Public International Law/Sources of International Law/Treaty Law





Author: Taxiarchis Fiskatoris and Marko Svicevic Required knowledge:

§ 1 History of International Law

§ 2 Nature and Purpose of International Law

Learning objectives:
 * § 2.2 Consent

Understand and define the term 'treaties' as international agreements in international law.

Explain key characteristics of treaties.

Understand how treaties are negotiated, drafted, and enter into force.

Understand how treaties are terminated or invalidated.

Examine how treaties are interpreted under the Vienna Convention on the Law of Treaties.

Explain what reservations entail and what their effect on treaties are.

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A. Introduction
Article 38(1) of the ICJ Statute names 'conventions, whether general or particular, establishing rules expressly recognized by… states' as the first source of public international law. More commonly known as 'treaties', they represent the most trusted and least controvertible avenue for States to express their consent to international legal rules. The United Nations Treaty Collection, which registers and publishes lists of treaties in accordance with Article 102 of the UN Charter, has over 250 000 treaties in its collection.

The basic international instrument of treaty law is the 1969 Vienna Convention on the Law of Treaties (VCLT), which is the main focus of this chapter. This chapter will therefore define and explain what treaties are and what role they play in international law, how treaties are negotiated and drafted, how they may be invalided or terminated, and how treaties are to be interpreted. As of June 2023, the VCLT has been ratified by 116 States. Most of its provisions have codified pre-existing customary international law, while other provisions have generated new custom. The VCLT only 'applies to treaties between States'. The rules regulating treaties between States and international organisations, and between international organisations have also been imprinted in a Convention, which has not yet entered into force. A third international convention with direct relevance to treaty law is the Vienna Convention on Succession of States in Respect of Treaties, which is in force but poorly ratified. All three have been drafted by the United Nations´ International Law Commission (ILC). The ILC is also responsible for several non-binding instruments which contribute to the overall study and scope of the law of treaties, such as the 2011 'Draft Articles on the Effects of Armed Conflicts on Treaties', the 2018 'Draft Conclusions on Subsequent Agreements and Subsequent Practice in Relation to the Interpretation of Treaties', and the 2021 'Draft Guide to Provisional Application of Treaties'.

1. Treaty law in context and of the times
Before going into more specific details on the rules of treaty law found in the VCLT, it is worth noting that the law of treaties is a complex and often delicate intervention in international law, given its very nature. Treaty law forms part and parcel of the 'nuts and bolts' of international law. As such, it is interwoven in almost every other discipline of international law. For example, while treaties are traditionally concluded between states, the role of non-state actors, broadly speaking has increasingly brought about questions. Non-governmental organisations, although without legal capacity to conclude treaties, have and continue to play a growing role in the drafting and negotiating of treaties. Likewise, as entities capable of legal personality, questions too arise as to what influence and obligations non-governmental organisations derive from general treaty law, such as those found in universal and regional human rights treaties. In turn this questions the broader role that non-state actors play in regard to the creation, development and clarification of the law of treaties. It is also worth noting that treaty law, although its progressive development and codification enhances clarity, is not without controversy nor ambiguity. Worthy of recollection is the fact that at the time the VCLT was negotiated and eventually adopted, not all states we see today were independent, and not all people's free from oppression of European and Western colonisation. Any consideration of the VCLT as a treaty regulating other treaties must therefore bear in mind its historical context. It has been well pointed out that, for example, applying a purely positivist approach to the VCLT would effectively if not entirely marginalise its role in international law.

These difficult issues more often than not transcend the VCLT itself, plaguing by extent the entirety of the law of treaties. Consider for example the effect of treaty law making long before the era of human rights and the adoption of the VCLT. The partitioning of Africa was in many ways effected through treaty law. These international agreements at the time were carefully negotiated with people's across regions. Despite what were in fact treaties which ultimately laid claim to territory and to the detriment of peoples of that territory, they were not necessarily directed at the various people's they were negotiated with, but rather as 'legal' symbols against rival European powers. While it is oftentimes easy to dismiss these practices and the corresponding effects of treaty law as relegated to the pages of history (once described as regulating 'common interests in international quasi-legislation, imposed on Africa from above'), the potential for these effects remains today. Indeed, while a fundamental principle of treaty law is that treaties are to be negotiated and implemented in good faith, there remain numerous cases even today where the law of treaties has fallen short of this expectation.

