A. Notion
1 The concepts of desuetudo or desuetude and obsoleteness or obsolescence have both been invoked as grounds for terminating treaties or certain norms in treaties (Treaties, Termination), but they do not appear as such in the Vienna Convention on the Law of Treaties (1969). Desuetudo concerns the discontinuance of conventional obligations because of practice inconsistent with the obligation, resulting in a subsequent rule of customary international law or amounting to a new tacit agreement due to lapse of time. However, the doctrine of desuetudo could also be invoked for the abrogation of other international norms, whether of customary origin or based on unilateral acts (Unilateralism/Multilateralism). Consequently, desuetudo could be more generally defined as the rejection of a rule through subsequent non-enforcement or non-compliance (Compliance). Despite this, desuetudo is mostly used in the context of the law of treaties and is linked to the issues of termination and modification of treaties and hierarchy in international law (Treaties, Amendment and Revision), in particular, the principle of lex posterior derogat legi priori.
2 The concept of obsolescence denotes the termination of a treaty or treaty obligation because of events other than lapse of time which make execution of the treaty unjust, detrimental, inopportune, or without sense, and therefore terminates or modifies the treaty. In particular, obsolescence covers the situation in which circumstances existing at the moment of the conclusion of the treaty have drastically changed so that the continued execution of the treaty is not presumed to be required (Treaties, Conclusion and Entry into Force). From this it is clear that obsolescence relates to the doctrine of rebus sic stantibus. Obsolescence is, however, more flexible than the doctrine of rebus sic stantibus, which generally may not be invoked and is subjected to stringent requirements. For instance, obsolescence could be invoked if from the change of circumstances it could be deduced that parties to the treaty have tacitly agreed to dispense with the continued application of the convention.
B. Historical Context
3 Desuetudo and obsolescence have been invoked in international law, but only to a limited extent. Notable examples in the past include assertions of desuetudo in the ‘Capitulation Treaties’ of 1604, 1673, and 1740 (Capitulations avec la France, Renouvellement des Capitulations avec la France, Capitulations renouvelées et augmentées avec la France) entrusting France with the protection of holy places and of the rights granted to French fishermen in Newfoundland by the Treaty of Utrecht of 1713. The Holy Alliance between Austria, Prussia, and Russia (Holy Alliance [1815]), which has never been formally terminated, has nevertheless been successfully terminated by desuetudo. The regime of the Congo of 1885 has also been terminated by desuetudo (General Act of the Conference respecting the Congo [signed 26 February 1885, entered into force 19 April 1886] (1885) 165 CTS 485).
4 A more recent example of desuetudo concerns Art. 18 Covenant of the League of Nations requiring registration of treaties for them to enter into force and to avoid the falling into desuetude of the minority protection treaties placed under the guarantee of the League of Nations (Treaties, Registration and Publication Minorities, International Protection). Since the Covenant of the League of Nations did not contain a provision concerning the protection of minorities, some States concluded treaties or made a declaration upon admission to the League of Nations in which they provided protection to minorities within their territory. All these instruments contained a clause that the obligations undertaken were of international concern and were placed under the guarantee of the League of Nations and could not be modified without the approval of the majority of the Council of the League of Nations. Only a member of the Council of the League of Nations could bring a claim of violation of the rights of a minority on the agenda. However, due to failure of the League of Nations to adequately address minority complaints and its final demise, the treaties and declarations were considered to be unenforceable and to have been fallen into desuetude.
5 The most famous instance of desuetudo is Art. 27 (3) Charter of the United Nations (United Nations Charter; United Nations (UN)). According to the wording of this provision, a UN Security Council resolution requires nine affirmative votes to pass(United Nations, Security Council), including the concurring votes of the permanent members of the Security Council (International Organizations or Institutions, Voting Rules and Procedures). However, the practice emerged that abstention by a permanent member does not bar the adoption of a Security Council resolution if it receives nine affirmative votes. This was recognized by the International Court of Justice (ICJ) in its advisory opinion in Legal Consequences for States of the Continued Presence of South Africa in Namibia (South West Africa) notwithstanding Security Council Resolution 276 (South West Africa/Namibia [Advisory Opinions and Judgments]). In this decision the ICJ dismissed the contention of South Africa that UN Security Council Resolution 284 of 29 July 1970 was invalid because two permanent members had abstained. The ICJ noted that there was abundant evidence that members of the UN Security Council, including the permanent members, had considered resolutions with the abstention of permanent members as validly adopted (see also International Organizations or Institutions, Membership). Furthermore, the practice had been generally accepted by the members of the UN, evidencing a general practice within the UN that such resolutions were valid.
