Silence is consent : Acquiescence and Estoppel in International Law

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J U R I D I C U M

Silence is consent

Acquiescence and Estoppel in International Law

Nathalie Holvik VT 2018

RV102A Rättsvetenskaplig magisterkurs med examensarbete, 15 högskolepoäng Examinator: Joakim Nergelius

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LIST OF ABBREVIATIONS

BYIL British Yearbook of International Law

Ch Chapter

Comm Commission

CUP Cambridge University Press

Ed Editor

Edn Edition

Eg For example

ICJ International Court of Justice

ICLQ International & Comparative Law Quarterly

Intl International

J Journal

L Law

No Number

OSAIL Oxford Scholarly Authorities on International Law

OUP Oxford University Press

PCA Permanent Court of Arbitration

PCIJ Permanent Court of International Justice

Rev Review

Rep Report

Supp Supplement

UK United Kingdom

UN United Nations

UNGA United Nations General Assembly

UNTS United Nations Treaty Series

US United States of America

VCLT Vienna Convention on the law of treaties

Vol Volume

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I. I

NTRODUCTION

A. Background

The 1900th century American humor writer Josh Billings once expressed that: “silence is one of the hardest arguments to refute”.1 Even though Billings probably did not consider silence in the context of public international law, this notion very accurately describes the legal value that silence may have within this particular legal field. Silence as a concept may for example affect the interactions between states and it may also play a predominant role in the settlement of disputes in international courts and tribunals.2

Against this background, various scholars have examined multiple areas of international law in which silence may have legal consequences.3_{The subject also occurs in reports and}

documents from the International Law Commission (ILC).4 Since it is a vast subject that can apply various situations, it may also be discussed from multiple viewpoints and situations within different aspects or fields of international law.

For example, in certain situations silence may indicate that there are gaps in the law. In connection to this, the issue of silences in the law can be examined from a theoretical point of view that relates to the perception of the international legal system as a whole.5 In other situations it can relate to state conduct, or the lack thereof, that eventually may have legal consequences.6 The latter, which is the focus of this paper, can be described as a negative form of state conduct with potential legal consequences.7

The legal value of silence is determined by the factual situation in which a state has remained inactive, or in other words, silent. Hence, silence on its own is of little value unless it is coupled with other legal concepts of international law. The concept that is most frequently

1_{Mira Balachandran, Quotations for all occassions (Emerald Publishers 2009).}

2_{Sophia Kopela, ‘The Legal Value of Silence as State Conduct in the Jurisprudence of International Tribunals’} (2010) 29 Australian Ybk Intl L 87, 87.

3_{See eg Sophia Kopela, ‘The Legal Value of Silence as State Conduct in the Jurisprudence of International} Tribunals’ (2010) 29 Australian Ybk Intl L 87; Helen Quane, ‘Silence in International Law’ (2014) 84 BYIL 240; Megan L Wagner, ‘Jurisdiction by Estoppel in the International Court of Justice’ (1986) 74 California L Rev 1777.

4_{See eg Intl L Comm, ‘Second report on subsequent agreements and subsequent practice in relation to the} interpretation of treaties’ [2014] Second Report of Special Rapporteur Georg Nolte, Sixty-sixth Session,

reprinted in UNGA Official Rec, Sixty-ninth Session, Supp No 10 A/69/10, 166, 66-76, 4 § 4; Intl L Comm,

‘Third Report on identification of customary international law’ [2015] Third Report of Special Rapporteur Michael Wood, Sixty-seventh Session, A/CN.4/682, 9-14.

5_{Hersch Lauterpacht, The Function of Law in the International Community (OSAIL 2011) 93-97; see also} Sophia Kopela, ‘The Legal Value of Silence as State Conduct in the Jurisprudence of International Tribunals’ (2010) 29 Australian Ybk Intl L 87, in its entirety.

6_{See eg Temple of Preah Vihear (Cambodia v Thailand) (Merits, Judgment) [1962] ICJ Rep 6, 23, 32.}

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associated with silence is acquiescence.8 This concept comes into play when a state, or states, remains silent in situations that generally call for a reaction.9_{The potential consequence of}

such inaction is that states risk becoming bound by their silence.10

Acquiescence can be applied in numerous contexts of international law, and has repeatedly been referred to in international cases.11 On this basis, the concept is well-established within international law, and the prerequisites for it to apply have been clearly outlined.12 However, in spite of this, there are still some prevailing uncertainties with regard to its relation and interaction with the concept of estoppel.

The latter concept can arise as a result of state silence, but it can also be applied under other circumstances.13 In essence, it is a concept that aims to promote stability in international relations,14 and which prevent states from going back on a previous representation in a given situation.15 It has sometimes been described as:

”a juridical wild card allowing one who plays it in international law to make it represent any legal notion he desires.”16

Despite the fact that estoppel has been referred to frequently in international jurisprudence there are many ambiguities surrounding the concept. These extend to its characterization, its application, and the necessary requirements for an estoppel to arise.17 Against this background, estoppel has prompted a lot of discussion within international law. It seems to be particularly unclear under which circumstances silence may give rise to estoppel.18 Very few attempts have been made to systemize the concepts of estoppel and acquiescence and on this basis the uncertainties surrounding them are still prevalent in international law.19

8_{Id, Kopela 87.}

9_{Id 87; Temple of Preah Vihear (Cambodia v Thailand) (Merits, Judgment) [1962] ICJ Rep 6, 23, 32.}

10_{Delimitation of the Maritime Boundary in the Gulf of Maine Area (Canada v United States of America)} (Merits, Judgment) [1984] ICJ Rep 246 § 130.

11_{See eg Temple of Preah Vihear (Cambodia v Thailand) (Merits, Judgment) [1962] ICJ Rep 6; Military and}

Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America (Jurisdiction,

Admissibility) [1984] ICJ Rep 392; Delimitation of the Maritime Boundary in the Gulf of Maine Area (Canada v United States of America) (Merits, Judgment) [1984] ICJ Rep 246.

12_{See eg Temple of Preah Vihear (Cambodia v Thailand) (Merits, Judgment) [1962] ICJ Rep 6, 23.}

13_{See Alexander Ovchar, ‘Estoppel in the Jurisprudence of the ICJ: A principle promoting stability threatens} to undermine it’ (2009) 21 BLR 1, in its entirety.

14_{Territorial Dispute (Libyan Arab Jamahiriya v Chad) (Merits) [1994] ICJ Rep 6 (Separate Opinion of Judge} Ajibola) 78 § 99; Alexander Ovchar, ‘Estoppel in the Jurisprudence of the ICJ: A principle promoting stability threatens to undermine it’ (2009) 21 BLR 1, 1.

15_{Ian C MacGibbon, ‘Estoppel in International Law’ (1958) 7 ICLQ 468, 468.}

16_{Brown C, ‘A Comparative and Critical Assessment of Estoppel in International Law’ (1996) 50 U Miami L} Rev 369, 385.

