An Introduction to Sources of International Law

Author: Donia Joevion Fuller, Research Associate

The clearest exposition on what one may consider as a source of law is to be found in Article 38(1) of the Statute of the International Court of Justice (ICJ), which states:

  • The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply:

a)      international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;

b)     international custom, as evidence of a general practice accepted as law;

c)      the general principles of law recognized by civilized nations;

d)     subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.

Put succinctly, sources of law fall into four categories, international treaties, customary international law, general principles of law and judicial decisions coupled with teachings of the most highly qualified publicists. This Article aims to show the operation of each source of law and how they may of potential benefit to any international tribunal called upon to settle disputes.

International Conventions and the theory of Pacta Sunt Servanda

The Vienna Convention on the Law of Treaties, 1969 (VCLT) stands as the authoritative position on the operation of international conventions, more commonly called treaties.  The Convention defines a treaty as, “ an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation ((Article 2 (1) (a) VCLT, 1969)).” Central to the operation of treaties, and arguably in keeping with the notion of State sovereignty is that states may contract themselves to any term; consequently, a treaty may have its own mechanism for enforcement and dispute resolution. In the absence of such clear provisions, the VCLT may act as a guide on how to operate.

The issue of state sovereignty also features in the discourse of why states adhere to their treaty obligations. In this regard, the principle of pacta sunt servanda is of utmost importance. Article 26 of the VCLT stipulates that, “Every treaty in force is binding upon the parties to it and must be performed by them in good faith.” Despite the fact that there is no way to manually coerce states into performing their international obligations as against another state, one must recognize that the operation of international politics is such that economic pressure by one party brought to bear on another may be a viable mechanism to secure compliance. In addition, particularly as it relates to developing countries that rely on aid from the European Union and other international organizations, compliance with human rights treaties such as the International Covenant on Civil and Political Rights features largely in the decision whether or not grants will be meted out to various territories. In recent months, Refugees International (RI), a Washington based international advocacy group for lifesaving assistance and protection for displaced people, urged the Obama administration to withhold assistance to the government of South Sudan over alleged human rights abuses by its army (SPLA) in Jonglei state. Similarly, the European Parliament threatened to halt funds to Egypt, citing Port Said death sentences, human rights violations, and lack of political consensus in Egypt.

Customary International Law- Opinio Juris and State Practice  

By virtue of being a treaty itself and having been widely accepted by the international community as the authority on the interpretation of treaties, the principles enshrined in the document illustrate the operation of customary international law ((Frederic L. Kirgis, “Reservations to Treaties and United States Practice,” ASIL Insights (May 2003), available at Customary international law as defined by the ICJ Statute is a general practice accepted as law. The ICJ itself stated in the North Sea Continental Shelf Case: “It is of course axiomatic that the material of customary international law is to be looked for primarily in the actual practice and opinio juris of States ((ICJ, Continental Shelf case (Libyan Arab Jamahiriya v. Malta), Judgement, 3 June 1985, ICJ Reports 1985, pp. 29–30, para. 27.)).” This profound statement is taken by many scholars to mean that in order to establish that there is a principle or rule of customary international law, there are two facets that must be observed. Firstly, there must be evidence of State practice (usus) as well as  a belief that such practice is required, prohibited or allowed, depending on the nature of the rule, as a matter of law (opinio juris sive necessitatis).

Reverting to the opening statement relating to the VCLT, for a fact, less than half of the recognized States of the international community are actually parties to the treaty. Yet, the Courts have consistently considered its provisions as codifying customary law ((Anthony Aust, Modern Treaty Law and Practice (2nd edn. CUP, Cambridge 2007) 7–8 and n7))and have therefore treated them as being of general application to all states irrespective of their status in relation to the treaty. For instance, in the Kasikili/Sedudu Island Case the ICJ held that Article 31 of the VCLT relating to the interpretation of treaties reflected customary international law. Consequently, it was applicable to the case notwithstanding that neither Botswana nor Namibia was parties to the VCLT ((Case concerning Kasikili/Sedudu Island. (Botswana / Namibia). Judgment of 13 December 1999. para 18)).

For this reason, it is understandable that a fairly high threshold must be met when asserting that both elements are met and two cases from the ICJ in particular illustrates this proposition. On the hand the Case of the SS Lotus expressly showed that State practice in the absence of opinion juris will not suffice and the Advisory Opinion on Nuclear Weapons ((Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion, I.C.J. Reports 1996, p. 226, International Court of Justice (ICJ), 8 July 1996, available at: [accessed 7 August 2013]))shows the converse to be true ((France. v. Turkey, 1927)). Despite the general acceptance of this definition however, it remains problematic when one considers that some international rules are permissive and reflect a belief in a right more so than an obligation. For this reason, the better view may be to consider opinion juris as an expression , assertion and acknowledgment of a legal right or obligation as opposed to a belief in one.