2. Defining treaties
Article 2(1)(a) of the VCLT defines a treaty as 'an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation'.

i. An international agreement concluded between States
'Every State possesses capacity to conclude treaties'. Although reference is made in the VCLT exclusively to States, the definition of treaties beyond the limited scope of the VCLT extends to international organisations, which have an autonomous international legal personality. Whether an international organisation has the capacity to conclude treaties depends on the rules of that organisation, and especially its constituent instrument. Until such a time as the 1986 Vienna Convention on the Law of Treaties between States and International Organisations or between International Organisations enters into force, which is admittedly very similar to the VCLT, such treaties are based on other sources of international law, in particular customary international law.

ii. In written form
For an international agreement to be called a treaty, it must be in written form, but not necessarily on paper. This is exactly the feature that renders treaties the most predictable and hence reliable source of public international law. Oral international agreements, as well as 'international agreements concluded between States and other subjects of international law or between such other subjects of international law' are not treaties per se, but they may still have legal effects. Such agreements are governed by other sources of public international law and not necessarily the provisions of the VCLT.

iii. Whether embodied in a single instrument or in two or more related instruments
Treaties are usually contained in a single document, but they do not always need to be. Exchange of letters (diplomatic notes), and even records of meetings between State representatives may constitute treaties if the intention of the parties was to create through them binding effects under international law.

iv. Governed by International Law
The intention to establish obligations and/or rights under international law is a key requirement for the characterisation of an international agreement as a treaty. States (and international organisations) are free to sign contractual agreements governed by national law (e.g. for leasing an embassy’s premises), which cannot be considered treaties. They are also free to enter international agreements not giving rise to obligations and/or rights under international law. Such agreements are often called “Memoranda of Understanding” (MoU). However, one should not pay too much attention to the headline of an agreement, as MoUs may in reality be proper treaties if the intention of the parties to give them binding effect under international law can be discerned. The intention to establish binding legal relations must be manifest within the text and context of the treaty.

v. Whatever its particular designation
As long as an international agreement fulfils the above four characteristics, it is from a legal point of view a treaty, whatever its name. Some of the most common names attached to a treaty are the following:


 * Article 38 of the ICJ Statute uses the term “Convention”. This is usually the name given to treaties prepared within an international organisation (e.g. UN Convention on the Law of the Sea; European Convention on Human Rights)
 * A “Protocol” is in most cases a treaty that supplements a pre-existing treaty with additional rights or obligations (e.g. Additional Protocols to the 1949 Geneva Conventions on International Humanitarian Law; Kyoto Protocol to the United Nations Framework Convention on Climate Change)
 * “Charter” is the label preferred for the constitutive treaties of international organisations (e.g. UN Charter, OAS Charter, OAU Charter, ASEAN Charter). The term may also designate a document that sets out a grant of rights or privileges (e.g. EU Charter of Fundamental Rights, African Charter on Humans and Peoples’ Rights).
 * The treaty establishing an international court or tribunal is often called a “Statute” (e.g. ICJ Statute; ICC Statute)
 * The word “Covenant” originates in religious scripts and traditionally refers to a solemn promise to engage in or refrain from a specified action. In international law it is used in the title of two major human rights conventions: International Covenant on Civil and Political Rights (ICCPR) and the International Covenant on Economic, Social and Cultural Rights (ICESCR), both adopted in 1966. It was also used in the case of the instrument establishing the League of Nations.
 * The label “Pact”, more common during the inter-war period, seems to connote a deal, that is not only legally but also morally binding (e.g. 1928 Kellogg-Briand Pact for Renunciation of War as an Instrument of National Policy)
 * The term 'Agreement' is used as an umbrella term covering both treaties and other instruments not meeting the VCLT criteria. In a narrow sense, the label 'agreement' is usually employed for treaties of a technical or administrative character between two States, or between a State and an international organisation.

3. Classification of treaties
Treaties establishing mutual rights and obligations between two parties are classified as bilateral. The great bulk of international treaties are bilateral in nature. A multilateral treaty is, on the other hand, a binding international agreement between many parties. A treaty between more than two but still not many parties can also be classified as plurilateral.

Most bilateral and plurilateral treaties merely create mutual rights and/or obligations for their parties, similarly to typical contracts of domestic law (contractual treaties). Although multilateral treaties also establish binding rights and/or obligations, most of them may eventually make, modify, elucidate and stabilize, or progressively develop international law more generally (law-making treaties). To be sure, several multilateral treaties purport to do so. In reality, the distinction between 'contractual treaties' and 'law-making treaties' is not always obvious.

4. Observance and application of treaties
The whole branch of international treaty law is premised on the fundamental legal principle 'pacta sunt servanda' (agreements must be respected). Article 26 of the VCLT enunciates that '[e]very treaty in force is binding upon the parties to it and must be performed by them in good faith'. A change of government does not release the State from its treaty obligations, unless the new government can raise a valid ground for the termination of the treaty. Besides, '[a] party may not invoke the provisions of its internal law as justification for its failure to perform a treaty'.

