From: Oxford Public International Law (http://opil.ouplaw.com). (c) Oxford University Press, 2023. All Rights Reserved.date: 25 April 2025
- Subject(s):
- Circumstances precluding wrongfulness
Published under the auspices of the Max Planck Institute for Comparative Public Law and International Law under the direction of Professor Anne Peters (2021–) and Professor Rüdiger Wolfrum (2004–2020).
A. Introduction and Development
1 The Articles on Responsibility of States for Internationally Wrongful Acts (‘ARSIWA’) of the International Law Commission [ILC] (‘Commission’) contain a list of six circumstances precluding wrongfulness (‘CPW’) in Arts 21–25. These are, respectively, consent, self-defence, countermeasures, force majeure, distress, and necessity (Necessity, State of). As the Commentary to these articles evidences, these circumstances have a long-standing history in international law and, aside from some (mostly doctrinal) controversies in relation to state of necessity (eg Heathcote [2007]) and distress (eg Paddeu [2018] Chapter 9), they are largely accepted as reflective of customary international law.
2 CPWs have been a part of international legal discourse and practice since the development of the law of responsibility as a discrete field in international law, which occurred roughly in the mid-19th century (see, eg, Paddeu [2011] 404ff). Indeed, provisions on defences have been a feature of most efforts of codification of the law of State responsibility, be they institutional (see, eg, Bases of Discussion 19, 21, 24, and 25, submitted to the League of Nation’s 1930 Hague Codification Conference) or private (eg Art. 4 Harvard Draft [1961]).
3 The ILC included CPWs in its work on State responsibility from early on (first Special Rapporteur on State Responsibility FV García Amador, Draft Art. 13 ‘International Responsibility: Third Report’ [1958] 72; Draft Art. 17 ‘International Responsibility: Sixth Report’ [1961] 48). In parallel to the ILC’s work on State responsibility, defences were also discussed during the Commission’s work on the law of treaties in the early 1960s. Sir Gerald Fitzmaurice, the ILC’s second Special Rapporteur on the law of treaties, proposed draft articles on ‘circumstances justifying non-performance’ of treaty obligations (‘Fourth Report on the Law of Treaties’ [1959] 43–47). In the late 1970s, Special Rapporteur on State Responsibility Roberto Ago, building on the work of García Amador and Fitzmaurice, formulated a list of six circumstances which the Commission adopted on first reading: consent, countermeasures, force majeure, distress, state of necessity and self-defence (‘Eighth Report on State Responsibility: The Internationally Wrongful Act of the State, Source of International Responsibility’ [1979]; Draft Articles [1996] Arts 29–34). The list of CPW adopted under Ago remained unchanged during the second reading of the ARSIWA, save for minor drafting changes (though note that there were controversies in respect of consent: UN ILC Report [1999] 74–75.)
B. Notion
4 There are three important features of the notion of CPWs in the law of State responsibility. Each is addressed in turn below.
5 First, the Commentary to the ARSIWA states that the CPWs are ‘to be distinguished’ from the ‘constituent requirements of the obligation, i.e. those elements which have to exist for the issue of wrongfulness to arise in the first place and which are in principle specified by the obligation itself’ (Part One Chapter V Commentary para. 7). The CPWs are, in other words, distinct and separate from the definition of the relevant obligation: in the ILC’s terminology, the obligation is a primary rule and the CPWs are secondary rules. Accordingly, the invocation of a CPW is not simply a denial of the claim of breach, but a claim which falls to be proven by the invoking State (ibid para. 8).
