theoretical claim in the guise of a procedural one.
This matters for how theory should be judged. A theory of international law is not a decorative frame around doctrine; it is a set of commitments that determine which arguments count as legal arguments at all.
1.2 Explanatory, normative, and critical claims
International legal theory operates in three registers that are easily confused. Explanatory work asks what international law is and how obligation arises within it. Normative work asks what the system ought to require and on what basis its authority can be justified. Critical work asks what the discipline's own categories obscure, and whose position they favor.
International law theories rarely belong to one register alone, and most writers move between them, which is legitimate. What is not legitimate is answering a claim made in one register with an argument drawn from another. A demonstration that states comply when compliance suits them does not refute a claim about the validity of a norm, and a demonstration that a norm is valid says nothing about whether anyone will observe it.
2. Natural Law and International Law
2.1 The scholastic foundations of the law of nations
The earliest sustained theorizing about a law governing relations between political communities came from Spanish theologians confronting the conquest of the Americas. Francisco de Vitoria, lecturing at Salamanca in 1539, rejected the argument that indigenous polities lacked dominion over their lands because they were not Christian, and reasoned instead from a universal law binding on all communities by virtue of their humanity (Vitoria, 1991).
The same lectures also supplied grounds on which Spanish presence could be defended, including a right of travel and commerce whose obstruction could justify war. Naturalist universalism, in its founding text, produced both a limit on conquest and a license for it.
Francisco Suárez took the analysis a step further by separating the natural law properly so called from a ius gentium that rested on the settled usage and tacit agreement of nations (Suárez, 1612). That separation opened the path toward a law of nations grounded in what states actually do rather than in what reason requires of them.
Hugo Grotius built on both. His treatise on the law of war and peace derived part of the law from reason and part from the will of nations, and its prologue advanced the claim that the foundations of that law would retain force even on the impossible supposition that God did not exist or took no interest in human affairs (Grotius, 1625). The argument freed the law of nations from theological premises without abandoning the idea that its content could be objectively determined.
2.2 Where naturalist reasoning survives in positive law
Naturalism is often described as historically superseded. Its structure persists in doctrines that cannot be explained by state consent alone, and the clearest of these is the category of peremptory norms. Article 53 of the Vienna Convention on the Law of Treaties voids a treaty that conflicts, at the time of its conclusion, with a norm accepted and recognized by the international community of States as a whole as one permitting no derogation (Vienna Convention on the Law of Treaties, 1969).
The definition is consensual in form and anti-consensual in effect. States identify the norm, but once identified it cannot be contracted around by the very states whose recognition constituted it. The International Law Commission's 2022 conclusions on peremptory norms annexed a non-exhaustive list drawn from the Commission's earlier work, including the prohibitions of aggression, genocide, crimes against humanity, slavery, apartheid and racial discrimination, and torture, together with the basic rules of international humanitarian law and the right of self-determination (International Law Commission, 2022).
A second survival is the Martens Clause, first placed in the preamble to the 1899 Hague Convention respecting the laws and customs of war on land. Where no specific treaty provision applies, populations and belligerents remain protected by principles of the law of nations derived from established usage, the laws of humanity, and the dictates of the public conscience (Hague Convention (II), 1899). The International Court of Justice confirmed in its advisory opinion on nuclear weapons that the clause continues to operate as part of customary international law (ICJ, 1996).
General principles of law recognized by civilized nations, in the dated formulation retained in Article 38(1)(c) of the Court's Statute, occupy similar ground (Statute of the International Court of Justice, 1945). They allow a tribunal to reason from principles common to legal systems where treaty and custom are silent, which is why the International Law Commission placed the topic on its programme of work in 2017 and has since sought to specify how such principles are identified.
3. Legal Positivism in International Law
3.1 Consent, sources, and the presumption of freedom
Legal positivism in international law holds that the content of the law is settled by what states have made, not by what morality prescribes. Its working apparatus is the doctrine of sources: Article 38 of the Statute of the International Court of Justice directs the Court to apply international conventions, international custom as evidence of a general practice accepted as law, and general principles of law, with judicial decisions and scholarly writings as subsidiary means for determining rules (Statute of the International Court of Justice, 1945).