The flipside to the principle 'pacta sunt servanda' is the principle 'pacta tertiis nec nocent nec prosunt' (agreements neither injure nor benefit third parties). This is enshrined in Article 34 of the VCLT, according to which a 'treaty does not create either obligations or rights for a third State without its consent'. For third parties, an agreement to which they have not consented constitutes 'res inter alios acta' (things done among others). When parties to a treaty intend to impose an obligation on third parties, the latter must accept the obligation in writing. However, when parties to a treaty intend to afford a right to third parties, the consent of the latter is presumed as long as they do not raise an objection. If a treaty provision acquires the status of a customary rule of international law, it then becomes binding on third parties, even without their expressed consent.

The question of whether treaties are directly binding on individuals or other non-State actors is of marked importance in the context of international human rights law and international criminal law, but has yet to be doctrinally settled. Finally, unless otherwise agreed by its parties, treaties do not apply retroactively.

1. Drafting and Negotiation
Before adopting a bilateral treaty, States normally hold a series of meetings of diplomats and legal experts who negotiate and draft the terms of the treaty. Multilateral treaties, especially 'law-making treaties', are negotiated at international conferences, usually summoned by international organisations. At international conferences, where negotiations are more difficult due to the number of participants, States often debate on the basis of optional draft texts prepared by permanent or special committees of experts, such as the UN International Law Commission. The drafting process of a treaty may take many years. States are free to decide the place, time frames, set up, and rules of procedure of a conference. The VCLT only regulates that, unless participants decide otherwise, the minimum requirement for the adoption of the text of a treaty at an international conference is a two-thirds majority of the 'States present and voting'. In practice, States resort to voting only if consensus appears impossible.

2. Signature
Successful negotiations conclude with the adoption of the text of the treaty, and its recognition as authentic and definitive. The most common way for authenticating the finalised text of a treaty is its signature. Signing a treaty is an expression of a government’s intention to render the treaty binding for its State in due course. Nonetheless, the signature alone seldom establishes the consent of States to be bound by the treaty. Ordinarily, it is evident from the text of the treaty when no further steps are required.

In any case, the signature entails the legal obligation of the signatory 'to refrain from acts which would defeat the object and purpose of a treaty' until the ratification of the treaty, or until the signatory 'shall have made its intention clear not to become a party to the treaty'. To be sure, such interim obligation is vague and open to contradictory interpretations.

3. Consent to be bound
As a matter of rule, States establish on the international plane their consent to be bound by a treaty through the acts of 'ratification', 'acceptance', 'approval' or 'accession', although 'any other [agreed] means' are an option.

A second step after signature offers to States the required time to reconsider the treaty as a whole, eventually to submit it to parliamentary scrutiny and approval, or to enact respective legislation. It may take many years between signature and ratification, as there are no general time limits, unless the treaty specifies them. After all, States are under no obligation to ratify a treaty that they have signed.

Ratification of bilateral treaties occurs through the exchange of documents called 'instruments of ratification', which are issued for that scope by the competent authorities of the contracting States. A mere mutual notification of completion of all domestic procedures that give effect to the treaty may in routine cases replace the ceremonial exchange of instruments.

Treaties remain commonly open for signature until an arranged date. States that did not exist or sign the treaty before that date can still adhere to the treaty if the treaty or its parties allow it. The international act with which a State avails itself of the opportunity to become a party to a treaty previously adopted by other is called 'accession'. 'Accession' is an expression of consent to be bound by a treaty and hence it has the same legal effect as ratification. It usually happens after the treaty has entered into force, but, depending on the treaty, it can also take place before.

Ratification, acceptance, approval or accession of multilateral treaties is accomplished with the deposit of the respective instruments with the depositary. The depositary is one or more States, an international organisation, or the secretary-general of an international organisation, especially the United Nations. The depositary is normally designated by the treaty, among others to keep custody of the original text of the treaty, to collect all documents or communications relating to it, and inform respectively all parties concerned.

4. Entry into force
Ratification signals a State’s consent to be bound by a treaty. However, it does not signify an immediate assumption of the obligations and/or rights emanating from the treaty, which only begins when the treaty enters into force. After the ratification and before the entry of the treaty into force, the States must still 'refrain from acts which would defeat the object and purpose of a treaty, 'provided that such entry into force is not unduly delayed. Although it is very infrequent, States that have ratified a treaty may freely withdraw their consent to be bound before the treaty becomes operative. There may also be transitional clauses, dealing for instance with the permissibility of reservations, that take effect as from the adoption of the treaty. Most treaties contain a clause specifying when and how they will come into force.

Bilateral treaties often enter into force at the time the two parties exchange the ratification instruments, while treaties in simplified form can readily come into force immediately after signature. In case of an accession, the treaty enters into force for the acceding party on the date of the deposit of the accession instrument, or after a short period of time, if there was a corresponding provision with respect to the initial entry into force of the treaty.