6 Obsolescence was invoked by Austria in 1991 in respect of the State Treaty for the Re-establishment of an Independent and Democratic Austria of 1955 (Staatsvertrag betreffend die Wiederherstellung eines unabhängigen und demokratischen Österreich [signed 15 May 1955, entered into force 27 July 1955] (1955) BGBl 725).
7 Despite the existence of some State practice and the general recognition of the doctrine, international case law has been scarce. One of the oldest cases concerning the recognition of desuetudo is the arbitral award in Affaire Yuille, Shortridge et Cie, rendered by the Senate of Hamburg, denying that certain privileges granted to British subjects in Portugal pursuant to the Treaty of Peace, Commerce, and Alliance between Great Britain and Portugal had fallen into desuetude. Desuetudo or obsolescence was also recognized but not retained in the arbitral award in respect of the Continental Shelf Arbitration (France v United Kingdom). The ICJ has never explicitly recognized desuetudo or obsolescence, although they have been touched upon in the nuclear test cases, namely Nuclear Tests Case (Australia v France) and Nuclear Tests Case (New Zealand v France), as well as the Aegean Sea Continental Shelf Case (Greece v Turkey) (Aegean Sea), which relates to the General Act for the Pacific Settlement of International Disputes of 1928. The opinions of judges in those cases discussed the issue, but were sceptical of making desuetudo a separate legal ground for terminating a treaty. Instead, they focused on the common consent of the parties to end the treaty, which might be inferred from the conduct of the parties. Similarly, the ICJ did not touch upon the submissions of Australia in East Timor (Portugal v Australia) that the resolutions of the UN General Assembly and the UN Security Council concerning East Timor had fallen into desuetude or had been nullified by subsequent events (United Nations, General Assembly;). In his dissenting opinion, however, Judge Weeramantry categorically denied that desuetudo and supervening events could nullify UN General Assembly or UN Security Council resolutions.
8 The International Law Commission (ILC) initially proposed including desuetudo or obsolescence within the codification of the law of treaties (Codification and Progressive Development of International Law). As such, Special Rapporteur Fitzmaurice proposed in his ‘Second Report on the Law of Treaties’ the inclusion of draft Art. 15 Law of Treaties considering desuetudo as a specific instance of an agreement terminating a treaty. In the end, the ILC came to the conclusion that desuetudo and obsolescence could either be regarded as a tacit agreement of the parties to the treaty to change or terminate the treaty, or a situation of rebus sic stantibus.
C. Applicable Law
1. Desuetudo and Obsolescence in the Law of Treaties
9 Article 42 (2) Vienna Convention on the Law of Treaties of 1969 establishes that treaties may only be terminated on the basis of the provisions of the Vienna Convention on the Law of Treaties. Hence, desuetudo and obsolescence cannot be considered legal grounds for terminating a treaty. Rather, both are factual causes for terminating treaties or conventional rules, covering various legal grounds for terminating entire treaties or conventional rules.
10 Concerning desuetudo, the non-execution of a treaty or some of its rules during a long period of time is never sufficient to terminate or modify the treaty. What is needed is either conduct of the parties from which a tacit agreement to terminate or modify the treaty may be inferred or a new inconsistent rule of customary international law. Regarding the first form of desuetudo, this means of terminating the treaty or parts thereof falls under Art. 54 (b) Vienna Convention on the Law of Treaties. Parties to the treaty may always terminate said treaty or certain conventional rules by consent. Sometimes the consent may be tacit and hence the termination of the treaty should be deduced from the actual inconsistent practice of the parties. Of course, the greatest caution is required in coming to this conclusion. The unequivocal intention of the parties to end the treaty has to be proven. Therefore, the conduct or claim of a single party is not sufficient to give rise to desuetudo. Rather, repeated incompatible practices of all parties should be the yardstick. This does not require all parties to perform the same inconsistent conduct. For instance, repeated instances of opposition by a party to the application of the treaty and a subsequent renunciation of the other parties of their rights to insist on performance of the treaty in question could be evidence of a tacit agreement that the convention or part thereof has come to an end.