17_{Alexander Ovchar, ‘Estoppel in the Jurisprudence of the ICJ: A principle promoting stability threatens} to undermine it’ (2009) 21 BLR 1, 1.

18_{Id 2.} 19_{Id 2.}

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A possible explanation to this could be that acquiescence or estoppel alone has rarely determined the outcome of a judgment on an international level. The concepts have most commonly been applied in a greater context and in relation to for example a treaty, declaration, or customary international law.20_{This width of application makes it very}

important to be able to make a distinction between them. Especially since silence may affect the process of the formation of customary international law, as well as treaty interpretation.21 Several scholars have pointed out that it is difficult to distinguish between the two concepts since they may apply to the same fact patterns.22 Some even go so far as to say that such a distinction is unnecessary since the result of their application is the same.23 However, cases such as the Military and Paramilitary Activities Case24 on admissibility and jurisdiction in the International Court of Justice (ICJ), clearly demonstrates that there is indeed a difference between the two concepts.25 A distinction is therefore, important since it will not only contribute to a better knowledge of the functioning of international law, but it may also provide guidance for states on how to act and what to expect in their international relations and affairs.

B.

Purpose and questions

This paper examines the situations in which silence may result in acquiescence or estoppel. Silence in this regard refers to a negative state conduct, namely the absence of positive actions or lack of protests. The reasons for choosing to focus on acquiescence and estoppel is because of the great impact acquiescence has had in international law,26_{and also because the there is}

no clear distinction between acquiescence and estoppel arising from silence.

On this basis this paper contains an assessment of which situations that may actualize acquiescence and/or estoppel arising from silence. For the purpose of clarity, a discussion regarding the necessary criteria in relation to the two concepts is included. This discussion

20_{Thirlway H, International Law: The Sources of International Law (Malcolm D Evans ed, 4th edn, OUP 2014)} 104.

21_{Intl L Comm, ‘Second report on subsequent agreements and subsequent practice in relation to the} interpretation of treaties’ [2014] Second Report of Special Rapporteur Georg Nolte, Sixty-sixth Session,

reprinted in UNGA Official Rec, Sixty-ninth Session, Supp No 10 A/69/10, 166, 66-76, 4 § 4; Intl L Comm,

‘Third Report on identification of customary international law’ [2015] Third Report of Special Rapporteur Michael Wood, Sixty-seventh Session, A/CN.4/682, 9-14.

23_{See eg Alexander Ovchar, ‘Estoppel in the Jurisprudence of the ICJ: A principle promoting stability threatens} to undermine it’ (2009) 21 BLR 1, 9, comment to footnote 59.

24_{Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America} (Jurisdiction, Admissibility) [1984] ICJ Rep 392.

25_{Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America} (Jurisdiction, Admissibility) [1984] ICJ Rep 392 §§ 39, 50-51.

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takes place in two parts; first the concepts have been examined separately, and then together. The aim of the latter is to highlight the differences and similarities between them.

Acquiescence and estoppel are both general principles, which is why it seemed prudent to also examine the role of general principles as a formal source of international law. This discussion is mainly included to broaden the understanding of the application of acquiescence and estoppel in international law, but also to draw attention to how the concepts may interact with other formal sources such as treaties and custom.

To substantialize this interaction, a section on how acquiescence and estoppel may affect the interpretation of subsequent practice in relation treaties is included. Furthermore, to contrast this, there is also a section discussing the potential effects of acquiescence and estoppel with regard to the formation of customary international law. The aim of these sections is to demonstrate how these two concepts can function in practice.

In order to properly understand the functioning of acquiescence and estoppel in the context of silence knowledge of their characterization and the mechanisms associated with them is important. On this basis, the following questions have been answered throughout the paper: 1. In what situations may silence give rise to acquiescence?

2. In what situations may silence give rise to estoppel?

3. Is it possible to distinguish between acquiescence and estoppel arising from silence?

C. Delimitations

As described above, this paper sets out to examine the situations in which silence may give rise to acquiescence and estoppel. Since these principles may apply in various contexts and in combination with other sources of international law some delimitations are necessary due to the space limitations of this study.

Therefore, this paper is limited to a general study of acquiescence and estoppel arising from silence. This includes an examination of necessary requirements for the principles to arise as well as an overview of general principles as a source of international law. With regard to estoppel, this also comprises a brief discussion of the principle in situations not including silence. The purpose of this is to provide a fuller understanding of the concept in its entirety, as well as to draw attention to the components of the principle that makes it distinct from acquiescence.

With regard to the parts of this paper that relate to the principles interaction with other sources of international law, it is limited to subsequent practice in relation to treaties and the role that they might have in the formation of customary international law. Its important to note that the mentioned principles may be highly relevant with regard to other areas of international law, such as the interpretation unilateral declarations. However, since this study is subject to space

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limitations, other areas in which acquiescence and estoppel may have legal consequences are only included insofar as they are relevant in the context of the main issues of this study. In addition, this study primarily focuses on acquiescence and estoppel arising as a result of negative state conduct. Therefore, the conduct of other international actors such as international organizations is also excluded from the scope of this paper.

D. Method and material

The legal issues in this paper have mainly been approached through the legal dogmatic method. Specifically, this means an examination of the current state of law surrounding the issues.27 In addition, for the purposes of this paper it is also important to distinguish between formal and material sources since the status of general principles as a source international law has been subject to debate.28 On the subject between the difference between formal and material sources Hugh Thirlway writes:

“The important distinction for our purposes is between the place, normally a written document, where the terms of the rule can be found conveniently stated: this is the material source; and the legal element that gives to the

rule its quality as law: this is the formal source.”29

The conclusion one can draw from this is that material sources are content-based, whereas formal rules legitimize the use of a material rule. The formal sources used in this paper stem from the universally accepted international law sources contained in Article 38(1) in the Statute of the International Court of Justice.30

These include international treaties, customary international law, general principles, judicial decisions, and the works of highly qualified scholars and publicists.31 The content of such sources are highly relevant for the analysis of the issues presented in this paper, thus they have also been used as material sources. This is the case since the formal perspective only explains where the source in question derives its force.32 In order to understand how the use of acquiescence and estoppel has been legitimized in international it s important to clarify from where the concepts derive its force. Having said that, most of the paper focuses on the content of the sources in question.

27_{Fredric Korling & Mauro Zamboni, Juridisk metodlära (Studentlitteratur AB 2014).}

28_{Bin Cheng, General Principles of Law as Applied by International Courts and Tribunals (CUP 1953,}

reprinted 2006) 1-5.

29_{Hugh Thirlway, Sources of International Law (OSAIL 2014) 3-4.}

30_{Statute of the International Court of Justice (deposited in the archives of the Government of the United States} of America, San Francisco 24 October 1945) UNTS 993, Article 38(1); Ademola Abass, International Law –

Text, Cases, and Materials (2nd_{ed, OUP 2014) 28; Hugh Thirlway, International Law: The Sources of}

International Law (Malcolm D Evans ed, 4th_{edn, OUP 2014) 92-93.}

31_{Statute of the International Court of Justice (deposited in the archives of the Government of the United States} of America, San Francisco 24 October 1945) UNTS 993, Article 38(d).

