Law of Treaties in International Law

The law of treaties is a fundamental part of public international law. Treaties allow States to create binding international obligations on matters ranging from peace and security to trade, human rights, environmental protection and diplomatic relations.

Key Takeaways

• A treaty is an international agreement governed by international law, ordinarily concluded between States in written form. It may be called a convention, agreement, protocol, covenant, charter or by another name.
• The Vienna Convention on the Law of Treaties, 1969, commonly known as the VCLT, provides the principal legal framework governing treaties between States. It deals with their conclusion, application, interpretation, amendment, validity and termination.
• A State may express its consent to be bound by a treaty through signature, ratification, acceptance, approval, accession, exchange of instruments or another agreed method.
• Once a treaty is in force, the principle of pacta sunt servanda requires the parties to perform it in good faith. A State generally cannot rely upon its internal law to justify failure to perform its treaty obligations.
• Treaties must generally be interpreted in good faith according to the ordinary meaning of their terms, their context, and their object and purpose. The rules are principally found in Articles 31 to 33 of the VCLT.
• Reservations allow a State, subject to legal limits, to exclude or modify the legal effect of particular treaty provisions in their application to that State.
• A treaty does not automatically end simply because circumstances have changed. Invalidity, withdrawal, termination or suspension can generally take place only on recognised grounds such as material breach, impossibility of performance, fundamental change of circumstances or conflict with a peremptory norm of international law.

What Is a Treaty in International Law?

A treaty is an international agreement through which subjects of international law, particularly States, undertake legally binding obligations towards one another. Treaties are one of the most important sources of international law because they allow States to expressly create legal rights and duties by consent.

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Article 2(1)(a) of the Vienna Convention on the Law of Treaties describes a treaty, for the purposes of the Convention, as an international agreement concluded between States in written form and governed by international law, whether contained in a single instrument or in two or more related instruments and whatever its particular designation.

Therefore, the name given to an instrument does not determine whether it is a treaty. Instruments called conventions, protocols, covenants, charters, agreements or exchanges of notes may constitute treaties when they satisfy the necessary legal requirements.

The central consideration is whether the parties intended the agreement to be governed by international law and to create international legal obligations.

What Is the Vienna Convention on the Law of Treaties, 1969?

The Vienna Convention on the Law of Treaties, 1969 is the principal international instrument dealing with treaties between States.

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The Convention was adopted on 22 May 1969 and entered into force on 27 January 1980. It developed from the work of the International Law Commission on the codification and progressive development of treaty law.

The VCLT deals with matters such as:

• Conclusion and entry into force of treaties: It lays down rules concerning adoption, authentication, consent to be bound and commencement of treaty obligations.
• Observance and application of treaties: It establishes important principles governing the performance and territorial application of treaties.
• Interpretation of treaties: Articles 31 to 33 provide the principal rules for determining the meaning of treaty provisions.
• Treaties and third States: The Convention explains when treaty rights or obligations may affect States that are not parties.
• Amendment and modification: It regulates circumstances in which States may change existing treaty obligations.
• Invalidity of treaties: It identifies circumstances such as fraud, coercion, error and conflict with jus cogens that may affect validity.
• Termination and suspension: It provides recognised grounds on which treaty obligations may be terminated or suspended.

Several rules reflected in the Convention are also recognised as rules of customary international law. This is particularly significant where the Convention does not directly apply to a particular treaty or State.

The 1969 Convention primarily concerns treaties between States. Treaties involving international organisations are separately addressed by the Vienna Convention on the Law of Treaties between States and International Organizations or between International Organizations, 1986.

Who Has the Capacity to Conclude Treaties?

Article 6 of the VCLT recognises that every State possesses the capacity to conclude treaties.

However, a State necessarily acts through natural persons. The authority of the individual representing the State is therefore important.

What Are Full Powers?

A person may represent a State for concluding a treaty when appropriate full powers have been produced or when the circumstances show that the States concerned intended to regard that person as representing the State without requiring full powers.

Certain officials, by virtue of their functions, normally do not need to produce full powers for specified treaty acts.

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These include:

• Heads of State;
• Heads of Government; and
• Ministers for Foreign Affairs.

These officials can generally perform acts relating to the conclusion of treaties on behalf of their State without producing full powers.