General Principles

It is submitted that in time, general principles of law may assume more importance than even international conventions and principles of customary law. The reason for this bold assertion is that in several areas, the law is being outstripped by technological developments. For instance, in the use of unmanned weapons or drones to combat acts of terrorism along a state’s border, the UN Charter and in particular Articles 2(4) and 51 may become problematic in their application.

On the one hand, the Charter outlaws the unlawful use of force, yet recognizes the inherent right of self-defence in respect of an armed attack. Where the latter is concerned, the there are certain procedures to be carried out before an act of self-defence is affected. The nature of unmanned weapons is such response to any such attack is immediate and would bring such a response outside the scope of the Charter. Yet, in the face of the 9/11 Terrorist Attack, is it reasonable to warrant that a State potentially compromise its security by following these procedures? Similarly, based on the principles enshrined in the Nicaragua Case it is doubtful whether the use of such technology fulfils the criteria laid out where anticipatory self defence is concerned. These questions are of course beyond the scope of this Article which merely aims to delineate the content of what are broadly considered sources of international law.  Nonetheless, to the extent that as Sir Gerald Fitzmaurice observed, a principle of law, as opposed to a rule, underlies a rule and explains the reasons for its existence. A rule answers the question ‘what’ whereas a principle answers the question ‘why ((Sir Gerald Fitzmaurice, ‘The General Principles of International Law’ (1957) 92 Collected Courses of the Hague Academy of International Law, p. 7.)), it is submitted that a principle may be more readily applied to emerging problematic areas of law, than a rule of for instance customary international law.

An apt example of the application general principles may be found in the Lena Goldfields Company Case where arbitrators recognized and indeed applied principles governing unjust enrichment as a general principle of law recognized by civilized nations ((H. C. Gutteridge, Transactions of the Grotius Society Vol. 38, Problems of Public and Private International Law, Transactions for the Year 1952 (1952), pp. 125-134- The Meaning and Scope of Article 38(1)(c) of the ICJ Statute)). In the landmark Chorzow Factory Case the Court categorically observed that, it is a principle of international law, and even a general conception of law, that any breach of an engagement involves an obligation to make reparation.  Whilst a comprehensive analysis of the precise scope and use of general principles supersedes the scope of the article, it may suffice to say that these principles essentially provide a mechanism to address international issues not already subject either to treaty provisions or to binding customary rules. Such general principles may arise both from municipal law and international law, and many may actually be described as procedural or evidential principles as well as those which affect with the machinery of the judicial process ((A; Bassiouni, M. Cherif, A Functional Approach to General Principles of  International Law,  Mich. J. Int’l L. 768 (1989-1990).)).

Judicial Decisions and Teachings of the Most Qualified Publicists

It is not surprising that the ICJ should recognize the utility of judicial decisions as a source of international law. In so far as there is no operation of binding precedent in international law, dissimilar to the operation of common law jurisdictions. Further, it is just as well that no distinction is made between judicial decisions of municipal and international courts. In relation to the latter, it is submitted that these too can be evidence of what is accepted as a general principle of law and even a customary practice.

In light of the growing problem of technology outstripping international customary principles and even broadly applicable treaties, the writings of qualified publicists are insightful on the dynamics that bring to bear on particular issues, and may also be reflective of the sentiment of that writer’s country on the issue. The benefits to an international tribunal are obvious, though such writings would never be seen as declaring the law. In view of the foregoing, an question which often emanates from these discussions is whether there is a hierarchy of sources of law.

A Hierarchy of Sources?

A simple answer to the question proposed would be in the negative. However such a sparse response would not adequately explain the matrices within which sources of law operate. To that end it is submitted that cognizance must be taken of rules of jus cogens which may be defined as customary international rules of the highest order. So much so, that Article 3 of the VCLT, 1969 explicitly states that even a treaty that seeks to enforce a practice contrary to such rules is void. Examples of such rules include the rule against torture, genocide and slavery.

However, if a rule of customary international law has not attained this status, nothing precludes States from contracting in a manner that contradicts such a customary rule. How the Court will deal with any dispute where treaty and custom conflict is another matter, but presumably the principle of pacta sunt servanda would prevail, particularly as a State may find itself bound by a customary practice that it arguably had no part in solidifying to the detriment of a treaty obligation. The latter, it is submitted is one of the ultimate expressions of State sovereignty, indeed at least theoretically, a State cannot be forced to subject itself to the terms of a treaty.  The extent to which a definitive hierarchy is even useful may be at best academic. Thus, it is respectfully submitted that for States, the best resolution in this case is not necessarily to try to find a hierarchy, but merely to attempt to discover the international rules and principles governing the legality or lack thereof of proposed state action coupled with an attempt to abide by them.