32_{Ademola Abass, International Law – Text, Cases, and Materials (2}nd_{ed, OUP 2014) 27; Sources of}

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Considering this, treaties, customary law norms, general principles, cases from both international and domestic courts and tribunals are discussed to illustrate the legal issues presented in the paper. Moreover, documents from the International Law Commission, books, and articles from highly qualified scholars are also used as references throughout this paper.

E. Disposition

Chapter II of this paper commences with an overview of general principles as a formal source in international law. This section also includes a brief examination of acquiescence and estoppel as general principles. The section was included in order to provide a deeper understanding of some of the underlying issues with regard to the application the two principles.

In the adjacent sections of this chapter, the concepts of acquiescence and estoppel are examined both separately and then together. The purpose of this division is to underline the requirements related to each of the two concepts as well as function as a demonstration of the complexities in related to distinction between them.

The part describing estoppel start with a general description of the principle, before estoppel arising as a result silence is dealt with. This is because the requirements remain the same regardless of whether it arose under silence or active conduct.

The first part of chapter III deals with treaty interpretation and subsequent practice. The second part examines the relation between subsequent practice, acquiescence, and estoppel. This format was chosen in order to explain the functioning of subsequent practice as a form of treaty interpretation and to emphasize impact that general principles, in this case acquiescence and estoppel, may have on other formal sources of international law.

Chapter IV discusses the influence that acquiescence and estoppel may have on the formation of customary international law. To contrast the effects that silence in the form of acquiescence and estoppel may have on this area of law, a brief discussion on the persistent objector rule is also included in this section.

Chapter V is the concluding section of this paper and it contains a summary of all the topics that have been examined in this paper. It also connects to the questions posed in the section B of this chapter.

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II.

G

ENERAL PRINCIPLES

,

ACQUIESCENCE

,

AND ESTOPPEL

A. General principles

General principles as a formal source of law was recognized within the international law spectra as early as 1899, and was also discussed during the Hague Conferences of 1899 and 1907. They appeared as a source of law in the Statute of the Permanent Court of International Justice (PCIJ),33 and are currently incorporated as a source of international law in Article 38(1)(c) in the Statute of the Court. According to various scholars, the reason for incorporating them as a source of international law was to deter the ICJ from declaring claims before it non liquet. In other words, to avoid that the lack of law on a particular subject would make it impossible for the Court to either uphold or reject a claim.34

The general principles existing in the international arena derive from national legal systems. Nevertheless, this does not mean that such principles have been transplanted into the international legal system in their current state. On the contrary, international courts and tribunals have adapted such principles through comparative analogies and used methods of general legal reasoning to make them coherent with the system itself.35 One example of this is the Right of Passage over Indian Territory case, in which the ICJ relied on comparative law studies concerning general principles.36

It is interesting to note that, even though general principles are incorporated as independent sources of law in Article 38(1)(c), that the ICJ and the PCIJ, have rarely based a decision solely on the basis of general principles.37 This notion could possibly be explained by the fact that there is a great uncertainty not only as to when general principles are applicable, but also what the criteria related to particular general principles are. Some writers believe that the use of general principles can only be legitimized in light of customary law, whereas others consider them as means of interpretation in relation to treaties and custom, or even as subsidiary sources of law.38

Against this background, numerous scholars have attempted to define general principles, both as a concept, but also the extent of the existence of such principles within international law. In spite of this, there is no exhaustive list, and as Maria Panzi writes:

33_{Maria Panezi, ‘Sources of Law in Transition – Re-visiting General Principles of International Law’ (2007)} 2007 ANCILLA IURIS 66, 67.

35_{See for example James Crawford, Brownlie’s Principles of Public International Law (8th edn, OUP 2012)} 34-38.

36_{Right of Passage over Indian Territory (Portugal v India) (Merits, Judgment) [1960] ICJ Rep 6.}

38_{Bin Cheng, General Principles of Law as Applied by International Courts and Tribunals (CUP 1953,}

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“In a sense of praetorian jurisprudence it seems that a court does not need to explain where it found the general principle, or how it established it exists. It can only invoke it, identify it as a general principle, that being enough

to provide for legitimacy.”39

However, regardless of what theory that is applied to general principles, it is clear in light of the above, that they form part of the formal sources of international law. It is also clear that Acquiescence and estoppel constitute general principles of international law. This notion has not always been uncontroversial since the concepts have also been discussed in light of customary international law. This inevitably raised the question of whether these doctrines may also be rules of a customary nature.40

Nevertheless, international jurisprudence clearly shows that both acquiescence and estoppel are general principles of international law.41_{For example, in the Chagos Case}42_the

Permanent Court of Arbitration (PCA) referred to estoppel as ‘a general principle of law’.43 The same conclusion can be drawn, albeit indirectly from the Gulf of Maine Case,44 in which the ICJ Chamber Court stated that estoppel and acquiescence stem from the principles of good faith and equity.45

Furthermore, this contention has been supported in the works of scholars such as Hugh Thirlway,46 Ian Brownlie,47 as well as others.48 Since international courts and tribunals have made this categorization and it has gained support in doctrine, it ought to be considered settled that, regardless of their status in relation to custom, acquiescence and estoppel are at the very least general principles of international law.

As such, acquiescence and estoppel has inevitably also been affected by the uncertainties and ambiguities described above. Furthermore, and as is demonstrated below, silence as a form of negative form of state conduct further strengthens the complexities surrounding the two concepts. This is because not all situations in which a state remains silent will give rise to

39_{Maria Panezi, ‘Sources of Law in Transition – Re-visiting General Principles of International Law’ (2007)} 2007 ANCILLA IURIS 66, 69.

40_{See eg Ian C MacGibbon, ‘Estoppel in International Law’ (1958) 7 ICLQ 468, 468.}

41_{Chagos Marine Protected Area Arbitration (Maritius v United Kingdom) (2015) PCA Case no 2011-03 § 435;}

Delimitation of the Maritime Boundary in the Gulf of Maine Area (Canada v United States of America) (Merits,

Judgment) [1984] ICJ Rep 246 §§ 25, 26, 79, 130. Argentina-Chile Frontier Case (Argentina, Chile) [1966] UNRIAA, vol XVI, 109-182 § 164.

42_{Chagos Marine Protected Area Arbitration (Maritius v United Kingdom) (2015) PCA Case no 2011-03.} 43_{Id § 435.}

44_{Delimitation of the Maritime Boundary in the Gulf of Maine Area (Canada v United States of America)} (Merits, Judgment) [1984] ICJ Rep 246.

45_{Id 130.}

46_{Hugh Thirlway, The Law and Procedure of the International Court of Justice: Fifty Years of Jurisprudence} (vol II, OUP 2013) 1122-1143.