Heads of diplomatic missions may represent their States for adopting the text of a treaty between the accrediting State and the State to which they are accredited. Similarly, representatives accredited to an international conference or organisation may perform certain treaty-related acts within the limits recognised by Article 7.

How Are Treaties Made?

Treaty-making normally involves several stages. The precise process depends upon the treaty and the intention of the participating States.

Negotiation

States first negotiate the proposed terms of the treaty. Representatives discuss their respective interests, obligations and the language that will ultimately form part of the agreement.

Negotiations may take place bilaterally between two States or through international conferences involving numerous States.

Adoption of the Text

After negotiations, participating States formally adopt the treaty text.

Article 9 of the VCLT contains rules concerning adoption. In an international conference, the text is ordinarily adopted by a two-thirds majority of States present and voting unless the participating States decide upon a different rule.

Authentication of the Text

Authentication establishes that the adopted treaty text is final and authoritative.

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It may occur through a procedure specifically mentioned in the treaty or otherwise agreed by the participating States. Signature, signature ad referendum or initialling may serve this purpose in appropriate circumstances.

Consent to Be Bound

Adoption of a treaty text does not necessarily mean that every negotiating State has become legally bound by it.

Under Article 11 of the VCLT, consent to be bound may be expressed through:

• signature;
• exchange of instruments constituting a treaty;
• ratification;
• acceptance;
• approval;
• accession; or
• any other agreed means.

The applicable method depends upon the treaty provisions and the intention of the States concerned.

What Is the Difference Between Signature, Ratification and Accession?

Signature, ratification and accession are connected with the process through which States express consent to treaties, but they perform different legal functions.

Signature

Signature may itself establish consent to be bound where the treaty provides for that result or where the intention of the negotiating States establishes it.

In many multilateral treaties, however, signature is followed by ratification before the State becomes legally bound.

Even where signature does not immediately make the State a party, Article 18 of the VCLT may require a signatory State to refrain from acts that would defeat the object and purpose of the treaty, subject to the conditions laid down in that provision.

Ratification

Ratification is the international act through which a State formally establishes its consent to be bound where ratification is required.

International ratification should be distinguished from domestic constitutional procedures that may have to be completed before a State undertakes the international act of ratification.

The position relating to treaties under the Indian Constitution is particularly relevant when examining how international treaty obligations interact with the constitutional and legislative framework in India.

Accession

Accession enables a State that did not sign a treaty during the period open for signature to subsequently become a party where the treaty permits accession or the relevant States otherwise agree.

Once effective, accession ordinarily produces the same treaty obligations for the acceding State as ratification does for a ratifying State, subject to the terms of the treaty.

When Does a Treaty Enter into Force?

A treaty enters into force in the manner and on the date provided by the treaty itself or as agreed by the negotiating States.

Multilateral treaties commonly require a particular number of ratifications, acceptances, approvals or accessions before entering into force.

A distinction must therefore be maintained between consent to be bound and entry into force. A State may have completed the required act expressing consent while the treaty itself is still awaiting the conditions necessary for its commencement.

Article 25 of the VCLT also recognises provisional application. A treaty or part of a treaty may be applied provisionally before formal entry into force where the treaty provides for provisional application or the negotiating States otherwise agree.

What Is Pacta Sunt Servanda?

Pacta sunt servanda means that agreements must be kept.

Article 26 of the VCLT provides that every treaty in force is binding upon its parties and must be performed by them in good faith. The principle of pacta sunt servanda is therefore one of the foundations of international treaty law.

Without such a rule, international agreements would have little stability because States could disregard treaty obligations whenever compliance became inconvenient.

Good faith requires States to perform treaty obligations genuinely and consistently with the commitments undertaken under the treaty.

Closely connected with this principle is Article 27 of the VCLT. It provides that a party may not generally invoke its internal law as justification for its failure to perform a treaty.

International treaty obligations and domestic implementation are therefore separate legal questions. Difficulties arising under national law do not ordinarily eliminate a State’s international obligations.

What Are Reservations to Treaties?

A reservation is a unilateral statement made by a State when signing, ratifying, accepting, approving or acceding to a treaty through which it seeks to exclude or modify the legal effect of certain provisions of the treaty in their application to that State.