6 The ILC’s position on this matter is jurisprudentially and factually more complex than it appears (see, among others, Williams [1988]; Campbell [1987]; Gardner [2007]; Duarte d’Almeida [2015]; Greasley [2020]). Particular difficulties arose in this respect during the second reading of the defence of consent, as ILC members disagreed over whether consent operated as a defence or was, instead, a ‘constituent requirement’ of every (non-absolute) international obligation (all of which would include, on this view, a requirement of ‘non-consent’). The matter was left unresolved in the ILC, which agreed to disagree on this point. Since then, some part of the scholarship has argued that whether consent is a defence or a ‘constituent requirement’ may depend on the specific obligation (see Duarte d’Almeida [2020]; Paddeu [2020]). Note that a similar difficulty arises in respect of the requirements of impossibility or emergency, which are at the core of the defences of force majeure, and necessity and distress, respectively—although it was not perceived as problematic in the ILC discussions. In some instances, these factual circumstances are already catered to by the relevant primary rule. For example, due diligence standards are limited by possibility, and obligations of best efforts by reasonableness (see Ollino [2022] at 220; Paddeu [2018] at 306). As such, situations of impossibility arising in connection with these rules are already addressed by the relevant primary rules, rendering the defence of force majeure unnecessary.
7 Second, the circumstances in the ARSIWA operate to exonerate the State in respect of conduct which is incompatible with its obligations. In the words of the ILC, they operate like ‘shields’ against otherwise well-founded claims of breach (Part One Chapter V Commentary para. 2). By the same token, the CPWs do not, as such, affect the continued existence or validity of the obligation affected: they do not suspend or terminate the relevant obligation. CPWs do not, as the metaphor goes, operate like ‘swords’. While the circumstances can only suspend the performance of the obligation for the time being, it is possible that the facts which fulfil the relevant CPW may determine the termination of the obligation (see the example at para. 23 below). The function of the circumstances as shields and not swords is the logical extension of the so-called ‘functional separation’ between the law of treaties and the law of responsibility (explained at paras 12–15 below). A consequence of this feature is that the State invoking a circumstance must resume compliance with the underlying obligation as soon as the facts giving rise to the CPW disappear (Art. 27 (a) ARSIWA).
8 Third, the circumstances in the law of responsibility may operate as either justifications or excuses. The expression ‘circumstances precluding wrongfulness’ used in the ARSIWA implies that the list of circumstances included in that document all operate in the same manner: to exclude the conclusion that conduct incompatible with an obligation of a State constitutes a breach of that rule. And yet, the various CPWs included in the text are very diverse, so diverse that they have been described as a ‘grab bag’ of rules (Rosenstock [2002] 794). Attempts have thus been made to classify these defences into two categories: justifications (which preclude wrongfulness) and excuses (which preclude the legal consequences of wrongfulness).
9 The Commission’s views on this question changed over time. On first reading, the Commission was of the view that all the circumstances operated to preclude wrongfulness (UN ILC Report [1979] 106). However, on second reading, Crawford raised this question again in response to both comments submitted to the ILC by States (France, UK, and Japan, see UN ILC Special Rapporteur J Crawford ‘Second Report on State Responsibility’ [1999] 58) and scholarly commentary (especially Lowe [1999]). While seemingly favourable to the classification, on balance Crawford (and along with him, the ILC) was not convinced that a strict classification of the circumstances into justifications or excuses should be made in the ARSIWA, and the matter could be left to subsequent development.
10 Since then, this question has arisen both in practice, especially in respect of the plea of necessity, and in scholarship. The CMS Annulment Committee, for example, characterized the plea of necessity as an excuse (CMS Gas Transmission Co v Argentine Republic [Decision on Annulment] [2007] paras 129, 134), and this holding has subsequently been followed by other tribunals (Sempra Energy International v Argentine Republic [Decision on Annulment] [2010] paras 114–115). Among scholars, several have been favourable to drawing the distinction between these two categories of defences (eg Christakis [2007]; Crawford [2013] 278–79; Aust [2014]; Paddeu [2018]), while others consider that the distinction remains a question of lex ferenda (eg Szurek [2010] 435–37). The debate whether, and if so which, circumstances can be characterized as justifications or excuses remains open. This is why some scholars prefer to refer to the circumstances as ‘defences’, since this term is agnostic as to potential categorization as justification or excuse (Paddeu [2018] 15–17).