47_{Ian Brownlie, Principles of Public International Law (6th ed, 2003) 616.}

48_{See eg Henrik Horn & Petros C Marvroidis eds, The WTO Case Law of 2004-2005 – Legal and Economic}

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acquiescence or estoppel. Hence, the inaction of a state must be interpreted in the light of the factual circumstances in which is has been maintained.

B. Acquiescence

In international law, silence is most frequently associated with acquiescence. This ought to be because the foundation of this concept is built on inaction.49 As a form of negative state conduct, acquiescence may be a determinant factor when it comes to determining the rights and obligations of states in situations which ‘calls for a reaction’.50 In the Gulf of Maine Case the ICJ Chamber Court described the principle by stating that:

“acquiescence is equivalent to tacit recognition manifested by unilateral conduct which the other party may interpret as consent.”51

Acquiescence is hence a negative concept that is generally associated with a lack of reaction in a situation that calls for a positive response, such as an objection.52 This means that silence may result in a tacit agreement. By extension this may also result in that strictly prohibited acts can be accepted if they are followed by repetition, as well as consent from other states.53

The concept often linked with the Latin maxim Qui tacit consentire videtur si loqui debuisset ac potuisset54 which translates to: “he who keeps silent is held to consent if he must and can speak.”55

Remaining silent in a given situation does not always amount to acquiescence. On the contrary, the circumstances must be of a special nature, or in other words the interests of the states involved must be at stake.56 This means that it is not actually the situation per se that calls for a reaction; instead it is the state’s interest in preserving its rights that determines whether the situation in question ought to require a positive reaction.57

50_{Id 87-89.}

52_{Ian Mcgibbon, ‘The Scope of Acquiescence in international law’ (1954) 31 Bristish Ybk Intl L 143, 143.} 53_{Id; Chan Phil, ‘The Scope of Acquiescence in international law p 143; Acquiescence/Estoppel in International} Boundaries: Temple of Preah Vihear revisited’ (2004) 3 Chinese J Intl L 421, 422-423, 425.

54_{See eg Temple of Preah Vihear (Cambodia v Thailand) (Merits, Judgment) [1962] ICJ Rep 6, 23; Nuno Sérgio} Marques Antunes, ‘Acquiescence’, Max Planck Encyclopedia of Public International Law (2006)

<http://opil.ouplaw.com/view/10.1093/law:epil/9780199231690/law-9780199231690-e1373>.

55_{Translation offered by Nuno Sérgio Marques Antunes in: Nuno Sérgio Marques Antunes, ‘Acquiescence’,}

Max Planck Encyclopedia of Public International Law (2006).

<http://opil.ouplaw.com/view/10.1093/law:epil/9780199231690/law-9780199231690-e1373>.

56_{Ian Mcgibbon, ‘The Scope of Acquiescence in international law’ (1954) 31 Bristish Ybk Intl L 143, 143; Chan} Phil, ‘The Scope of Acquiescence in international law p 143; Acquiescence/Estoppel in International

Boundaries: Temple of Preah Vihear revisited’ (2004) 3 Chinese J Intl L 421, 422-23.

57_{James Crawford, Brownlie’s Principles of Public International Law (8th edn, OUP 2012) 419; see also the} reasoning in Temple of Preah Vihear (Cambodia v Thailand) (Merits, Judgment) [1962] ICJ Rep 6 and in the

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In order to be able to react to a set of facts, a state must have had necessarily knowledge of the situation. Hence, when it comes to acquiescence, a tacit acceptance of a situation will only occur if the state in question had knowledge, or ought to have known about the circumstances that called for a reaction.

For example, in the Temple of Preah Vihear Case the ICJ concluded that in order for silence to have the effect of being interpreted as consent, the situation must not only call for a reaction, but also be known to a competent authority.58

The situation that the ICJ concluded called for a reaction in the Temple of Preah Vihear Case, revolved around the fact that the Thai authorities had remained silent with regard to a map, which placed the disputed temple in question within the borders of Cambodia. The map had been produced by French topographical officers at the request of the Siamese (now Thai) authorities since they lacked the means to do so.59

The map was later communicated to the Siamese authorities by the French authorities. It was referred to as being part of the work by a joint-commission that was composed of representatives from both countries.60 The commission in question derived its competences from a bilateral treaty between the two states.61

During the proceedings in the ICJ Thailand claimed that the joint-commission had exceeded its competence when placing the temple in Cambodia since the underlying treaty did not contain any provisions as to the delimitation of the temple area.62 As a response to this the ICJ concluded that such a possible error was beside the point,63 and stressed that the Thai authorities had not only remained silent with regard to the map, but they had also used it themselves.64 In light of this, the ICJ concluded that the map did not have binding force at the time of its production,65 but had acquired such a status in light of the subsequent events. The Court’s extensive reasoning with regard to whether the Thai authorities had knowledge of the disputed map clearly demonstrates that knowledge of the situation at hand is an important element when it comes to acquiescence. Furthermore, the passage of time may also play a determinant role when it comes to assessing whether a state has acquiesced to a certain fact or not.

Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America

(Jurisdiction, Admissibility) [1984] ICJ Rep 392.

58_{Temple of Preah Vihear (Cambodia v Thailand) (Merits, Judgment) [1962] ICJ Rep 6, 23-27.} 59_{Id 20.} 60_Id. 61_Id. 62_{Id 20-23.} 63_{Id 22.} 64_{Id 22-23.} 65_{Id 21.}

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According to Ian McGibbon, the presumption that silence equals tacit consent is fortified by the period of time that passes before the concerned states react to the situation in question.66_It

is an essential element when it comes to for example historic and prescriptive rights,67 but is highly also relevant in relation to treaties as well as customary international law.68_{In spite of}

this, there is no explicit time limit that has been laid down with regard to this criterion.

To a certain extent this ought to be explained by the fact that an assessment of whether a state’s silence has given rise acquiescence is to be determined in casu. This notion is supported by the fact that it is the rights at stake that will determine whether a situation calls for a reaction or not. Consequently, the factual circumstances of each case must be taken into account in order to determine whether a tacit acceptance exists or not. Therefore, there is a possibility that the time limit for raising an objection in a given situation may vary depending on the both the rights in question and the factual circumstances of each case.

Support for this can also be found in international jurisprudence. For example, the disputed map in the Temple of Preah Vihear Case gained a lot of publicity and was communicated to multiple other countries.69 According to the Court, these circumstances in combination with the fact that the maps had official standing ‘called for some reaction, within a reasonable period’.70 It was pointed out that Thailand had waited 50 years to raise an objection as to the validity of the map.71 On this basis, the Court concluded that Thailand had acquiesced with regard to the binding character of the map.72

Similar stances to acquiescence have been reiterated in numerous other international cases such as the Continental Shelf Arbitration,73 Land, Island and Maritime Dispute Case,74 and in the Military And Paramilitary Activities Case.75

In the Continental Shelf Arbitration the UK argued that France had acquiesced to a practice that made a certain rock ‘the base-point for the measurement’ of the UK’s fishing zones and territorial waters.76_{It evidenced its argument by claiming that competent French authorities}

66_{Ian Mcgibbon, ‘The Scope of Acquiescence in international law’ (1954) 31 Bristish Ybk Intl L 143, 143.} 67_Id.