5. Registration and Publication
Article 102 of the Charter of the United Nations requires that 'every treaty and every international agreement entered into by any Member of the United Nations… shall as soon as possible be registered with the Secretariat and published by it'. The provision pertains to both treaties that fulfil the aforementioned criteria, and other kinds of international agreements. Registration and publication with the United Nations Treaty Series is meant to eradicate the conflictual dynamic of secret diplomacy, and to enable public access. The UN Charter warns that unregistered international agreements cannot be invoked before any organ of the United Nations, including obviously the International Court of Justice. The practice of UN organs is less strict than the rule.

Registration should not be confused with a deposit of a ratification instrument with the Secretary-General of the United Nations. Treaties and international agreements can only be registered with the UN after their entry into force. The registration and publication duty extends to cases of treaty amendments. The registration by just one party to the treaty is adequate, while multilateral treaties are registered by their depositary. The UN does not impose any time constraints for registration. More importantly, 'non-registration or late registration… does not have any consequence for the actual validity of the agreement, which remains no less binding upon the parties'. Vice versa, the act of registration cannot turn a non-binding international agreement into a binding treaty.

i. Amendment
An “amendment” is a change of one or more treaty provisions, usually with the aim of updating or enhancing the treaty regime. Given that amendments affect all parties to the treaty, they must obtain the consent of parties to be bound by the amended provision. Thus, amendments are negotiated, signed, ratified, brought into force, registered and published. Some treaties require unanimity for an amendment to pass. If amendments can pass with a majority, parties that do not express their consent to be bound by the amendment remain bound by the previous provision, in conformity with the principles pacta tertiis nec nocent nec prosunt, and pacta sunt servanda. However, new parties acceding to the treaty accept the treaty as amended. Between the parties that have ratified the amendments or acceded to the amended treaty, and those that have not ratified them, it is the old provision that remains effective. The more parties to a treaty, the more difficult its amendment. This is why many multilateral treaties lay down specific amendment procedures and requirements, which may deviate from the above canon.

ii. Review, Revision
Some treaties provide an alternative 'review' or 'revision' procedure, which refers to updating the whole or part of the treaty at a new diplomatic conference with the participation of all parties to the treaty. Review or revision takes place after a provided number of years, or following a majority vote.

iii. Modification
Furthermore, 'two or more of the parties to a multilateral treaty may conclude an agreement to modify the treaty as between themselves alone if: (a) the possibility of such a modification is provided for by the treaty; or (b) … not prohibited by the treaty'. The original treaty provisions remain applicable between those few parties and all other parties. The modification must not affect the rights and/or rights of other parties under the treaty, and must not be incompatible with the object and purpose of the treaty as a whole.

1.Termination and Suspension
A treaty may be terminated or suspended in several ways described below. The termination permanently releases the parties from any obligation to perform the treaty. The suspension releases them from their treaty obligations temporarily. However, the termination 'does not affect any right, obligation or legal situation of the parties created through the execution of the treaty prior to its termination'. It is also immaterial when there exists a parallel customary rule of international law, which continues being biding on States. Besides, '[d]uring the period of the suspension the parties shall refrain from acts tending to obstruct the resumption of the operation of the treaty'.

i. Consent-based
A treaty may be terminated or suspended with the consent of all its parties. Likewise, if all parties to a treaty adopt a new substitute treaty, the earlier treaty impliedly loses its effect. However, the old treaty remains effective if not all of its parties adhere to the new one.

Should only some of the parties no longer feel like being bound by a treaty, they may denounce it or withdraw from it, but only if such a possibility is expressly allowed, implied by the nature of the treaty, or predicated on the established consensual intention of the parties. In any event, a party wishing to exit a treaty must give notice of its intention at least one year in advance. The term 'denunciation' is mostly used with reference to bilateral treaties, which are thus terminated. The term 'withdrawal' better describes the retreat from a multilateral treaty, which continues being in force among the rest of its parties.

There is also the possibility that the treaty itself contains an expiration date, or a clear goal, the achievement of which terminates the agreement. Nonetheless, a treaty does not terminate merely because it has not reached the required ratifications number for its entry into force.

ii. After a material breach of the treaty
The operation of a treaty can also be terminated or suspended as a consequence of its material breach. The VCLT defines a material breach as '(a) a repudiation of the treaty not sanctioned by the present Convention; or (b) the violation of a provision essential to the accomplishment of the object or purpose of the treaty'.

It should be noted though, that a material breach, regardless of its gravity, does not in itself terminate or suspend the treaty. It only entitles innocent parties to pursue the termination or suspension of the treaty in whole or in part, in accordance with a predetermined procedure. The consequences of a material breach depend on the bilateral or multilateral nature of the treaty. A breach of a multilateral treaty is more probable to temporarily render the treaty ineffective between the injured and the defaulting State, rather than leading to a comprehensive termination of the treaty.