C. The Generality of the Circumstances Precluding Wrongfulness
11 The CPW are, like the ARSIWA, of general application. Pursuant to Art. 12, the ARSIWA apply to the breach of any international obligation regardless of its conventional or customary character. It follows that the CPW are also applicable to the breach of any obligation regardless of its source (Part One Chapter V Commentary para. 2). This said, the ARSIWA recognizes certain limits to the applicability of the CPW. This section discusses the applicability of the CPW to treaty breaches (1) and then reviews the limits of their applicability (2).
1. Circumstances Precluding Wrongfulness and Treaty Breaches
12 The applicability of CPW to treaty breaches was contested on at least two occasions in international dispute settlement proceedings, first by New Zealand in the Rainbow Warrior arbitration and then by Slovakia in the Gabčíkovo-Nagymaros case (Rainbow Warrior, The; Gabčíkovo-Nagymaros Case [Hungary/Slovakia]), and it has been criticized in scholarly commentary (Bowett [1991]). The legal issue is nowadays settled, but it is nevertheless worth addressing it in some detail as the ultimate result was not a given and the debate is helpful in understanding the development of the law (note, as mentioned earlier, that the ILC at one point included defences to non-performance of treaty obligations in its work on the law of treaties).
13 The dispute in Rainbow Warrior concerned the unilateral removal of two French agents, responsible for the sinking of the Rainbow Warrior at Auckland Harbour, from the island of Hao in contravention with a ruling by the UN Secretary General (United Nations, Secretary-General). Facing the claim by New Zealand, France sought to justify its actions by reference to the CPW in the law of responsibility. An issue thus arose as to the applicable law to the dispute. For New Zealand only the customary law of treaties applied to the dispute: since the law of treaties contained specific grounds for the termination and suspension of treaties, a State wishing to justify its non-performance of treaty commitments could not rely on the general law of State responsibility. Indeed, it was not a ‘credible proposition’ to contend that, even though the law of treaties includes a ‘number of lawful excuses for non-performance’, there may be other ‘excuses … derived from the customary Law of State Responsibility’ (para. 73).
14 Slovakia made a similar argument in the Gabčíkovo-Nagymaros case before the International Court of Justice (ICJ) (though note that this argument changed throughout the proceedings, Dupuy [1997] 18, fn 43). The dispute concerned the suspension and unilateral termination of the 1977 Treaty between Hungary and the then Czechoslovakia for the joint construction of a barrage system in the Danube. After intermittent performance on both sides for approximately 10 years, in 1989 Hungary suspended and later abandoned works under the Project. In May 1992, after Czechoslovakia had resolved to continue the works on a unilateral modification of the project to be implemented in October 1992 (the so-called ‘Variant C’), Hungary notified Czechoslovakia of its intention to terminate the Treaty. The dispute was submitted to the ICJ, where Hungary invoked the plea of necessity to justify its suspension of the works on the joint project as from 1989 and the termination of the Treaty (Memorial of Hungary 267; Counter-Memorial of Hungary 207–11). Slovakia, succeeding to Czechoslovakia’s rights and obligations under the Treaty, objected to Hungary’s reliance on the defences from the law of responsibility. It argued that the defences in the law of responsibility could not be invoked to justify the non-performance of treaty obligations, insofar as the consequences of treaty breaches were exclusively governed by the Vienna Convention on the Law of Treaties (1969) (‘VCLT’) in accordance with Art. 42 (2) VCLT (Memorial of Slovakia 315–20).