11 The second form of desuetudo concerns the emergence of a new rule of customary international law which is incompatible with the existing treaty or part thereof. This might be a rule of customary international law that is only binding on the parties to the treaty, but could equally constitute a new rule of general customary international law. Although this means of terminating a convention or some of its obligations is not listed in the Vienna Convention on the Law of Treaties, it is considered a valid way of modifying or terminating a treaty. The ILC considered changes to treaties by a subsequent custom as falling out of the scope of the law of treaties. Nevertheless, since conventional rules and customary rules are at the same hierarchical level, later inconsistent customary rules may abrogate from previous conventional rules and vice versa. Pursuant to the principle of lex posterior derogat legi priori, the later norm should be applied. Hence, a later customary norm will abrogate a treaty or some of its rules if it is inconsistent with that treaty or its rules. In this respect, it has to be pointed out that a later general rule of customary international law will not terminate or modify a more specific treaty, pursuant to the maxim of lex posterior generalis non derogat legi priori speciali.
12 Concerning obsolescence, the mere occurrence of an event does not normally result in the complete or partial termination of a treaty. In the first place, it will be the conduct of the States Parties to the convention after the event which will determine their fidelity to the treaty or parts thereof. If from their conduct it can be inferred that the parties do not want to continue the treaty, the treaty will be ended or modified by a tacit agreement and not by the event as such.
2. Desuetudo and Obsolescence outside the Law of Treaties
13 Although desuetudo and obsolescence have frequently been invoked and applied in the law of treaties, both concepts could also be applied to other sources of international law, in particular, customary international law and unilateral acts. In this context desuetudo entails subsequent non-enforcement or non-compliance of a previous rule from which it can be inferred that the rule or obligation has ceased to exist.
14 Concerning customary international law, desuetudo is the normal way in which customary norms come to an end. A mere inconsistent practice will not be sufficient. Opinio iuris should be deduced from the conflicting practice in respect of the extinction or modification of the previous norm (see also Treaties, Conflicts between). Consequently, a small amount of conflicting practice will not amount to desuetudo if the practice is considered to be in breach of the rule by the overall majority of States to which the rule applies. On the other hand, widespread inconsistent practice will often demonstrate that the opinio iuris of the previous rule has ceased to exist, leading to desuetudo.
15 Obsolescence could occur if, due to new circumstances, States consider the previous rules of customary international law inappropriate to regulate the new situation. Subsequently, opinio iuris could cease to exist and States could start adopting a new practice. Nevertheless, obsolescence as such will not constitute the legal terminating ground, but rather the disappearance of opinio iuris and the new inconsistent practice. Exceptionally, obsolescence could be the ground for terminating a customary rule when the object to which the customary rule relates disappears or is destroyed.
16 Finally, unilateral acts might equally be subject to desuetudo or obsolescence, with the exception of binding unilateral acts in the framework of international organizations (International Organizations or Institutions, General Aspects), which would have to be replaced by a subsequent binding decision unless execution becomes materially impossible or senseless. It is doubtful whether desuetudo will ever constitute the legal ground for terminating a unilateral act. In most instances, recourse can be had to acquiescence or estoppel. Obsolescence will normally not terminate the obligation flowing from the unilateral act unless the event leads to the material impossibility of honouring the obligation for the future.
D. Evaluation
17 The doctrine of desuetudo and obsolescence of international rules, particularly conventional rules, have been rarely invoked and it can be expected that they will be rarely applied in the future law of treaties since they do not constitute legal grounds for terminating a conventional rule, but only describe the factual cause of changes in, and termination of, treaty rules. Therefore, a treaty or its rules can only be terminated by grounds recognized in the Vienna Convention on the Law of Treaties or by a new rule of customary international law. This is more than justified since the stability of treaty relations requires that treaties should not become terminated merely because of some inconsistent conduct of the parties or an event making the application or the execution of the treaty more cumbersome. Outside the law of treaties, desuetudo is highly relevant since it is the legal ground par excellence for terminating rules of customary international law. Concerning unilateral acts, however, desuetudo will not be significant since acquiescence or estoppel will be the legal grounds on which the assertion will rest that the obligation has terminated. Obsolescence is rightly not a means of terminating customary norms or obligation flowing from unilateral acts. The concept is too flexible to be applied and risks endangering the stability of international relations. Therefore, an event making the application of a rule of international law more difficult, unjust, more detrimental, or inopportune is not sufficient to terminate or modify existing international law. Exceptionally, when the event makes the application of a customary rule or an obligation flowing from a unilateral act impossible or senseless for the future, obsolescence could result in the termination or modification of the rule in question.
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