Of course, the party that breached the treaty cannot invoke its own wrongdoing to terminate or suspend the treaty. Neither can a party pursue the termination or suspension of a treaty invoking the material breach of another treaty. Besides, 'treaties of a humanitarian character', including human rights treaties, cannot be terminated or suspected on such grounds. If a material breach of a treaty causes harm to a State, secondary rules of State responsibility apply, irrespectively of whether the harmed State pursues the termination or suspension of the breached treaty.

iii. Due to a fundamental change of circumstances
On demand of several drafting States, the VCLT did not exclude the termination of or withdrawal from a treaty due to a fundamental change of circumstances which has rendered the execution of treaty obligations unexpectedly onerous or unfair. However, in order to keep it in line with the primordial principle of the sanctity of treaties (pacta sunt servanda), the VCLT sets a high threshold for the application of the so-called rebus sic standibus (so long as things stand) clause. The ICJ has also consistently upheld a very restrictive approach.

Thus such a pleading can only be made if cumulatively

a) the change is fundamental;

b) could not have been foreseen;

c) has 'radically' transformed the extent of obligations still to be performed under the treaty into something different from what originally agreed; and

d) the specific circumstances at the time of the conclusion of the treaty constituted an essential basis of the consent of the parties to be bound by the treaty.

Additionally, this ground of termination is inapplicable to treaties establishing a boundary, as well as unavailable to any party that induced the fundamental change by not performing its duties towards the other treaty parties. 'A boundary established by treaty thus achieves a permanence which the treaty itself does not necessarily enjoy'.

iv. Due to supervening impossibility of performance
A less controversial ground for termination/withdrawal is 'the permanent disappearance or destruction of an object indispensable for the execution of the treaty', which unexpectedly renders its performance not simply onerous or unfair, but impossible. If the supervening impossibility of performance is temporary, it can only lead to the suspension of the treaty. Once again, if a party contributed to the occurrence of such a situation, it cannot itself pursue the termination/suspension of the treaty on this ground. The submergence of an island under the sea level, or the natural desiccation of a river as an effect of climate change may be scenarios giving rise to such a termination/suspension ground.

v. Because of an armed conflict
The VCLT sets forth that '[t]he severance of diplomatic or consular relations between parties to a treaty does not affect the legal relations established between them by the treaty except insofar as the existence of diplomatic or consular relations is indispensable for the application of the treaty'. However, the effects of armed conflicts on treaties fall outside the scope of the Convention. The ILC has attempted though to prepare a set of non-binding Draft Articles on the matter. The general principle is that the outbreak of an international armed conflict, or a non-international armed conflict in which governmental authorities take part, may terminate or suspend a treaty as between States parties to the conflict or as between a State party to the conflict and a State that is not; but not necessarily.

When the treaty itself is silent on the matter, the answer can primarily, but not exclusively, be sought in

'a) the nature of the treaty, in particular its subject matter, its object and purpose, its content and the number of parties to the treaty; and

b) the characteristics of the armed conflict, such as its territorial extent, its scale and intensity, its duration and, in the case of non-international armed conflict, also the degree of outside involvement'.

However, there is a number of treaties, 'the subject matter of which involves an implication that they continue in operation, in whole or in part, during armed conflict'. Such are, by way of illustration, multilateral 'law-making treaties', treaties creating permanent regimes, especially treaties establishing boundaries, treaties for the international protection of human rights, treaties on international criminal justice, treaties relating to the international protection of the environment or to international watercourses and aquifers, treaties creating international organisations, treaties relating to diplomatic and consular relations, treaties relating to the international settlement of disputes, and of course treaties regulating the conduct of hostilities.

vi. Other grounds
Art 64 of the VCLT postulates an additional termination ground, namely the emergence of a new peremptory norm of general international law (jus cogens). In such an event, 'any existing treaty which is in conflict with that norm becomes void and terminates'. A jus cogens norm is defined by the VCLT as 'a norm accepted and recognized by the international community of States as a whole as a norm from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character'. To be sure, the exact content of such norms remains unclear. If the supervening jus cogens norm is not in contrast but with a treaty clause that may stand independently from its context, the norm only severs the said clause without implications for the rest of the treaty.

Art 42 of the VCLT suggests that the enumerated termination/suspension grounds are exclusive. It is debatable though whether by means of customary international law, or as forms of implied consent, desuetude or obsolescence constitute additional grounds. The former refers to a consistent practice of the parties to a treaty, that runs counter to their treaty obligations. The latter refers to the expiration of the treaty through disuse.

