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Part 5 · The law of treaties

Treaties are bargains between legal equals, and the Vienna Convention on the Law of Treaties is their rulebook. Every problem asks the same four questions in the same order: is it a treaty, is the state bound, what does the text require, and is there a way out.

Ideas and cases at a glance

Authority	In a line
Art 2(1)(a) VCLT; Qatar v Bahrain	A treaty is a written agreement between states governed by international law, whatever it is called
Arts 11 to 18, 24 VCLT	Consent to be bound by signature, ratification or accession; entry into force; and the signatory's interim good faith duty
Arts 26, 27, 34 VCLT	Pacta sunt servanda, no internal law excuse, and no obligations or rights for third states without consent
Arts 19 to 21 VCLT; Genocide Reservations (1951)	Reservations are permissible unless prohibited or incompatible with the treaty's object and purpose
Arts 31 and 32 VCLT; Bosnia v Serbia (2007)	Interpret in good faith, by ordinary meaning in context and in light of object and purpose, with travaux as supplementary means
Arts 53, 54, 60 to 62 VCLT; Gabcikovo-Nagymaros	Invalidity and termination: jus cogens, consent, material breach, impossibility, and a change of circumstances read so narrowly it virtually never succeeds

What counts as a treaty

Treaties are the most important source of international law: created consciously by states, able to cover any subject, and binding only between their parties. The threshold question in any problem is whether the document in front of you is a treaty at all, and the answer turns on substance, never on the label.

RULE. arrowA treaty is an international agreement concluded between states in written form and governed by international law, whatever its particular designation (Art 2(1)(a) VCLT). Label and subjective intent are not decisive (Qatar v Bahrain), so a document titled a memorandum of understanding can be a treaty, and vice versa.

Trap. The label trap runs both ways. Examiners plant instruments called minutes, communiques or memoranda. Ask whether the parties committed to obligations governed by international law: obligation language, definite terms and machinery for performance point to a treaty; aspiration and political assurance point away from one.

Becoming bound

Being a treaty and binding this state are different questions. The VCLT separates the moment of signature from the moment of consent, and the space between them is a favourite examination site.

RULE. checklistConsent to be bound may be expressed by signature, exchange of instruments, ratification, acceptance, approval or accession (Arts 11 to 16 VCLT), and the treaty enters into force as it provides or once all negotiating states consent (Art 24). Heads of state, heads of government and foreign ministers can bind without producing full powers (Art 7(2); Eastern Greenland).

RULE. twistA state that has signed subject to ratification is not yet bound to perform, but it must refrain from acts which would defeat the object and purpose of the treaty pending ratification (Art 18 VCLT). The twist is the gap: signature does not bind, yet it is not legally weightless either.

Once the treaty is in force for a party, the core discipline takes over. Pacta sunt servanda: every treaty in force is binding on the parties and must be performed by them in good faith (Art 26). A state cannot invoke its internal law to justify a failure to perform (Art 27), and on the international plane the state answers as a single unit for all its organs, so a breach by a sub-national government is attributable to the state, which cannot plead its federal structure as an excuse. Conversely, the treaty creates neither obligations nor rights for a third state without its consent, pacta tertiis (Art 34): a third state takes on obligations only by express written acceptance (Art 35), while rights may be accepted by conduct (Art 36).

Trap. The custom backstop. A non-party can still be bound by the same norm as custom, since customary rules keep their separate existence (North Sea Continental Shelf). Never end the analysis at "not a party": ask whether the rule binds anyway through the two-element test in customary international law.

Reservations

A reservation is a unilateral statement, however phrased or named, by which a state purports to exclude or modify the legal effect of certain provisions of the treaty in their application to that state (Art 2(1)(d)). Characterise by effect, not label, and distinguish a mere interpretative declaration, which states a reading without modifying anything.

RULE. arrowA reservation is permissible unless the treaty prohibits it, permits only other specified reservations, or the reservation is incompatible with the object and purpose of the treaty (Art 19 VCLT, codifying Reservations to the Genocide Convention).

The mechanics then favour the reserving state. Silence for twelve months is deemed acceptance (Art 20(5)). An objection does not prevent the treaty entering into force between the two states unless the objector definitely says so (Art 20(4)(b)). And either way, the reserved provision simply drops out between the two states to the extent of the reservation (Art 21). Accept, and the reservation applies; object, and the provision does not apply between you: the reserving state gets what it wants no matter what the response is.

Trap. Who decides compatibility? Under the Vienna scheme each state judges for itself and responds by acceptance or objection. For human rights treaties there is a rival view: Belilos and Human Rights Committee General Comment 24 claim power for the supervisory body to assess compatibility and sever an impermissible reservation, leaving the state bound without it. States objected vehemently. In a problem, run both views and say which the facts favour.

Interpretation

RULE. staircaseA treaty shall be interpreted in good faith in accordance with the ordinary meaning to be given to its terms in their context and in the light of its object and purpose (Art 31(1) VCLT). Context includes subsequent agreement and subsequent practice (Art 31(3)). The travaux preparatoires are supplementary means only: to confirm a meaning, or to determine it where Art 31 leaves the meaning ambiguous or manifestly absurd (Art 32). These rules themselves reflect custom.

Notice the architecture. Articles 31 and 32 blend all three schools of interpretation: text, through ordinary meaning; party intent, through context, subsequent agreement and practice, and the travaux; and teleology, through object and purpose, with the intent school deliberately subordinated to the supplementary role of Art 32. If you have met the text, context and purpose method of Australian statutory interpretation, the family resemblance is deliberate and worth one sentence in an essay, but the treaty rule is its own regime with its own text.