The classical statement of the consensual premise came from the Permanent Court of International Justice in the Lotus case, where the Court reasoned that rules binding on states emanate from their own free will and that restrictions upon their independence cannot be presumed (PCIJ, 1927). The practical consequence is a residual freedom: conduct not prohibited is permitted.
That reasoning is neither obsolete nor uncontested. In the Kosovo advisory opinion the Court answered the question put to it by finding no prohibition in general international law on the issuing of declarations of independence, a mode of analysis that Judge Simma, in a declaration appended to the opinion, criticized as an outdated approach that ignores the possibility of conduct being tolerated or merely permitted rather than simply unprohibited (ICJ, 2010).
3.2 Hart and Kelsen: obligation without a system
H. L. A. Hart's contribution to this debate is frequently misread as a restatement of Austin. It was the opposite in method: Hart rejected both the command theory and the idea that sanctions are what make rules legal. His doubt about international law concerned structure. A developed legal system, on his account, combines primary rules of obligation with secondary rules of recognition, change and adjudication; international law appeared to him to consist of primary rules without a rule of recognition capable of validating them (Hart, 1961).
Hart did not conclude that international law is not law. He concluded that it resembles the simpler social structure of a community without formal institutions, and left open whether it might develop the missing features. Later positivists have replied that the sources doctrine performs the work of a rule of recognition, since courts, foreign ministries and legal advisers converge in practice on the criteria of Article 38. The reply is plausible but incomplete, because the acceptance of those criteria is itself a matter of practice rather than of authoritative enactment.
Hans Kelsen approached validity differently. A norm is valid because it was created in accordance with a higher norm, and the chain of authorization must terminate in a presupposed basic norm rather than in any fact (Kelsen, 1967). For the international legal order, he located that terminus in the norm requiring that agreements be kept, and later in a norm treating custom as a law-creating fact.
The account is formally elegant and purchases its coherence at a known price. The basic norm is presupposed rather than demonstrated, so the question of why international law binds is answered by declaring the question outside legal science. That move satisfies few readers who came to the problem because they wanted it answered.
3.3 Custom and the strain on consent
Customary international law puts the consensual account under its heaviest load. A rule of custom requires general practice accompanied by opinio juris, the acceptance of that practice as law. The requirement generates a familiar circularity: the states whose conduct first establishes the practice must already regard themselves as legally bound by a rule that does not yet exist.
The Court's treatment in North Sea Continental Shelf remains the standard reference. It held that the equidistance method in Article 6 of the 1958 Continental Shelf Convention had not passed into custom, and insisted that acts amount to settled practice only where they are accompanied by a belief that the practice is rendered obligatory by a rule of law, as distinct from motives of courtesy or convenience (ICJ, 1969).
In Nicaragua, the Court applied the test with a lighter touch, drawing opinio juris substantially from state attitudes expressed through General Assembly resolutions, and holding that conduct inconsistent with a rule does not defeat the rule where the state concerned defends its conduct by invoking exceptions rather than denying the rule (ICJ, 1986). The reasoning shows how far the evidentiary basis of custom can move toward what states say and away from what they do.
The persistent objector rule allows a state that has opposed a rule consistently from the period of its formation to escape its application. Its usual authority is the Fisheries judgment, where the Court observed that the Norwegian system of straight baselines could in any event not be opposed to Norway, which had always opposed any attempt to apply the contrary rule to its coast (ICJ, 1951); whether that passage establishes the rule or merely assumes it remains disputed. It preserves consent formally while confirming how narrow the preservation is, since it offers nothing to states that did not exist when the rule crystallized. The Commission's 2018 conclusions on the identification of customary international law restate the two-element approach and specify the forms of evidence that count for each element, without resolving the underlying theoretical difficulty (International Law Commission, 2018).
4. Process, Legitimacy, and Compliance
4.1 Policy-oriented jurisprudence
American legal realism reached international law through the work of Myres McDougal and Harold Lasswell at Yale, whose policy-oriented approach recast law as a process of authoritative and controlling decision rather than a body of rules (McDougal and Lasswell, 1959). The analyst begins by specifying an observational standpoint, then maps the claims advanced, the participants, the base values at stake and the outcomes produced, appraising them against a preferred order of human dignity.