15 This argument was rejected on both occasions: by the Tribunal in Rainbow Warrior (para. 75) and by the ICJ in Gabčíkovo-Nagymaros ([1997] para. 48). The two tribunals recognized that the law of treaties and the law of responsibility were both applicable to the disputes, though each in its own field: while the law of treaties governed the existence, suspension or termination of treaties, the law of responsibility dealt with the question of their breach. In other words, the law of treaties regulated the suspension or termination of treaties (codified in Arts 60–62 VCLT) and not the issue of justification or excuse for the non-performance of treaty obligations (Treaties, Suspension; Treaties, Termination). In turn, the CPW in the law of responsibility applied to the question of exoneration for the non-performance of treaty obligations, but not to the question of their suspension or termination. Both tribunals thus endorsed the ‘functional separation’ (Dupuy [1997] 11, 29) between these two fields of international law and confirmed the application, as a matter of principle, of the defences to treaty breaches.
2. Limits to the Generality of the Circumstances Precluding Wrongfulness
16 While the defences are in principle of general application, there exist some limits to their applicability. First, the defences are unavailable in relation to breaches of obligations under peremptory rules; second, individual defences may be specifically excluded in relation to certain categories of obligations; and, third, the defences may be set aside by the relevant special law. These three limitations are addressed in turn.
(a) Peremptory Rules
17 Pursuant to Art. 26 ARSIWA, the CPW do not preclude ‘the wrongfulness of any act of a State which is not in conformity with an obligation arising’ under a peremptory rule. Thus, insofar as the prohibition of torture is recognized as peremptory, torture can never be justified (or excused) by the plea of necessity—not even in so-called ‘ticking time-bomb’ scenarios (on which see, generally, Shany [2007] and Farrell [2013]). Indeed, the breach of a peremptory rule could not give rise to a defence involving the breach of another peremptory rule: to paraphrase the ICJ, a genocide may not justify a State engaging in a counter-genocide (Application of the Convention on the Prevention and Punishment of the Crime of Genocide [Counter-Claims] [Order of 17 December 1997] para. 35; Application of the Convention on the Prevention and Punishment of the Crime of Genocide Case [Bosnia and Herzegovina v Serbia and Montenegro]; ‘Bosnian Genocide case’). This exclusion constitutes an extension of the non-derogable character of peremptory rules to the field of State responsibility: if peremptory rules are non-derogable by agreement (and treaties inconsistent with ius cogens rules are void), a fortiori they are also non-derogable unilaterally by way of application of one of the defences in the law of responsibility. Article 26 applies to all the CPW and to all peremptory rules, and was endorsed by the tribunals in Border Timbers v Republic of Zimbabwe (Award [2015] para. 656) and von Pezold v Republic of Zimbabwe (Award [2015] para. 657). In both cases, the tribunal rejected Zimbabwe’s defence of necessity, among others, on the grounds that the measure in question violated the obligation of racial non-discrimination.
(b) Individual Exclusions
18 Some of the defences in the ARSIWA also specify categories of obligations to which they are not applicable. For example, self-defence may not justify the breach of obligations of ‘total restraint’, namely obligations intended to impose absolute restrictions on the conduct of armed hostilities by States even when these States are acting in self-defence (Legality of the Threat or Use of Nuclear Weapons [Advisory Opinion] para. 30; Art. 21 ARSIWA Commentary, paras 3–4); countermeasures may not affect the prohibition of force, obligations of a humanitarian character prohibiting reprisals, fundamental human rights, dispute settlement obligations, or diplomatic or consular inviolability (Art. 50 ARSIWA); and state of necessity may not be invoked if ‘the international obligation in question excludes the possibility of invoking necessity’ implicitly or explicitly (Art. 25 (2) (a) and Commentary at para. 19), such as international humanitarian law (Humanitarian Law, International; ‘IHL’).
(c) The lex specialis Principle
19 Pursuant to Art. 55 ARSIWA, the CPW are also not applicable ‘where and to the extent’ that there exists a lex specialis governing the issue (Art. 55 Commentary, para. 2). For the rule in Art. 55 to apply two sequential conditions are necessary. First, the two rules in question must ‘have at least the same legal rank as those expressed’ in the ARSIWA (Art. 55 Commentary, para. 2); namely, both the CPW and the special rule in question must have the ‘rank’, to use the language in the Commentary, of secondary rules (Binder [2009] 620). Second, if both rules have the same rank then there must be ‘some actual inconsistency between’ the two or a ‘discernible intention that one provision is to exclude the other’ (Art. 55 Commentary, para. 4).