Reservations are especially important in multilateral treaties because States may accept the general framework of a treaty while having concerns about particular obligations.

When Is a Reservation Permitted?

Article 19 of the VCLT generally permits a State to formulate a reservation unless:

1. the treaty prohibits reservations;
2. the treaty permits only specified reservations and the proposed reservation is not among them; or
3. the reservation is incompatible with the object and purpose of the treaty.

The object-and-purpose test has significant importance in international law.

In the Reservations to the Convention on the Prevention and Punishment of the Crime of Genocide Advisory Opinion, 1951, the International Court of Justice considered the compatibility of reservations with the object and purpose of the Genocide Convention.

The decision influenced the later development of the rules on reservations reflected in the VCLT.

What Happens When Another State Objects to a Reservation?

Other contracting States may accept or object to a reservation.

The precise legal effect depends upon the nature of the reservation, the treaty provisions and the response of other States. An objection does not necessarily prevent treaty relations from arising between the objecting State and the reserving State unless the objecting State clearly expresses an intention that the treaty should not enter into force between them.

How Are Treaties Interpreted?

Treaty interpretation determines the meaning and legal effect of treaty provisions.

Articles 31 to 33 of the VCLT provide the central framework for interpretation.

General Rule of Interpretation Under Article 31

Article 31 requires a treaty to be interpreted:

• in good faith;
• according to the ordinary meaning of its terms;
• in their context; and
• in light of the treaty’s object and purpose.

These factors form part of an integrated process of interpretation rather than completely independent tests.

The context includes the treaty text as a whole together with its preamble and annexes. Certain agreements and instruments connected with the conclusion of the treaty may also form part of the relevant context.

Article 31 further requires consideration, where relevant, of subsequent agreements between the parties regarding interpretation, subsequent practice establishing agreement regarding interpretation and relevant rules of international law applicable between the parties.

Supplementary Means of Interpretation Under Article 32

Article 32 permits recourse to supplementary means of interpretation, including:

• the preparatory work of the treaty; and
• the circumstances surrounding its conclusion.

Supplementary means may be used to confirm the meaning reached through Article 31.

They may also assist where application of Article 31 leaves the meaning ambiguous or obscure or produces a result that is manifestly absurd or unreasonable.

Treaties Authenticated in More Than One Language

International treaties are frequently authenticated in several languages.

Article 33 contains rules for interpreting treaties authenticated in two or more languages. Unless the treaty provides otherwise, each authentic text is equally authoritative.

Where comparison reveals a difference in meaning that cannot be resolved through Articles 31 and 32, the meaning that best reconciles the texts having regard to the treaty’s object and purpose is to be adopted.

Do Treaties Create Obligations for Third States?

The general rule is commonly expressed through the principle pacta tertiis nec nocent nec prosunt, meaning that treaties generally neither impose obligations nor confer rights upon third States without the conditions recognised by international law.

Article 34 of the VCLT states that a treaty does not create either obligations or rights for a third State without its consent.

Obligations for Third States

An obligation may arise for a third State under a treaty provision where:

• the parties to the treaty intend that provision to establish an obligation for the third State; and
• the third State expressly accepts the obligation in writing.

Rights for Third States

A treaty provision may also create a right for a third State where the parties intend to confer such a right and the applicable conditions under the VCLT are satisfied.

The rules concerning third States preserve the consent-based nature of treaty obligations.

Can Treaties Be Amended or Modified?

Treaties are not necessarily permanent in their original wording. States may agree to amend treaty provisions as political, economic, technological or legal circumstances develop.

The VCLT distinguishes between an amendment of a treaty and, particularly in multilateral treaties, modification of treaty relations between certain parties.

Modification between particular parties is subject to important restrictions. It should not adversely affect the rights of other parties or interfere with obligations whose performance is essential to the effective execution of the treaty as a whole.

When Can a Treaty Be Invalid?

International law recognises limited circumstances in which a State’s consent to a treaty or the treaty itself may be legally defective.

The VCLT does not permit States to disregard treaty obligations merely because they later consider the agreement disadvantageous.

Important grounds affecting validity include error, fraud, corruption and coercion.

Error

Article 48 permits a State, in limited circumstances, to invoke an error relating to a fact or situation that it assumed existed when the treaty was concluded and that formed an essential basis of its consent.