68_{See eg Temple of Preah Vihear (Cambodia v Thailand) (Merits, Judgment) [1962] ICJ Rep 6; Ian Mcgibbon,} ‘The Scope of Acquiescence in international law’ (1954) 31 Bristish Ybk Intl L 143, 143.

69_{Temple of Preah Vihear (Cambodia v Thailand) (Merits, Judgment) [1962] ICJ Rep 6, 23.} 70_{Id 23.}

71_{Id, based on the author’s own calculation in light of the years provided in the case.} 72_{Id 23.}

73_{Delimitation of the Continental Shelf between the United Kingdom of Great Britain and Northern Ireland, and}

the French Republic (United Kingdom, France) (Award) (1978) UNRIAA, Vol XVIII, 3-413

§§ 127-144.

74_{Land, Island and Maritime Frontier Dispute Case (El Salvador v Honduras: Nicaragua Intervening) (Merits,} Judgment) [1992] ICJ General list no 75 §§ 68-81, 348-355.

75_{Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America} (Jurisdiction, Admissibility) [1984] ICJ Rep 392 §§ 31-39.

76_{Delimitation of the Continental Shelf between the United Kingdom of Great Britain and Northern Ireland, and}

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knew of this usage, and that it had never been questioned until the dispute arose.77 Namely, the French authorities had not raised any objection for more than 10 years.

The arbitration tribunal agreed with this reasoning. Even though it made no reference to a situation that called for a reaction, such reasoning ought to be considered implicit in the judgment since the Court of Arbitration stressed the fact that the French authorities had not challenged the UK’s contentions for a prolonged period of time.78

The Land, Island and Maritime Dispute Case regarded a boundary dispute between Honduras and El Salvador.79 Negotiations regarding the demarcation of the boundary had commenced in 1881.80 The ICJ Chamber Court found that Honduras’ conduct; or rather inactivity from 1881-1972, when the demarcation was questioned by Honduras, amounted to acquiescence.81 This decision was based on the fact that Honduran authorities had knowledge of the circumstances, but no action was taken until 91 years later.

The jurisdiction and Admissibility phase of the Military and Paramilitary Activities Case concerned whether Nicaragua’s declaration to the Court’s compulsory jurisdiction had binding force although it lacked a signature.82_{In spite of its procedural default, the Court}

came to the conclusion that Nicaragua’s declaration was valid by virtue of its subsequent conduct.83 Nicaragua had for a period of almost 40 years appeared in numerous ICJ Yearbooks as one of the states having accepted the Courts compulsory jurisdiction.84

In the face of this, Nicaragua had remained silent and neither contested nor approved these statements.85 It was pointed out that Nicaragua must have had knowledge of the situation, and according to the Court, its silence towards the situation amounted to acquiescence, whereby Nicaragua’s declaration to the Court was considered to have binding force.86 This was further strengthened by the fact that no states had objected to the validity of Nicaragua’s declaration to the Court’s jurisdiction up until the proceedings of the case in question.87

The cases referred to above illustrate diversity of situations that can be considered to call for a reaction in order to avoid becoming bound to a certain fact by remaining silent. It is however, evident that the it is the rights at stake that functions as the determinant factor as to whether a

77_{Id § 129.} 78_{Id §§ 141-144.}

79_{Land, Island and Maritime Frontier Dispute Case (El Salvador v Honduras: Nicaragua Intervening) (Merits,} Judgment) [1992] ICJ General list no 75 §§ 74-75.

80_{Id §§ 74-76.} 81_{Id § 80.}

82_{Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America} (Jurisdiction, Admissibility) [1984] ICJ Rep 392 §§ 32-36.

83_{Id §§ 31-39.} 84_{Id §§ 37-38.} 85_{Id § 39.} 86_{Id § 38-39.} 87_Id.

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situation actually call for some kind positive action. The importance of awareness with regard to the situation at hand is also greatly emphasized in all of the above-mentioned cases. Furthermore, the passage of time varies from case to case which implies that is indeed the factual situation of each case that will determine what constitute “a reasonable period” in which a state may raise objections.

While all of these cases concerned acquiescence, the Temple of Preah Vihear Case and the Military and Paramilitary Activities Case also dealt with the issue of estoppel. Hence, further analysis of these particular cases is provided for in the adjacent sections.

C. Estoppel

1. Estoppel, preclusion, and debarment

Estoppel is a well-recognized concept in international law and has been used by multiple international courts and tribunals to promote stability in international relations.88 It is derived from the common law legal systems in which there are multiple variances of the concept.89 Nevertheless, according to Sir Hersch Lauterpacht, concepts similar to estoppel exist in all legal systems of private law.90

Estoppel is often associated with the Latin dictum ‘allegans contraria non audiendus est’, which means that ‘one should not benefit from his or her own inconsistency’91 Some other principles linked to this Latin maxim are the civil law concepts of preclusion, debarment and foreclusion.92 This is important to mention because it could be one of the reasons why international courts’ and tribunals’ have used very different terminologies in cases that appears to have dealt with issues of estoppel.93

The reasoning of the ICJ Chamber Court in the Gulf of Maine Case also strengthens this contention. In this case it is stated that estoppel originates from the principles of good faith and equity, and that it is closely related to the notion of preclusion.94

88_{Alexander Ovchar, ‘Estoppel in the Jurisprudence of the ICJ: A principle promoting stability threatens} to undermine it’ (2009) 21 BLR 1, 1-3.

89_Id.

90_{Sir Hersch Lauterpacht, Private Law Sources and Analogies of International Law (Logmans, Green & Co Ltd} 1927) 204.

91_{Alexander Ovchar, ‘Estoppel in the Jurisprudence of the ICJ: A principle promoting stability threatens} to undermine it’ (2009) 21 BLR 13, 3, this is the translation used in this paper, but is also, according to Ovchar, the translation that has been most endorsed in the international law community. See footnote 13 in the same article.

92_{Id 3.} 93_{Id 1-3.}

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According to Alexander Ovchar, the concept of estoppel in international law consists of three main criteria. First, a statement or representation must have been made by a state; second, the former must be clear and unconditional and made by a competent authority; third, the state claiming estoppel must have relied on the representation in question.95_{On a similar note}

James Crawford writes, by referring to Derek Bowett, that the necessary requirements of estoppel are:

“(a) an unambiguous statement of fact; (b) which is voluntary, unconditional, and authorized; and (c) which is relied on in good faith to the detriment of the other party or to the advantage of the party making the

statement.”96

Despite the fact that the requirements of estoppel in international law have been identified, it is important to note that the institution has not been applied with uniformity.97 Hence, it is difficult to fully understand its functioning in the international sphere. This is not the only issue surrounding the subject. Since international courts and tribunals have used different terminology when applying the principle it may in some cases be difficult to determine whether the case in question actually deals with estoppel or some other intuition.