20 The principle in Art. 55 has been at issue in various disputes relating to the effect on foreign investors of the measures adopted by Argentina to address its financial crisis of the early 2000s. In response to those claims, Argentina has justified its measures by reference to the security exception or non-precluded-measures (‘NPM’) clause contained in some of its bilateral investment treaties (eg Art. XI Argentina–USA BIT) and, alternatively, the plea of state of necessity under customary international law. The question thus arose as to the relationship between these two rules, to which tribunals have given radically different answers (for a taxonomy of the approaches adopted by these tribunals see Kurtz [2011]).
21 The question was discussed at some length by the Annulment Committee in CMS v Argentina. Following the observations contained in the Commentary to Art. 55, the Committee held that the NPM clause was a primary rule, insofar as it operated to exclude the breach of the treaty, and that state of necessity was a secondary rule, since it excluded the legal consequences (responsibility proper) of a wrongful act. Since the two rules at issue had different ranks, there was no normative conflict to be solved by resorting to the lex specialis principle (Annulment para. 134). The Committee also added that if both rules had belonged to the same ‘rank’ (either both having primary or secondary rule character) then they would have covered ‘the same field’ requiring the application of the lex specialis principle (ibid para. 133). Other tribunals have subsequently endorsed the CMS annulment decision on this point (eg Casualty Company v Argentine Republic [Award] [2008] paras 163–64; Sempra v Argentina [Annulment] para. 115).
D. Consequences of Invocation
22 The ARSIWA also contain a ‘without prejudice clause’ in Art. 27 which lists two consequences of the invocation of CPW.
23 First, pursuant to Art. 27 (a) the successful invocation of a CPW is without prejudice to the obligation of the invoking State to resume compliance with the obligation once the CPW no longer exists. This is a logical consequence of the CPW’s character as ‘shields’ and not ‘swords’: the CPW justifies non-compliance for the time being, without striking down the obligation (see above). The principle has been affirmed by tribunals on numerous occasions, including in Rainbow Warrior ([1990] para. 75), Gabčíkovo-Nagymaros (para. 101), CMS v Argentina (Award [2005] paras 379–82), LG&E v Argentina (Decision on Liability [2006] para. 261), and EDF v Argentina (Award [2012] para. 1171). This obligation to resume compliance exists only where the effect of the facts which give rise to a circumstance precluding wrongfulness do not also ‘give rise to the termination of the obligation’ (Art. 27 Commentary, para. 1). Thus, if the obligation is to return a work of art which is subsequently destroyed by an unforeseeable and irresistible natural event (such as a sudden fire or an earthquake) giving rise to a material impossibility of performance justified by force majeure, there is no surviving obligation to comply with. Moreover, the obligation may be fully reinstated or restored, but modalities for resumption of compliance may need to be settled (Art. 27 Commentary, para. 3). These are matters that are not resolved by Art. 27, and may need case-by-case assessment.
24 The Commentary to this provision also adds that: ‘The words “and to the extent” are intended to cover situations in which the conditions preventing compliance gradually lessen and allow for partial performance of the obligation’ (Art. 27 Commentary, para. 2). Article 27 (a) is a without prejudice clause, so there is no (general) obligation to resume partial compliance as far as this is possible (compliance as far as possible may be required by the relevant substantive rules: for an example from IHL, see Paddeu and Trapp [2022]). Indeed, partial non-compliance would still amount to a breach of the relevant obligation (Art. 12 Commentary, para. 2). Nevertheless, partial return to performance as soon as this is possible is both desirable and practically valuable. It is desirable to minimize harm caused: indeed, situations in which a CPW applies may cause harm to another State or individuals (as recognized in Art. 27 (b)). It is practically valuable, too, since the extent of harm caused while a CPW lasted may have an impact on any potential compensation that may be due, either for material loss by reference to Art. 27 (b) or, arguably, in situations where excuses may only operate partially (thus to exclude some, but not all, of the consequences of the wrongful act).