Another debatable termination ground is the full performance of a treaty, when the treaty itself does not explicitly provide for such eventuality.

Last but not least, it is only logical that a bilateral treaty comes to an end when one of the two State-parties loses its international legal personality, unless of course there is a successor State.

2. Invalidity
Under specific circumstances, treaties may lose their validity, although this occurs very rarely. Invalidity has different legal consequences compared to termination. While the latter releases the parties from their treaty obligations from the point of the termination on, invalidity exonerates the injured parties from the legal effects from the point of conclusion of the treaty. Practically, acts having been performed in execution of a void treaty before its invalidation may need to be reversed. However, claims of reversal cannot be made by a party that has generated the grounds for the invalidity.

i. Absolute grounds for invalidity
The VCLT enumerates three absolute grounds for invalidity, which automatically render the treaty null and void:

a) when the consent of a State to be bound by the treaty has been a product of coercion of a representative of a State through acts or threats directed against him or her;

b) when the consent is a product of coercion of the State itself by the illegal threat or use of force 'in violation of the principles of international law embodied in the Charter of the United Nations'. The last words exclude any legal threat or use of force after an authorisation of the UN Security Council or in self-defence. Besides, only the military use of force gives rise to invalidity. A treaty cannot be invalidated if a State has been compelled to ratify it, say, under the pressure of economic sanctions, or the political pressure from a former coloniser. Such forms of pressure have merely been condemned by the drafters of the VCLT in a non-binding declaration annexed to the Final Act of the Vienna Conference;

c) Thirdly, a treaty is void if, at the time of its conclusion, it conflicted with an existing jus cogens rule.

Even if the act of coercion pertained to a single treaty clause, or even if only a single treaty clause is against a pre-existing peremptory norm, it is still the whole treaty that automatically becomes null and void.

ii. Relative grounds for invalidity
The VCLT also lists five relative grounds for invalidity, which do not render the treaty void ab initio, but voidable. Put differently, relative grounds do not immediately nullify the treaty, but rather give to an affected State the right to retrospectively annul its consent to be bound by that treaty. This would practically mean the nullification of a bilateral treaty, or a withdrawal of the victim State from a multilateral treaty with retrospective effect. In the latter scenario, though, the rights and obligations of other treaty parties would remain unaffected. Contrary to the consequences of absolute grounds, there is the possibility for severing the clauses to which the relative grounds are related, instead for nullifying the whole treaty.

Relative grounds, potentially invalidating the consent of an affected State, are the following:

a) A 'manifest' violation of 'fundamental' internal law provisions regarding competence to conclude treaties. The VCLT goes on to clarify that '[a] violation is manifest if it would be objectively evident to any State conducting itself in the matter in accordance with normal practice and in good faith'. The term 'fundamental' points to constitutional or equivalent rules.

b) Omission by a State representative to observe specific restrictions on authority to express the consent of his/her State, on the precondition that the other negotiating parties had been duly notified.

c) An error that 'relates to a fact or situation which was assumed by [the affected] State to exist at the time when the treaty was concluded and formed an essential basis of its consent to be bound by the treaty. Had the error been foreseeable or caused by the affected State itself, it cannot be invoked as a ground for invalidity. Likewise if the error relates only to the wording of treaty text.

d) Fraudulent conduct of another negotiating State.

e) Corruption of a representative of the affected State, directly or indirectly by another negotiating State.

1. Reservations
Article 2(1)(d) of the VCLT defines a reservation as 'a unilateral statement, however phrased or named, made by a State, when signing, ratifying, accepting, approving or acceding to a treaty, whereby it purports to exclude or to modify the legal effect of certain provisions of the treaty in their application to that State'. Reservations to treaties are one way in which States express their disagreement with certain provisions and exclude their legal effect to such States. In this way, a State that wishes to be party to a specific treaty may nonetheless adopt and ratify the treaty without compromising the entirety of the instrument because of one or several provisions it disagrees with. For such reasons, reservations provide a compromise whereby, especially for multilateral treaties, they can achieve widespread adoption and acceptance by numerous States.

While reservations are a useful tool for States in excluding or modifying a treaty's legal effects, there are certain cases where reservations are prohibited. In principle, three situations prohibit reservations:


 * if the reservation is incompatible with the object and purpose of a treaty
 * if the reservation is prohibited by the treaty
 * if the treaty provides only for specified reservations and the reservation is question fall beyond the scope of such specified reservation.

These limitations to reservations which States may make are for the most part reasonable. For example, reservations which are incompatible with the object and purpose of a treaty are prohibited in that, if made, could render the entirety or very logic behind the treaty void.

Treaties focusing on issues of climate change and the environment are one example where reservations are often explicitly prohibited. For example, Article 25 of the Kyoto Protocol provides that '[n]o reservations may be made to this Protocol'. Another example of a treaty which prohibits reservations is Article 120 of the Rome Statute of the International Criminal Court.