In fact the cases that have contained the most consistent terminology in relation to the subject are those in which the ICJ has ruled that the facts did not actually give rise to an estoppel.98 For example, in the North Sea Continental Shelf Cases the ICJ examined inter alia whether Germany was estopped from claiming not to be bound by Article 6 in the 1958 Geneva Continental Shelf Convention.99 Germany had previously signed the Convention in question, but had not ratified it.100_{In light of this the Court stated that:}

“the existence of a situation of estoppel could suffice to lend substance to this contention, that is to Say if the Federal Republic were now precluded from denying the applicability of the conventional régime, by reason of past conduct, declarations, etc., which not only clearly and consistently evinced acceptance of that régime, but

95_{Alexander Ovchar, ‘Estoppel in the Jurisprudence of the ICJ: A principle promoting stability threatens} to undermine it’ (2009) 21 BLR 13, 4-7; see also eg North Sea Continental Shelf Cases (Federal Republic of Germany v Denmark and Netherlands) (Judgment) [1969] ICJ Rep 3§ 26.

96_{James Crawford, Brownlie’s Principles of Public International Law (8th edn, OUP 2012) 420; Derek Bowett,} ‘Estoppel Before International Tribunals And Its Relation To Acquiescence’ (1957) 33 British Ybk Intl L 176, 202.

97_{James Crawford, Brownlie’s Principles of Public International Law (8th edn, OUP 2012) 420-421; Alexander} Ovchar, ‘Estoppel in the Jurisprudence of the ICJ: A principle promoting stability threatens

to undermine it’ (2009) 21 BLR 13, 5.

98_{Compare eg North Sea Continental Shelf Cases (Federal Republic of Germany v Denmark and Netherlands)} (Judgment) [1969] ICJ Rep 3 and the Land and Maritime Boundary between Cameroon and Nigeria (Cameroon v Nigeria: Equatorial Guinea intervening) (Merits, Judgment) [2002] ICJ Rep 303 with the Temple of Preah

Vihear (Cambodia v Thailand) (Merits, Judgment) [1962] ICJ Rep 6 and the Arbitral Award Made by the King of Spain on 23 December 1906 (Honduras v Nigeria) (Merits, Judgment) [1960] ICJ Rep 192.

99_{North Sea Continental Shelf Cases (Federal Republic of Germany v Denmark and Netherlands) (Judgment)} [1969] ICJ Rep 3; Convention on the Continental Shelf (adopted 29 April 1958, entered into force 10 June 1964) 499 UNTS 311 (1958 Geneva Convention) art 6.

100_{North Sea Continental Shelf Cases (Federal Republic of Germany v Denmark and Netherlands) (Judgment)} [1969] ICJ Rep 3 § 39.

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also had caused Denmark or the Netherlands, in reliance on such conduct, detrimentally to change position or suffer some prejudice. Of this there is no evidence whatever in the present case.”101

These requirements have been endorsed in other cases such as the in the judgment on Nicaragua’s application to intervene in the Land, Island and Maritime Frontier Dispute,102 the Cameroon v Nigeria Case,103 and in the Military and Paramilitary Activities Case.104 However, the common denominator in all of these cases is that there was no evidence of an estoppel in the given situations.

Even though these cases can serve as proof that estoppel is a recognized concept in international law, there are no cases in which either the PCIJ or the ICJ have explicitly applied it. Instead they have used estoppel-like theories and managed to avoid make a clear pronunciation of its practical substance as well as necessary requirements.

For example, the Legal Status of Eastern Greenland Case is frequently referred to as an estoppel-case. However, estoppel is never mentioned in the case, instead the PCIJ used the term “debarred”.105 By contrast, in the Genocide Case the ICJ refer to both the phrase “debar” and estoppel as belonging to the same institution, although the requirements for an estoppel to arise was not considered to be fulfilled.106

However, in light of the fact that institutions such as preclusion, estoppel, and debarment all stem from the same Latin dictum it must be assumed that they are indeed different expressions of the same institution. This can be further supported by how they have been applied international courts and tribunals.

2. Estoppel in international courts and tribunals

In the Chagos Case the PCA found that the circumstances of that case gave rise to an estoppel.107 The reasoning in this judgment provides a systematic examination of the concept and how it may function in international law. When defining estoppel, the tribunal relied on the words of Judge Spencer:

101_{Id § 30.}

102_{Land, Island and Maritime Frontier Dispute (El Salvador v Honduras) (Application to Intervene, Judgment)} [1990] ICJ Rep 92 § 63.

103_{Land and Maritime Boundary Between Cameroon v Nigeria Case (Cameroon v Nigeria) (Preliminary} Objections, Judgment) [1998] ICJ Rep 275 § 57.

104_{Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America} (Jurisdiction, Admissibility) [1984] ICJ Rep 392 § 51.

105_{Legal Status of Eastern Greenland (Denmark v Norway) (Merits, Judgment) [1933] PCIJ General list 43 §} 186.

106_{Application of the Convention on the Prevention and Punishment of the Crime of Genocide (Bosnia &} Herzegovina v Serbia & Montenegro) (Merits, Judgment) [2002] ICJ Rep 43 § 85, 104.

107_{Chagos Marine Protected Area Arbitration (Maritius v United Kingdom) (Award) (2015) PCA Case no} 2011-03 § 435.

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”estoppel operates to prevent a State contesting before the Court a situation contrary to a clear and unequivocal representation previously made by it to another State, either expressly or impliedly, on which representation the other State was, in the circumstances, entitled to rely and in fact did rely, and as a result the other State has been

prejudiced or the State making it has secured some benefit or advantage for itself.”108

In light of this, the PCA clearly recognized that estoppel might arise from silence since a ‘representation’ can be made impliedly. In this case it is also emphasized that estoppel is designed to safeguard states’ legitimate expectations with regard to the conduct of one another. On this basis, and by virtue of good faith, a state making a representation that creates reliance cannot suddenly start to act contrary to its previous representation.109

With regard to silent representations, other cases such as the Elettronica Sicula SpA Case and the Temple of Preah Vihear Case recognize that negative conduct may give rise to estoppel.110

For example, in the Temple of Preah Vihear Case the ICJ concluded that Thailand’s silence with regard to the map also resulted in that it was precluded (estopped) from disputing its validity.111

Furthermore, the Fisheries Case is commonly referred to as a case where estoppel arose as a result of silence even though the principle is never explicitly mentioned in the reasoning of the Court.112 In this case the United Kingdom (UK) questioned whether Norway’s use of baselines were in accordance with international law.113 Norway admitted to the fact that this usage was to be considered a derogation from general international law, but contended that it was justified since it had been practiced for a significant amount of time and because no states had raised any objections against it.114 The ICJ concluded that Norway’s delimitation of the baselines in question had been consistently applied from 1869 until the date that the UK seized the Court.115

Furthermore, this practice had been tolerated, without objections by other states, for over sixty years.116_{In light of this, the UK claimed that its lack of objection depended on the fact that it}

108_{Id § 435; Temple of Preah Vihear (Cambodia v Thailand) (Dissenting Opinion, Judge Spencer) [1962] ICJ} Rep 101, 143-144.