25 Second, there may be a potential obligation to make compensation for the material loss suffered by the State to which the obligation was owed. This possibility is envisaged in Art. 27 (b), which indicates that the successful invocation of a CPW is without prejudice to the ‘question of compensation for any material loss caused by the act in question’. While the text refers to compensation, the Commentary to Art. 27 explains that this is not to be understood as a form of reparation (on which see Art. 34); rather, compensation under Art. 27 (b) concerns the question whether a State which has successfully invoked a CPW must ‘make good any material loss suffered by any State directly affected’ (para. 4). The Commentary also clarifies that the notion of ‘material loss’ is narrower than the notion of damage used elsewhere in the ARSIWA (ibid).
26 There are two main uncertainties concerning the obligation referred to in Art. 27 (b). First, it is unclear in which circumstances the (eventual) obligation to make compensation arises. The first draft of this provision limited the eventual obligation to make compensation to the successful invocation of force majeure, necessity, distress and consent (Art. 35 Draft Articles 1996). The clause was later expanded and now does not distinguish between CPWs that may give rise to this obligation and circumstances which do not. Potentially, the invocation of any defence could give rise to this obligation to make compensation for material loss. But the Commentary does not assist in determining the circumstances in which such an obligation might arise. It simply states that the question of compensation (its possibility and extent) should be agreed as between the State invoking the defence and the State which has suffered the material loss (para. 6). The tribunal in Enron v Argentina endorsed the ILC’s position, stating that it would give preference to a ‘negotiated settlement between the parties’, and adding that only in the absence of such an agreement the determination is ‘to be made by the Tribunal to which the dispute has been submitted’ (Award [2007] para. 345).
27 Second is the absence of a clear legal basis for the eventual obligation to make compensation. Historically, it was not uncommon for States to provide compensation in situations characterized as force majeure, for example, for which they bore no (legal) responsibility; though this was done on an equitable or gratuitous basis and not as a matter of legal obligation (eg French Decree of 1848 on indemnities for the February 1848 revolution, cited by Calvo [1880] 443; see further practice in Paddeu [2011] 409–11). The ILC struggled to identify a legal basis for the potential obligation to make compensation for material loss. During the first reading of the ARSIWA, some members of the Commission suggested that the potential obligation of compensation was an instance of liability arising from conduct not prohibited under international law (eg, Robert Q Quentin-Baxter in the 1615th and Julio Barboza in the 1617th ILC meeting on State responsibility, UNYBILC [1980] vol I, at 168 and 176 respectively). On second reading, the Commission clarified that this rationale did not have a ‘sufficient basis in international law’ (UN ILC Report [1999] para. 406). The ARSIWA as finally adopted do not take a position on this question.
28 In the literature, Christakis has argued that these two difficulties may be solved by reference to the distinction between justification and excuse. Since excuses do not affect the wrongfulness of the relevant conduct, the obligation to compensate arises only for this category of defences and can be grounded on wrongfulness (Christakis [2007] 235–40). But Crawford has countered that not all excuses should generate a duty of compensation. In his view, force majeure (an excuse) should not entail a duty of compensation, since the breach of the obligation is, in these cases, involuntary (Crawford [2013] 319). There is some merit to both views. Excuses can (at least theoretically) be partial, and exclude some, but not all, of the consequences of the wrongful act. A distinction could be drawn between, on the one hand, excuses that leave the State no choice whether to comply with the obligation, such that the breach is involuntary (like force majeure); and, on the other, excuses that only partially constrain the State and thus leave some degree of choice (even if the choice is unpalatable) as to compliance with the obligation, such that the breach can be deemed voluntary (necessity, distress). Compensation could be due in the latter case only: in this case the State would have voluntarily breached its obligation (Paddeu [2018] 77–94).