Where a treaty expressly provides for reservations, there is no need for other States party to the treaty to accept the reservation made by one of the State parties. In some cases however, State parties to a treaty may need to accept or even object to reservations made to such treaty. If, for example, there is a limited number of negotiating States and the object and purpose of the treaty requires its application to all the parties as an essential condition of their consent, then reservations made to such treaty need to be accepted by all parties. Another example where reservations needs to be accepted is where the treaty in question is a constituent instrument of an international organisation, in which case, a competent organ of that organisation needs to accept the reservation.

Reservations and objections to reservations may be withdrawn at any time and do not require the consent of any State which had previously accepted such reservation.

2. Interpretative declarations
Another unilateral statement which a State can make when joining a treaty is an interpretative declaration. Interpretative declarations are statements in which a State indicates or clarifies what it understands to be the scope or nature of specific treaty provisions. Interpretative declarations do not modify the provisions of a treaty but may later be followed by other States in how they too interpret certain provisions of that treaty.

Beyond the standard interpretative declaration, a State may also make a conditional interpretative declaration. Such a declaration has the effect that a State does not wish to be bound by certain provisions unless a specific interpretation is accorded to those provisions. Conditional interpretative declarations are therefore subject to the same rules as reservations.

Although the distinction between reservations, interpretative declarations and conditional interpretative declarations may seem simple, in practice their distinction is not as easy. States sometimes use ambiguous language when entering these unilateral statements, ultimately making it difficult to determine their intention.

1. Interpretative authorities
One of the starting points in discussing treaty interpretation is precisely who has the authority to interpret treaties. Given they are legal texts distinct from the domestic laws of States, it is necessary to understand both who may interpret them and precisely how they are to be interpreted. In principle, the application of a treaty implies interpretation; since it would not be possible to apply the provisions of a treaty without first reading and interpreting its provisions. On this basis therefore, all entities concerned with the treaty in question engage in its interpretation. Subjects of international law who therefore have the competence to interpret treaties and their provisions include:


 * States
 * International organisations and their organs
 * International courts, tribunals and forums
 * Domestic courts, tribunals and forums

2. General rule of Interpretation
Article 31 of the VCLT provides the general rule of interpretation of treaties and reflects customary international law. The rules embodied in Article 31 of the VCLT adopt a multifaceted approach to interpreting treaties. Treaties are first and foremost to be interpreted in good faith and the ordinary meaning of terms are to be used, in context and in light of a treaty's object and purpose. The interpretation of a treaty includes its context, which in addition to the main text, preamble and annexes also includes other agreements which relate to the treaty made between the parties in connection with the treaty, or, an instrument made between one or more parties in connection with the treaty accepted by other parties as an instrument to the treaty.

In addition, further context taken into account when interpreting the provisions of a treaty are the subsequent agreement and subsequent practice of State parties regarding the interpretation of a treaty. In other words, subsequent agreement and subsequent practice in the application of the provisions of a treaty, may clarify how they interpret it and may even indicate they consider such an interpretation effective for purposes of apply its provisions. It also makes perfect sense that given the wording of Article 31(1), special meanings are given to terms only if the parties so intended. In practice, most treaties usually start off with a section defining terms used with the treaty, in this way clarifying how such terms are not only understood in the context of the treaty, but how they are applied throughout its provisions.

One of the reasons behind the interpretation of treaties suggested by Article 31 is that, naturally by examining the very text and context of a treaty, it is presumed that a treaty constitutes an authentic expression of the intentions of its parties. Only by examining the treaty itself can one ascertain the intention of its drafters.

3. Other theories of treaty interpretation
In addition to those rules of interpretation mentioned above, there exist a number of theories of treaty interpretation.


 * Teleological interpretation: requires that the meaning of words and terms be interpreted in light of the object and purpose of a treaty. In such cases, a teleological interpretation aims to give effect to the overall aims and objectives of a treaty.
 * Systematic interpretation: requires a treaty to be interpreted with the ordinary meaning of words and that all parts of a treaty as well as corresponding documents produced between the parties be taken into account. Such documents would include the negotiation and drafting history of a treaty.
 * Textual interpretation: requires that the ordinary meaning of words be used to interpret treaties, that such meaning be clear, and that upon interpretation, does not lead to unreasonable or absurd outcomes.
 * It is worth mentioning that other theories of interpretation may differ to those found in the VCLT. Briefly, there is no concrete position as to which theory one should adopt when interpreting treaties. Some authors take the VCLT as a point of departure, whereas others consider either the complimentary or exclusive position of other theories of interpretation.