109_{Chagos Marine Protected Area Arbitration (Maritius v United Kingdom) (Award) (2015) PCA Case no} 2011-03 § 435.

110_{Elettronica Sicula SpA (United States of America v Italy) [1989] ICJ Rep 15 § 44; Temple of Preah Vihear} (Cambodia v Thailand) (Merits, Judgment) [1962] ICJ Rep 6, 32.

111_{Temple of Preah Vihear (Cambodia v Thailand) (Merits, Judgment) [1962] ICJ Rep 6, 32.}

112_{Alexander Ovchar, ‘Estoppel in the Jurisprudence of the ICJ: A principle promoting stability threatens} to undermine it’ (2009) 21 BLR 13, 10-11; Fisheries Case (United Kingdom v Norway) (Merits, Judgment) [1951] ICJ Rep 116.

113_{Fisheries Case (United Kingdom v Norway) (Merits, Judgment) [1951] ICJ Rep 116, 125.} 114_{Id 127-130, with regard to the lapse of time the phrase ‘time immemorial’ is used.} 115_{Id 138.}

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had not had knowledge of the situation in question.117 The Court however, did not accept this contention since the UK amongst other things:

”As a coastal State on the North Sea, greatly interested in the fisheries in this area, as a maritime Power traditionally concerned with the law of the sea and concerned particularly to defend the freedom of the seas, the

United Kingdom could not have been ignorant of the Decree of 1869 which had at once provoked a request for explanations by the French Government”118

The Court also pointed out that this practice had occurred openly and ‘in the face of’ other states, which evidenced that the attitude towards the situation was that it should be considered in accordance with international law.119 On this basis it was concluded that the UK could not object to this after having refrained from raising any objections for such a long period of time.120

The conclusion that can be drawn from this is that the ICJ based its decision on the presence of acquiescence, and that the UK in the face of its subsequent conduct also was estopped from claiming that Norway’s practice with regard to its baseline’s was not in accordance with international law. It is clear that the requirements of acquiescence are fulfilled since the UK remained silent in a situation that according to the ICJ called for a reaction. It was presumed that the respondent also ought to have been aware of the situation, as well as had an interest in in it. Even though the Court did not explicitly pronounce it, it must be induced that this resulted in a tacit acceptance of the circumstances.

Furthermore, the requirements of estoppel ought to be considered fulfilled despite the lack of an explicit reference to the term itself. This conclusion can be evidenced by the fact that the ICJ considered the UK’s subsequent conduct and its demonstrated attitude towards the facts of the case. It was also stressed that Norway had relied on the acceptance of the UK and other states for over 60 years. A decision to the contrary would inevitably have meant that Norway would have suffered some prejudice.

In light of this, it is clear that acquiescence and estoppel are two concepts that may be heavily intertwined and very hard to distinguish between. The adjacent section deals with this issue, and attempts to highlight the existing differences and similarities between the two principles.

D. Acquiescence and estoppel

The distinction between acquiescence and estoppel in international law is complex, and according to some authors, sometimes impossible to assess.121 Nevertheless, the Gulf of

117_{Id 138-139.} 118_Id. 119_Id.

120_{Id 138-140.}

121_{See eg Alexander Ovchar, ‘Estoppel in the Jurisprudence of the ICJ: A principle promoting stability threatens} to undermine it’ (2009) 21 BLR 1, 9, comment to footnote 59.

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Maine Case can serve as guidance in this context. In the instant case the ICJ Chamber Court pointed out that the same facts can apply to both acquiescence and estoppel, even though the concepts are based on different legal reasoning.122 The Chamber Court further stated that:

“acquiescence is equivalent to tacit recognition manifested by unilateral conduct which the other party may interpret as consent, while estoppel is

linked to the idea of preclusion.”123

It also explained that the most fundamental difference between the two concepts is the element of reliance, which is prerequisite for an estoppel to arise, but not required for Acquiescence to apply.124 However, James Crawford among others argue that another disparity in relation to the relevant institutions is the fact that tacit consent is required with regard to Acquiescence, but not for estoppel arise.125

There are two cases that are particularly relevant when it comes to highlighting the complex relation between acquiescence and estoppel, namely: the Temple of Preah Vihear Case and the Military and Paramilitary Activities Case. As discussed previously, in the former judgment the Court found that both acquiescence and estoppel applied to the same fact pattern,126 at least if one accepts the argument that preclusion is a consequence of estoppel. In the Military and Paramilitary Activities Case on the other hand, the Court opted for acquiescence and ruled out estoppel.127

As mentioned previously, the Court concluded that Nicaragua had acquiesced to the Court’s compulsory jurisdiction. However, there was also a claim of estoppel in this case. The respondent, namely, the US argued that even if Nicaragua could rely on its declaration in general, it was estopped from doing so in relation to the US. According to the US, in their diplomatic relations Nicaragua had acted towards the former as though she was never bound by her declaration.128_{The US further alleged it had relied on this representation which had}

been clear and consistent.129

The Court did not agree with this contention. It stated that in light of Nicaragua’s acquiescence to be bound by the optional-clause and the fact that the US failed to produce sufficient evidence with regard to Nicaragua’s representation towards it, and therefore an

123_Id. 124_Id.

125_{James Crawford, Brownlie’s Principles of Public International Law (8th edn, OUP 2012) 422; Temple of}

Preah Vihear (Cambodia v Thailand) (Seperate Opinion, Judge Fitzmaurice) [1962] ICJ Rep 6, 63.

126_{Temple of Preah Vihear (Cambodia v Thailand) (Merits, Judgment) [1962] ICJ Rep 6, 23, 32.} 127_{Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America} (Jurisdiction, Admissibility) [1984] ICJ Rep 392 §§ 39, 50-51.

128_{Id § 48.} 129_{Id §§ 48-49.}

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estoppel could not arise.130 Neither could it be proven that Nicaragua’s ‘reliance’ on its own declaration was contrary to ‘good faith or equity’.131

This certainly explains why Nicaragua could rely on its declaration against the US, but it does not explain why Nicaragua, in addition to having acquiesced to the validity of the relevant instrument, was not also estopped from claiming that it was not legally binding upon it. As mentioned formerly, in the Temple of Preah Vihear Case Thailand was not only considered to have acquiesced to the map placing the temple in the border of Cambodia, but was also precluded from denying this.132

The difference between the two cases ought to be that Nicaragua never denied the validity of its declaration. In addition, Nicaragua was the party that seized the Court for the purpose of adjudication in the dispute.133 It is however possible that the outcome would have been different if Nicaragua had been the party denying this acceptance, maybe then an estoppel would have prevented it from making such a claim.