The rule is subject to restrictions. A State cannot ordinarily rely upon error where its own conduct contributed to the error or where the circumstances should have placed it on notice of the possibility of error.

Fraud

Under Article 49, where a State has been induced to conclude a treaty by the fraudulent conduct of another negotiating State, it may invoke the fraud as invalidating its consent to be bound.

Corruption of a State Representative

Article 50 deals with circumstances where the consent of a State has been obtained through corruption of its representative, directly or indirectly, by another negotiating State.

Coercion of a Representative

Under Article 51, the expression of a State’s consent has no legal effect where it has been procured through coercion of its representative by acts or threats directed against that representative.

Coercion of a State

Article 52 provides that a treaty is void where its conclusion has been procured by the threat or use of force in violation of the principles of international law embodied in the Charter of the United Nations.

What Is the Effect of Jus Cogens on Treaties?

A particularly important limitation upon treaty-making arises from jus cogens under international law, also known as peremptory norms of general international law.

Article 53 of the VCLT provides that a treaty is void if, at the time of its conclusion, it conflicts with a peremptory norm of general international law.

A peremptory norm is a norm accepted and recognised by the international community of States as a whole as one from which no derogation is permitted and which may be modified only by a subsequent norm of general international law having the same character.

Examples commonly associated with peremptory norms include prohibitions relating to genocide, slavery, torture and aggression, although identifying the precise content and scope of jus cogens requires careful examination of international law.

Article 64 addresses the emergence of a new peremptory norm. Where a new jus cogens norm emerges, an existing treaty that conflicts with that norm becomes void and terminates.

Thus, States cannot use treaty-making power to validly contract out of fundamental peremptory rules recognised by the international legal order.

How Can a Treaty Be Terminated or Suspended?

Treaty obligations may be terminated or suspended in accordance with the treaty itself, by consent of the parties or on grounds recognised under international law.

Termination must be distinguished from invalidity. Invalidity concerns circumstances affecting the legal validity of consent or the treaty, while termination generally concerns bringing an otherwise valid treaty to an end.

Termination According to the Treaty

A treaty may itself specify its duration and the circumstances in which a party may withdraw or terminate participation.

The applicable treaty provisions must ordinarily be followed.

Termination by Consent

The parties to a treaty may agree to terminate it or suspend its operation.

International law generally respects such consensual arrangements because treaty obligations themselves arise principally through State consent.

Material Breach

Article 60 recognises material breach as a ground that may affect the operation of a treaty.

A material breach includes:

• repudiation of the treaty that is not sanctioned by the VCLT; or
• violation of a provision essential to the accomplishment of the object or purpose of the treaty.

The consequences differ between bilateral and multilateral treaties.

For a bilateral treaty, a material breach by one party may entitle the other party to invoke the breach as a ground for terminating the treaty or suspending its operation.

Multilateral treaties involve more complex rules because the rights and obligations of several States may be affected.

Supervening Impossibility of Performance

Article 61 recognises supervening impossibility of performance in restricted circumstances.

The principle may apply where impossibility results from the permanent disappearance or destruction of an object indispensable for the execution of the treaty.

Temporary impossibility may justify suspension rather than termination in appropriate circumstances.

The ground is generally unavailable where the impossibility results from the invoking State’s own breach of the treaty or another international obligation owed to a treaty party.

Fundamental Change of Circumstances

The doctrine traditionally called rebus sic stantibus is reflected in Article 62 of the VCLT.

A fundamental change of circumstances may be invoked only under strict conditions.

The circumstances existing when the treaty was concluded must have constituted an essential basis of the parties’ consent, and the effect of the change must radically transform the extent of the obligations still to be performed.

The doctrine cannot ordinarily be invoked in relation to a treaty establishing a boundary. It is also unavailable where the fundamental change results from a breach by the State seeking to rely upon it.

The restrictive nature of Article 62 reflects the importance international law attaches to stability in treaty relations.

Important Cases on the Law of Treaties

Judicial decisions have played an important role in explaining the application of treaty rules, particularly in relation to reservations, interpretation and termination.

Reservations to the Genocide Convention Advisory Opinion, 1951

The International Court of Justice considered reservations made to the Convention on the Prevention and Punishment of the Crime of Genocide.