29 Nevertheless, this approach does not answer the question of whether justifications too can give rise to compensation. The issue is most relevant with respect to the defence of necessity (but not only: see arguments about compensation for material loss in respect of self-defence against non-State actors: Blum and Goldberg [2022]). States have, on the whole, endorsed the classification of necessity as a justification (they have invoked it as such: Paddeu [2018] 414–21), and invocations of this defence are often accompanied by demands for compensation by the affected parties. To the extent that conduct covered by a justification is permissible, any subsequent duty to pay compensation for material loss must be grounded in positive law: either treaty, custom or—potentially—a general principle (Treaties; General Principles of Law). Assessments about the positive law basis of such a duty vary significantly (cf Bücheler [2015] 290–96; Binder and Janig [2019] at 677–78 in favour; Díaz Inverso [2015] 54; and Paddeu [2018] 421–25). Unsurprisingly, the relevant case law on compensation in cases of necessity is also inconsistent. Some international tribunals have accepted—at least in obiter—an obligation to pay compensation: for example, CMS v Argentina (Award para. 390), South American Silver Ltd v The Plurinational State of Bolivia (Award [2018] para. 620), and the annulment decisions in Mitchell v Democratic Republic of Congo (Annulment [2006] para. 57, fn 30), and EDF v Argentina (Annulment [2016] para. 330). In contrast, the Tribunal in LG&E (Decision on Liability paras 260–64) held that no such obligation arises in respect of a successful plea of necessity. The issue remains open.
E. Procedural Aspects of Invocation
30 There are two main procedural issues that arise in connection with the invocation of defences in international dispute settlement. First is the question of jurisdiction. International tribunals have jurisdiction to apply the law of State responsibility to determine the existence of a breach of international law (Bosnian Genocide case [Judgment] [2007] para. 149). By implication, tribunals will have jurisdiction to apply the defences under the law of State responsibility as well. This much is uncontroversial.
31 Difficulties may arise in respect of defences which are triggered by prior wrongful acts, in particular self-defence and countermeasures, where this prior wrongful act does not fall within the subject-matter jurisdiction of the international court or tribunal. In the ICAO Council cases, Qatar complained that the respondents’ measures affected its rights under the Chicago Convention (1944). The respondents argued that these were countermeasures in response to Qatar’s breach of, among others, terrorism-prevention obligations (Terrorism). The question arose whether the ICAO Council had jurisdiction over the respondents’ countermeasures defence insofar as the alleged prior wrongful act involved obligations that fell outside the jurisdiction of the ICAO. The ICJ’s judgment in the ICAO Council cases confirmed that a tribunal may, in the course of deciding the legality of a countermeasure, make a ruling on the existence of a prior wrongful act (triggering the countermeasure) even if this determination falls outside the jurisdiction of the tribunal as constituted for that dispute—namely, even if the wrongful act involves the violation of obligations over which the tribunal has no subject-matter jurisdiction (Appeal Relating to the Jurisdiction of the ICAO Council under Article 84 of the Convention on International Civil Aviation [Bahrain, Egypt, Saudi Arabia and United Arab Emirates v Qatar] [Judgment] [2020] para. 49). However, other tribunals have taken a narrower view of this question, holding that they lacked jurisdiction over a countermeasures defence where the triggering wrongful act involved obligations falling outside their subject-matter jurisdiction (see Corn Products International Inc v The United Mexican States [Decision on Responsibility] [2008] para. 181; Archer Daniels Midland Company and Tate & Lyle Ingredients Americas Inc v The United Mexican States [Award] [2007] para. 128); see further Jansen Calamita [2011]).