The framework captures something the rule-based accounts miss, namely that decision in international affairs is distributed among foreign ministries, tribunals, secretariats and advocacy networks rather than concentrated in courts. Its weakness was equally visible to contemporaries. Once rules become one input among many into a decision oriented toward values chosen by the observer, the analysis loses its capacity to tell a state that it has broken the law, and critics noted how often the school's conclusions tracked United States policy during the Cold War.
4.2 Legitimacy and internalization
Thomas Franck reframed the question by asking why states obey rules they could break with impunity. His answer located the compliance pull of a rule in its legitimacy, which he analyzed through determinacy of content, symbolic validation, coherence with related rules, and adherence to a hierarchy of secondary rules governing rule-making (Franck, 1990).
Abram Chayes and Antonia Handler Chayes argued from the practice of treaty regimes that most noncompliance is not deliberate defection. It arises from ambiguity in the obligation, limits on the capacity of the state to perform, and the passage of time between undertaking and implementation, which makes a managerial response of reporting, technical assistance and negotiated adjustment more effective than sanctions (Chayes and Chayes, 1995).
Harold Koh added the domestic dimension. Transnational legal process describes how repeated interaction among states, international institutions, courts and private actors generates interpretations that are internalized into domestic legal and political systems, after which compliance follows from ordinary internal legal obligation rather than from calculation about external consequences (Koh, 1997).
4.3 Rationalist accounts and the evidentiary dispute
Jack Goldsmith and Eric Posner pressed the opposite case. Observed compliance, they argued, is explained by coincidence of interest, by coordination on a focal point, by bilateral cooperation sustained through repeated play, and by coercion, so that international law exerts no independent influence on state behavior (Goldsmith and Posner, 2005).
Andrew Guzman accepted the rationalist premises and reached a different result, treating reputation, reciprocity, and retaliation as costs that make legal commitments meaningful precisely because breaking them is expensive (Guzman, 2008). The disagreement between the two is less about states than about evidence: what would show that a rule, rather than an interest that happens to coincide with it, produced the conduct in question. Neither side has produced a test the other accepts.
5. Critical Approaches to International Law
5.1 Indeterminacy and the structure of legal argument
The most influential critical account is Martti Koskenniemi's analysis of the structure of international legal argument. He described a discipline caught between two justifications it cannot reconcile: an ascending argument that derives obligation from state will, which risks reducing law to whatever states want, and a descending argument that derives it from an objective normative order, which risks imposing the theorist's preferences on states (Koskenniemi, 2005).
Each position is vulnerable to the criticism generated by the other, so lawyers oscillate between them, and competent counsel can construct a professionally respectable argument on either side of most contested questions using the same authorities. The claim is not that legal argument is meaningless. It is that the choice between available arguments is not itself determined by law, and that presenting it as determined conceals a political decision behind a technical one. David Kennedy had earlier reached a related conclusion by examining how doctrinal writing in the fields of sources, process and substance repeats the same structural moves (Kennedy, 1987).
5.2 Third World approaches and the colonial encounter
Antony Anghie's historical argument challenges the standard account in which a European law of nations, developed among sovereign equals, was later extended to the rest of the world. On his reading, central doctrines of sovereignty were formed in the colonial encounter itself, through a recurring pattern in which a difference was posited between civilized and uncivilized communities and then bridged by legal techniques that subordinated the latter (Anghie, 2005).
The techniques changed over time while the pattern held: the standard of civilization governing recognition, capitulations and unequal treaties, the mandate system of the League of Nations, and conditionality in later institutional practice. B. S. Chimni extended the analysis to contemporary economic governance, arguing that trade, investment and financial institutions reproduce the hierarchy through ostensibly neutral rules (Chimni, 2006).
Third World approaches to international law bear directly on consent-based theory. States that gained independence after 1945 inherited a body of custom and a structure of institutions formed without their participation, which is the strongest historical objection to the claim that the system rests on the will of those it governs.