4. Supplementary means of interpretation
Beyond the general rule of interpretation in Article 31 of the VCLT, Article 32 provides supplementary means of interpretation. According to Article 32, certain elements may be used in furthering the precise meaning of provisions if upon application of the rules of interpretation in Article 31 prove unsatisfactory (to the extent that ambiguities remain or the application of Article 31 leads to manifest absurdity or unreasonableness).

Supplementary means of interpretation under Article 32 most commonly include the preparatory work of a treaty, including documents related to negotiation history between the State parties and drafting history of the treaty. Preparatory works are usually available to the negotiating parties, thereby excluding unilateral sources and confidential sources, that may not necessarily have been introduced or made available to other negotiating States parties.

Supplementary means of interpretation carry less weight as they are in effect meant to complement and clarify the application of Article 31.

Further Readings

 * Anthony Aust, Modern Treaty Law and Practice (3rd ed, 2013 CUP)
 * Enzo Cannizzaro (ed), The Law of Treaties: Beyond the Vienna Convention (2011 OUP)
 * Christine Chinkin, Third Parties in International Law (1993 Clarendon Press; OUP)
 * Olivier Corten and Pierre Klein (eds), The Vienna Conventions on the Law of Treaties: A Commentary (2011 OUP)
 * Oliver Dörr and Kirsten Schmalenbach (eds), Vienna Convention on the Law of Treaties: A Commentary (2nd ed, 2018 Springer)
 * Richard Gardiner, Treaty Interpretation (2nd ed, 2017 OUP)
 * Paul Gragl and Malgosia Fitzmautice, `The Legal Character of Article 18 of the Vienna Convention on the Law of Treaties´, (2019) 68(3) ICLQ 699
 * Duncan B. Hollis (ed), The Oxford Guide to Treaties (2nd ed, 2020 OUP)
 * Alina Kaczorowska, Public International Law (Routledge, 2012), 3rd ed.)
 * Jan Klabbers, The Concept of Treaty in International Law (1996 Martinus Nijhoff)
 * Robert Kolb, The Law of Treaties: An Introduction (2016 Edward Elgar)
 * Ulf Linderfalk, On the Interpretation of Treaties: The Modern International Law as Expressed in the 1969 Vienna Convention on the Law of Treaties (2007 Springer)
 * UN Treaty Section of the Office of Legal Affairs, Treaty Handbook, (Rev ed, 2012 UN Publications)
 * Benjamin Mulamba Mbuyi, Droits des Traités Internationaux: Notes de Cours à l'Usage des étudiants en Droit (2009 L'Harmattan)

Conclusion

 * Summary I
 * The law of treaties is a fundamental aspect of international law regulating the entry into, conclusion, validity application, and interpretation of treaties. The VCLT codifies much of customary international law on the law of treaties.
 * A treaty is, under Article 2(1)(a) of the VCLT 'an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation'.
 * Ordinarily, treaties are negotiated, drafted and signed by their parties. Following adoption, a treaty then goes through the ratification process before it comes into force.
 * Treaties can be amended, modified and reviewed. Treaties can also be terminated. Grounds for suspension or termination include, by consent of the parties, following a material breach of the treaty, based on a fundamental change in circumstances, due to supervening impossibility of performance or because of an armed conflict.
 * Treaties can be invalidated on two broad grounds: Absolute grounds for invalidity and relative grounds for invalidity. Absolute grounds for invalidity include: the conclusion of a treaty is a product of coercion of a representative of a State through acts or threats directed against him or her; when the consent is a product of coercion of the State itself by the illegal threat or use of force 'in violation of the principles of international law embodied in the Charter of the United Nations'; if, at the time of its conclusion, it conflicted with an existing jus cogens rule. Relative grounds for invalidity include: a “manifest” violation of “fundamental” internal law provisions regarding competence to conclude treaties; omission by a State representative to observe specific restrictions on authority to express the consent of his/her State, on the precondition that the other negotiating parties had been duly notified; an error that “relates to a fact or situation which was assumed by [the affected] State to exist at the time when the treaty was concluded and formed an essential basis of its consent to be bound by the treaty”; had the error been foreseeable or caused by the affected State itself, it cannot be invoked as a ground for invalidity. Likewise if the error relates only to the wording of treaty text; fraudulent conduct of another negotiating State; corruption of a representative of the affected State, directly or indirectly by another negotiating State.
 * In so far as reservations concern, they are unilateral statements made during the negotiation, signing, adoption or accession of State party to a treaty modifying or excluding legal effect of certain provisions of that treaty.
 * Regarding the interpretation of treaties, Articles 31 and 32, considered to reflect customary international law, provide the rules of interpretation. Article 31 provides the general rule of interpretation, and includes that terms be assigned their ordinary meaning and that treaty provisions be interpreted in good faith.