The Court emphasised the relevance of the convention’s object and purpose when determining the legal implications of reservations.

The advisory opinion became highly influential in the development of the modern international law governing reservations to multilateral treaties.

Gabčíkovo-Nagymaros Project, 1997

The Gabčíkovo-Nagymaros Project (Hungary/Slovakia) concerned a treaty concluded in 1977 relating to the construction and operation of a system of locks on the Danube.

The ICJ considered several treaty-law arguments, including necessity, impossibility of performance, fundamental change of circumstances, material breach and termination.

The Court adopted a restrictive approach towards fundamental change of circumstances. The circumstances relied upon were not considered sufficient to satisfy the requirements for terminating the treaty on that ground.

The case illustrates the strong international-law preference for maintaining the stability of treaty obligations.

Kasikili/Sedudu Island Case, 1999

The Kasikili/Sedudu Island case concerned interpretation of an 1890 treaty in determining the boundary between Botswana and Namibia.

The ICJ reaffirmed that the rule of treaty interpretation reflected in Article 31 of the VCLT represents customary international law.

The decision is significant because it demonstrates that treaty interpretation principles reflected in the VCLT may operate as customary rules even where the Convention itself does not directly govern the treaty concerned.

What Is the Position of Treaties in Indian Law?

The international validity of a treaty and its operation within India’s domestic legal system involve related but distinct questions.

The Constitution contains provisions relevant to international relations and implementation of international obligations. Article 51 encourages respect for international law and treaty obligations, while Article 253 gives Parliament power to make laws for implementing treaties, agreements and conventions with other countries and decisions made at international conferences, associations or other bodies.

The relationship between a treaty, international law and constitutional rights also requires consideration of India’s constitutional framework. An international commitment cannot by itself override the Constitution, and domestic implementation may require legislation where treaty obligations affect existing municipal law or require changes that cannot be achieved solely through executive action.

Indian courts have also referred to international conventions and international legal principles in appropriate cases, particularly where they are consistent with domestic law and constitutional guarantees.

Importance of the Law of Treaties

Treaties are among the most important instruments through which States organise international relations. They establish rights and duties in fields including international trade, environmental protection, diplomatic relations, armed conflict, human rights, maritime law and international cooperation.

The law of treaties performs several important functions:

• Legal certainty: Treaty law establishes rules for determining when States become legally bound, when treaties enter into force and what obligations arise from them.
• Stability of international relations: Principles such as pacta sunt servanda protect international agreements from arbitrary disregard and strengthen confidence between States.
• Protection of State consent: Rules governing signature, ratification, accession, reservations and third States ensure that treaty obligations generally rest upon legally recognised consent.
• Orderly interpretation: Articles 31 to 33 of the VCLT provide a structured method for determining the meaning of treaty provisions and resolving interpretative disputes.
• Control over termination: Strict rules governing withdrawal, suspension and termination prevent States from escaping treaty obligations merely because continued performance has become inconvenient.
• Protection of fundamental international norms: The rules relating to jus cogens ensure that the freedom of States to conclude treaties does not extend to agreements contrary to peremptory norms of international law.
• Peaceful international cooperation: Treaty law enables States to establish predictable legal frameworks for cooperation across political, economic, humanitarian, environmental and security matters.

Conclusion

The law of treaties provides the legal framework through which international agreements are created, interpreted, performed, modified and brought to an end. The Vienna Convention on the Law of Treaties, 1969 is central to this framework and reflects several principles that are also recognised as customary international law.

A treaty begins with negotiation and State consent, but treaty law extends far beyond its formation. Principles relating to good-faith performance, reservations, interpretation, third States, amendment, invalidity and termination determine how treaty obligations operate throughout their existence.

Among these rules, pacta sunt servanda remains fundamental because international cooperation depends upon States respecting the agreements they have undertaken. At the same time, international law recognises carefully limited circumstances in which treaty consent may be defective or continued performance may cease to be legally required.

The law of treaties therefore balances two important considerations: respect for the sovereign consent of States and the need for stability, predictability and good faith in international legal relations.

Note: This article was originally written by Shubham Shukla (Christ University) and published on 07 June 2021. It was subsequently updated by the LawBhoomi team on 30 September 2026.