5.3 Feminist analysis and the reach of the public sphere
Hilary Charlesworth, Christine Chinkin and Shelley Wright argued that both the institutions that make international law and the substance of its rules are structured around male experience, and that the exclusion is not incidental to the doctrine but built into it (Charlesworth, Chinkin and Wright, 1991). The organizing mechanism they identified is the distinction between public and private: international law addresses the conduct of states in a public sphere, while much of the harm suffered by women occurs in domains classified as private and therefore outside its reach.
Charlesworth and Chinkin later developed the analysis across the fields of statehood, sources, institutions, human rights and the use of force (Charlesworth and Chinkin, 2000). Its doctrinal effects are traceable in the treatment of sexual violence in armed conflict, and in the expansion of due diligence obligations, which require states to act against private conduct they did not commit.
Marxist analysis proceeds from a different premise but shares the refusal to treat form as neutral. China Miéville argued that the legal form of sovereign equality does not restrain coercion so much as organize it, since disputes between formally equal sovereigns are settled by the relative force available to each (Miéville, 2005).
6. Theoretical Commitments Inside Legal Argument
Treaty interpretation is the clearest illustration. Articles 31 and 32 of the Vienna Convention require a treaty to be interpreted in good faith in accordance with the ordinary meaning of its terms in their context and in the light of its object and purpose, with preparatory work available as a supplementary means (Vienna Convention on the Law of Treaties, 1969). The rule combines textual and purposive elements without specifying their relative weight, and the weighting an interpreter adopts follows from a view about whether treaties record bargains or establish regimes.
The Court has drawn out that view where a term's meaning changes over time. In the dispute over navigational rights on the San Juan River it held that a generic term used in an 1858 treaty of unlimited duration must be given the meaning it bears at the time of application rather than at the time of conclusion, on the presumption that the parties intended it to evolve (ICJ, 2009). The presumption is not derived from the text of the treaty. It comes from a theory about what parties to a perpetual instrument can be taken to have intended.
The structure of obligation is a second illustration. In Barcelona Traction the Court distinguished obligations owed to particular states from obligations owed to the international community as a whole, in which all states have a legal interest (ICJ, 1970). That distinction, stated in passing and without immediate consequence for the outcome, has since supported standing for states with no individual injury, in proceedings concerning the obligation to prosecute or extradite (ICJ, 2012) and in the genocide case brought against Myanmar (ICJ, 2022). A conception of the international legal order as a community rather than a network of bilateral bargains produced a procedural result.
Fragmentation raises the question at the level of the system. The Commission's study group concluded that the proliferation of specialized regimes in trade, investment, human rights and environmental protection is not resolved by any hierarchical master rule, and that conflicts are managed through interpretative techniques including lex specialis and the systemic integration required by Article 31(3)(c) of the Vienna Convention (International Law Commission, 2006). Whether that amounts to a system depends on the criteria for systemic unity, which is the question Hart raised.
The limits are visible where the Court has declined to answer. In the nuclear weapons opinion, it found the threat or use of such weapons generally contrary to the rules applicable in armed conflict, yet stated that it could not conclude definitively whether the threat or use would be lawful or unlawful in an extreme circumstance of self-defense in which the very survival of a state was at stake (ICJ, 1996). Neither the presumption of freedom inherited from Lotus nor the assumption that the legal order is complete survives that passage intact.
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Conclusion
No single theory accounts for the whole of the practice. Consent explains the law of treaties well and the formation of custom badly. Naturalist reasoning explains peremptory norms and the residual protections of humanitarian law, and offers little guidance on the technical rules that occupy most of the field. Process and compliance scholarship explains behavior and cannot say what the law requires. The critical literature explains why the discipline keeps reproducing the same arguments without supplying an alternative ground for obligation, which its leading exponents have not claimed to do.
The recurring search for one foundation is what keeps generating unstable foundations. A more productive question is which account explains which part of the practice, and what each leaves unexplained. Sources doctrine functions, as Crawford observed of the field generally, because a professional community applies it with sufficient consistency for arguments to be tested against shared criteria, not because its validity has been demonstrated from outside (Crawford, 2019).
That is a modest basis, and it is the basis the system has. Theory earns its place in international law by making visible what a legal argument is actually resting on, which is also what allows an argument to be contested rather than merely asserted.
References
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