32 One key difference between these cases concerns the jurisdiction of the relevant dispute settlement body over the State author of the (alleged) prior wrongful act. In the ICAO Council cases, the alleged wrongful act triggering the countermeasure was one that implicated the same States that were parties to the dispute before the ICAO Council: and all of these parties had consented to the jurisdiction of the relevant dispute settlement body (the ICAO Council) (Dispute Settlement Mechanism: 1944 Convention on International Civil Aviation). In the NAFTA cases, instead, the triggering wrongful act had been committed by a State not party to the proceedings. In these cases, the claims were brought by investors against Mexico, who argued that Mexico’s measures had affected their rights under NAFTA Chapter XI. Mexico’s measure had been a response to an alleged prior wrongful act by the United States, which was not a party to the proceedings before these tribunals (the point was made explicitly in Corn Products [Decision on Responsibility] para. 181; but it may be implicit also in the tribunal’s statement in Archer Daniels Midland [Award] at para. 128).
33 The second procedural incident concerns the burden of proof. As a general rule, claimants must prove their claims and respondents their defences (ARSIWA Part One Chapter V Commentary para. 8; Gabčíkovo-Nagymaros paras 41–45; Riddell and Plant [2009] 87). Some difficulties may arise in respect of some defences, where it is not clear whether the facts that give rise to those defences constitute instead elements of the definition of the relevant rule (on which see Stone [1944]; Pauwelyn [2020]). For example, since consent is usually seen as part of the definition of the prohibition of force, the claimant should bear the burden of disproving its presence (but the Court in DRC v Uganda did not specify the burdens in relation to the argument of consent; Armed Activities on the Territory of the Congo Cases). Likewise, situations of emergency or impossibility may be required to prove the defences of necessity or force majeure, but may also be relevant in assessing the degree of diligence displayed by a State in the circumstances. Thus, where the claimant alleges the breach of a due diligence obligation in circumstances of national emergency or other crisis, the burden of (dis)proving the existence of an emergency or of impossibility could potentially fall on the claimant.
F. An Exhaustive List?
34 The list of circumstances in the ARSIWA is not intended to be exhaustive. Indeed, the Commentary to Chapter V of Part One explicitly indicates that the Chapter ‘sets out the circumstances precluding wrongfulness presently recognized under general international law’ (para. 9 [emphasis added]), implying that new defences may develop and achieve general recognition.
35 During the revision of the draft Articles adopted on first reading Special Rapporteur Crawford discussed the possibility of adding certain CPWs, including performance in conflict with a peremptory norm, the exception of non-performance and the clean hands doctrine (‘Second Report’ [1999] 76–83; Clean Hands, Principle). The Commission eventually discarded all three additional justifications for a variety of reasons, including their not operating like CPWs or their non-recognition as a matter of customary law (ILC Report [1999] 78–80, 85–86).
36 Of these additional justifications, the exception of non-performance was discussed in the Application of the Interim Accord of 13 September 1995 case between the then Former Yugoslav Republic of Macedonia (‘FYROM’ or North Macedonia, as it is now known) and Greece before the ICJ. Greece argued, in response to North Macedonia’s claim that it had breached its conventional obligation not to object to North Macedonia’s membership in international organizations of which Greece was a member, that it had acted in response to a prior breach by North Macedonia and that the wrongfulness of its conduct was accordingly precluded by virtue of the exception of non-performance (Judgment [2011] para. 114) (International Organizations or Institutions, Membership). The ICJ rejected the plea on the facts since the conditions for the application of the exception (a previous breach of the Accord by North Macedonia) were not met, but it avoided pronouncing on the legal recognition of the exception of non-performance (Judgment [2011] para. 161). Judge Simma—who had been critical of the introduction of the exception as a CPW when he was a member of the ILC (UN ILC [1999] UNYBILC vol I, 147)—criticized the Court for avoiding pronouncing on and rejecting the ‘controversial’ status of the exception non-performance (Separate Opinion para. 6).
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