Bosnia v Serbia shows the method working. Article I of the Genocide Convention has the parties "undertake to prevent and to punish" genocide, and nowhere expressly prohibits a state from committing it. Reading the ordinary meaning with the Convention's humanitarian object and purpose, the ICJ held that the obligation to prevent genocide necessarily implies the prohibition of committing it ([166]). And the word "including" in the Article IX compromissory clause confirmed that disputes about state responsibility for genocide fall within it ([167]), with the travaux consulted as supplementary means ([163] to [164]). One judgment, every interpretive tool, in the Art 31 then Art 32 order.

Invalidity and escape

The final question is whether the state can get out, and the Convention's answer is: rarely.

RULE. checklistConsent procured by coercion of a state's representative (Art 51) or by the threat or use of force (Art 52) is void, as is a treaty conflicting with jus cogens (Art 53). Termination or withdrawal occurs in conformity with the treaty or by consent of all parties (Art 54), or unilaterally only for material breach (Art 60), supervening impossibility of performance (Art 61), or a fundamental change of circumstances (Art 62).

Trap. Rebus sic stantibus almost never works. Art 62 is construed so narrowly that it virtually never succeeds (Gabcikovo-Nagymaros). A state pleading changed circumstances because the treaty became inconvenient or expensive fails; the change must be fundamental, unforeseen, and transform the extent of obligations still to be performed. Treat Art 62 as an argument to defeat, not a lifeline.

The attack plan

Attack plan. Five steps, in this order. 1. Is it a treaty? Art 2(1)(a), substance over label. 2. Is this state bound? Trace consent under Arts 11 to 16, check entry into force under Art 24, and apply Art 18 to a mere signatory. 3. What does it require? Interpret under Art 31, reaching Art 32 only to confirm or to cure ambiguity, and apply any reservation through Arts 19 to 21. 4. Is there an escape? Invalidity, termination, material breach, impossibility, changed circumstances. 5. Conclude on bound or not, breach or not, remembering the custom backstop for non-parties.

Worked example

State A ratifies a multilateral wildlife convention "subject to the reservation that Article 4 (the ban on trade in listed species) shall not apply to trade conducted under traditional practices." The convention is silent on reservations. State B objects, saying the reservation guts the convention, but says nothing about treaty relations. State A then permits traditional trade. Advise State B.

A Pass answer says. Reservations are allowed unless contrary to the object and purpose of the treaty (Art 19). The trade ban looks central, so the reservation is probably impermissible and State A is in breach of Article 4. This reaches a conclusion, but it skips the machinery: it never asks who decides compatibility, what State B's objection actually achieved under Arts 20 and 21, or what follows if the reservation stands.

An H1 answer says. The statement excludes the legal effect of Article 4 in part, so it is a reservation whatever its phrasing (Art 2(1)(d)). The treaty is silent, so permissibility turns on the object and purpose test in Art 19(c) (Genocide Reservations): if the trade ban is the convention's core, a carve-out for a significant trade class is arguably incompatible; if the convention's scheme is conservation through regulated exceptions, the reservation may be compatible. Under the Vienna scheme State B judges for itself, but its objection did not definitely oppose entry into force, so the treaty operates between A and B with Article 4 disapplied to the extent of the reservation (Arts 20(4)(b), 21): the objection changes almost nothing, which is the objection paradox. On the rival General Comment 24 approach a supervisory body could sever the reservation and hold A to Article 4 in full. Conclusion: on the Vienna analysis A is not in breach as against B; B's real remedies are the compatibility argument and collective pressure, and the answer says which characterisation of the convention's object the facts better support.

Critical pitfalls and counter-arguments

Treating signature as consent. A signatory pending ratification is not bound to perform; only the Art 18 good faith duty applies. Check the entry into force clause before declaring anyone bound.
Letting the label decide. Art 2(1)(a) looks through the title both ways. Argue from obligation language and intended legal effect.
Forgetting the objection paradox. Objecting to a reservation, without definitely opposing treaty relations, leaves the objector in nearly the same position as an acceptor. The Vienna regime favours the reserver, and saying so shows command.
Using travaux first. Art 32 is supplementary. Start with ordinary meaning, context and purpose under Art 31, and reach the travaux only to confirm or to cure ambiguity or absurdity.
Pleading internal law or inconvenience. Art 27 bars the internal law excuse, and Art 62 is read so narrowly it virtually never succeeds. The strong essay point runs the other way: the Convention's rigidity is what makes treaty commitments credible.

Take it to the practice bank

The Principles of Public Law practice bank has ten law of treaties problems, each with a Pass answer, an H1 answer and the examiner's comments. Problem 8 drills consent to be bound and the Art 18 interim obligation, Problem 1 works the Art 31 and 32 method, and Problem 3 runs a reservation through the object and purpose test. Then stress the escape routes: Problem 4 on material breach, Problem 5 on fundamental change of circumstances, Problem 2 on the internal law excuse, Problem 7 on third states, and Problems 9 and 10 on jus cogens and the Genocide Convention.

Practice prompts

Take the worked example and rewrite it with State B's objection stating that it "opposes the entry into force of the convention as between itself and State A." Trace exactly which article changes the result and what State A can still do against every other party.
Write the interpretation paragraph for a clause obliging parties to "take appropriate measures to prevent pollution," where one state argues the clause requires only legislation and another argues it requires enforcement, using the Art 31 elements in order and Bosnia v Serbia as your method authority.
Draft the two sentences that kill an Art 62 argument for a state seeking to leave a fisheries treaty because stocks have moved: the narrow rule with Gabcikovo-Nagymaros, then its application, and then add the one fact pattern that might genuinely satisfy the article.

Check your understanding

Auto-marked drills. Answer, then see the authority in the feedback.