Further guidance as to the differences between acquiescence and estoppel can be found in Sir Gerarld Fitmaurice separate opinion to the Temple of Preah Vihear Case in which he clarifies in what situations acquiescence may be accompanied by estoppel. He recalls that acquiescence and estoppel stem from different theories, but acknowledges that the former can function as the latter in certain situations.134 On this subject he states that:

“for instance where silence, on an occasion where there was a duty or need to speak or act, implies agreement, or a waiver of rights, and can be regarded as a representation to that effect”

The conclusion that can be drawn from this is that acquiescence can operate as an estoppel when tacit acceptance is also perceived as a representation, which as mentioned previously is the first requirement of estoppel. This reasoning is also in line with the ICJ Chamber Courts statement in the Gulf of Maine Case that the two principles can apply to the same fact pattern.135

This inevitably raises the question of how these institutions operate in practice. On this note, Sir Gerald Fitzmaurice makes an excellent distinction between the legal effects of the two principles in his separate opinion to the Temple of Preah Vihear Case. He explains that acquiescence is equivalent to accepting a certain obligation or becoming bound by ‘a certain

130_{Id §§ 50-51.}

131_{Id § 51.}

132_{Temple of Preah Vihear (Cambodia v Thailand) (Merits, Judgment) [1962] ICJ Rep 6, 32.}

133_{Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America} (Jurisdiction, Admissibility) [1984] ICJ Rep 392.

134_{Temple of Preah Vihear (Cambodia v Thailand) (Seperate Opinion, Judge Fitzmaurice) [1962] ICJ Rep 6, 62.} 135_{Delimitation of the Maritime Boundary in the Gulf of Maine Area (Canada v United States of America)} (Merits, Judgment) [1984] ICJ Rep 246 § 130.

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instrument’, whereas estoppel precludes a party from denying the existence of such an obligation.136

Furthermore, an estoppel is not dependent on the acceptance of a party. It might very well be that there was no acceptance to begin with, but an estoppel may still arise due to a state’s subsequent conduct in relation to a certain situation. To clarify, if it can be shown that a state did not accept an obligation, or if it cannot be shown that it did, the state in question can still be estopped from claiming that it is not bound by the obligation by virtue of its succeeding actions.137

Another author that agrees with this logic is James Crawford. He writes that acquiescence does not require reliance, but that it should be interpreted as ‘a promise implied in the context of lapse of time.’138 With regard to estoppel he argues that it is a general concept that consists of representations that were not intended to be binding. Instead it is the surrounding circumstances in which the representations occurred that ‘crystallizes’ their binding force.139 Thus, for an estoppel to arise, an implied promise is not a necessary requirement.140 In light of this it is possible to draw the conclusion that the concept of estoppel is conduct-based,141 whereas acquiescence possesses a mental element in the form of acceptance that can be validated by the surrounding circumstances.

In light of this one can draw the conclusion that the greatest differences between acquiescence and estoppel are that the former requires acceptance, which the latter does not. In addition, estoppel requires a representation that has created reliance, which is not necessary for acquiescence. The next sections aim to further underline these differences by demonstrating how the principles can apply in relation to subsequent practice to treaties and the formation of customary international law.

III. T

REATY LAW

A. Treaty interpretation

As mentioned previously, a treaty or a convention is one of the recognized formal sources enshrined in Article 38(1)(a) Statute of the Court.142 Rules relating to treaties, and their interpretation have, to a great extent, been codified in the 1969 Vienna Convention on the

136_{Temple of Preah Vihear (Cambodia v Thailand) (Seperate Opinion, Judge Fitzmaurice) [1962] ICJ Rep 6, 63.} 137_Id.

138_{James Crawford, Brownlie’s Principles of Public International Law (8th edn, OUP 2012) 422.} 139_{Id 421.}

140_Id.

141_{Hugh Thirlway, The Law and Procedure of the International Court of Justice: Fifty Years of Jurisprudence} (vol II, OUP 2013) 1125.

142_{Statute of the International Court of Justice (deposited in the archives of the Government of the United States} of America, San Francisco 24 October 1945) UNTS 993.

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Law of Treaties (VCLT).143 According to Articles 1 and 2(a) of the VCLT, a treaty, for the purposes of the Convention, is a written agreement, concluded between states, which is governed by international law.144

The binding force of treaties or conventions is premised on the principle pacta sunt servanda. In other words, parties to a treaty should be able to rely on the notion that the obligations set out in the instrument will be followed by those whom have chosen to be bound by it.145 Treaties are highly important sources of law within the international legal system, and there are areas in which treaties are almost entirely dominant, such as environmental law.146 Treaties have also played a pivotal role in the development and progression in fields such as human rights, as well as international investment law.147

The VCLT recognizes every state’s capacity to conclude treaties. However the conclusion must be made by a competent authority, such as heads of states or ministers of foreign affairs.148 Furthermore, means of expressing consent to be bound by a treaty includes inter alia signature, acceptance or ratification.149 Consent can be given in numerous ways. However, it is clear that the conclusion to be bound by a treaty requires some kind of active conduct. In other words, states cannot become parties to treaties through silence. Nevertheless, when it comes to treaty interpretation, silence may have legal implications, especially in light of subsequent practice.

Before moving on to the subject of treaty interpretation, it must be acknowledged that it is a vast and complex issue, in which there are multiple rules and approaches that may be relevant.150 The general rules on treaty interpretation have been codified in Articles 31-33 VCLT. In Article 31(1) it is stated that treaties shall be:

“interpreted in good faith in accordance with the ordinary meaning to be given to the terms of the treaty in their context and in the light of its object and purpose”

From the first paragraph of Article 31 it can be understood that focus is placed primarily on literal interpretation. This is also evident from the jurisprudence of the ICJ, which has placed

143_{Vienna Convention on the Law of Treaties (adopted 23 May 1969, entered into force 27 January 1980) 1155} UNTS 331 (VCLT); Malgosia Fitzmaurice, International Law: The Practical Wokring of the Law of Treaties (Malcolm D Evans ed, 4th_{edn, OUP 2014) 166.}

144_{Vienna Convention on the Law of Treaties (adopted 23 May 1969, entered into force 27 January 1980) 1155} UNTS 331 (VCLT), Article 1 and 2(a).

145_{Thirlway H, International Law: The Sources of International Law (Malcolm D Evans ed, 4}th_{edn, OUP 2014)} 95.

146_{Malgosia Fitzmaurice, International Law: The Practical Wokring of the Law of Treaties (Malcolm D Evans} ed, 4th edn, OUP 2014) 166.

147_Id.

148_{Vienna Convention on the Law of Treaties (adopted 23 May 1969, entered into force 27 January 1980) 1155} UNTS 331 (VCLT), Articles 6-7.

149_{Id Article 11, see also Articles 12-17.}

150_{Malgosia Fitzmaurice, International Law: The Practical Wokring of the Law of Treaties (Malcolm D Evans} ed, 4th edn, OUP 2014) 178.

Silence is consent : Acquiescence and Estoppel in International Law