Basis of International Law — the Theories — Public International Law Notes

Basis of International Law — the Theories

Here is a puzzle. A sovereign State, by definition, has no one above it. So how can anything bind it against its will? Anzilotti, an Italian positivist, thought he had the final answer in two Latin words — pacta sunt servanda (“agreements must be kept”) — which he called the fundamental, unprovable postulate on which the whole of international law rests. Every theory in this topic is really a different answer to that one puzzle: why is a sovereign bound at all?

The competing answers — why a State is bound

The “basis” of international law means the ultimate reason States are obliged to obey it. There are several theories; learn each with its content and its criticism.

A. The naturalist theory

International law binds because it is part of the law of nature — a higher law of reason and morality, binding on all, discoverable by human reason (Grotius, Vitoria, Pufendorf). Criticism: “law of nature” is vague and subjective; different writers found opposite rules in it. It cannot alone explain the detailed, technical rules of modern IL.

International law binds only because States have consented to it — expressly through treaties, tacitly through custom (Anzilotti, Triepel). Will of the State is the source of all obligation. Criticism: it cannot explain how a new State is bound by existing customary law it never consented to, nor how a rule can bind a State that objected. Consent alone is too narrow.

C. Jellinek’s auto-limitation (self-limitation) theory

A refinement of consent. A sovereign cannot be bound by an outside will, so it binds itself: the State, by its own free will, limits its own sovereignty and thereby accepts international law. Criticism: if the State limited itself, it can un-limit itself at will — which would make international law depend on each State’s continuing pleasure, i.e. no real obligation at all.

D. The fundamental-rights theory

Because a State is sovereign, it possesses certain fundamental rights prior to international law — existence, self-preservation, equality, independence, self-defence. International law exists to protect these. Criticism: it exaggerates sovereignty and can be used to justify a State breaking the law in the name of “self-preservation”.

E. Pacta sunt servanda as the ultimate postulate (Anzilotti)

Anzilotti located the basis in a single norm that cannot itself be proved but is assumed: pacta sunt servanda (“agreements must be kept”). Treaties bind because of this norm; custom binds because of an implied agreement. Criticism: it explains treaty obligation but strains to explain custom, and one cannot indefinitely ask “and why is that norm binding?”

The practical takeaway: no single theory fully explains international obligation. The best answer today combines consent (the ordinary working basis — States are bound by what they agree to and practise) with the recognition that some norms (jus cogens — peremptory norms like the prohibition of aggression, genocide, slavery) bind regardless of consent.

Article 26, Vienna Convention on the Law of Treaties (1969): “Every treaty in force is binding upon the parties to it and must be performed by them in good faith.” (Pacta sunt servanda.)

In Simple Terms: The most concrete anchor of obligation in modern law is Article 26 of the VCLT: once you have agreed to a treaty, you must keep it, in good faith. This is the codified form of pacta sunt servanda — the postulate Anzilotti said the whole system rests on.

🧩 WORKED EXAMPLE — sorting the theories in an answer

Facts. “State A signs a disarmament treaty, then argues that as a sovereign it can never be bound against its present will. Which theory answers A, and how?”

Rule. Consent theory + auto-limitation: A did consent, and pacta sunt servanda (Art. 26 VCLT) makes that consent binding.

Apply. A’s “no sovereign can be bound” is the auto-limitation trap — that a self-limitation is freely revocable. The reply: consent, once given in a treaty, is fixed by pacta sunt servanda; the State cannot un-bind itself at pleasure or treaties would be worthless.

Conclusion. A is bound. Sovereignty explains why consent is needed; it does not let a State escape a consent already given.

flowchart TD
    A["Why is a sovereign State bound?<br/>(Basis of International Law)"]
    A --> B["Naturalist<br/>law of nature / reason"]
    A --> C["Consent / positivist<br/>treaties + custom"]
    A --> D["Auto-limitation (Jellinek)<br/>State limits itself"]
    A --> E["Fundamental rights<br/>rights prior to IL"]
    A --> F["Pacta sunt servanda (Anzilotti)<br/>ultimate postulate"]
    classDef box fill:#e8f0fe,stroke:#333,color:#111;
    class A,B,C,D,E,F box;

Case Laws

S.S. Lotus (France v. Turkey, PCIJ, 1927) — restrictions on States “cannot be presumed”; obligations emanate from the free will of States (a strong statement of the consent basis).
North Sea Continental Shelf Cases (1969, ICJ) — a rule binds as custom only where there is settled practice and opinio juris (a sense of legal obligation), showing consent/acceptance at work in custom.

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Basis of International Law — the Theories — Public International Law Notes