21.10.2025 Lecture 3 Sources of Public International Law by Dr. Omar Grech

Sources of Public International Law
Article 38(1) of the Statute of the International Court of Justice

The International Court of Justice (ICJ) has the primary responsibility to make decisions in accordance with international law on disputes that are submitted to it. Article 38(1) of its Statute enumerates the principal and subsidiary sources of law that the Court shall apply:

  • (a) International conventions, whether general or particular, establishing rules expressly recognized by the contesting states.

    • These are essentially treaties, which are legally binding agreements between states. "General" conventions often refer to multilateral treaties with wide participation (e.g., UN Charter), while "particular" conventions refer to bilateral treaties or those between a limited number of states.

  • (b) International custom, as evidence of a general practice accepted as law.

    • Customary international law arises from the consistent and widespread practice of states, coupled with a belief that such practice is legally obligatory (opinio juris). It is unwritten law.

  • (c) The general principles of law recognized by civilized nations.

    • These are fundamental principles common to major legal systems around the world, such as good faith, proportionality, and the principle that no one can be a judge in their own cause (nemo judex in causa sua). They fill gaps where treaty or customary law might not exist.

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

    • These are not primary sources of law but are used to interpret and clarify existing law. Article 59 specifies that ICJ decisions have no binding force except between the parties and in respect of that particular case.

Treaty Law

Treaty Law is primarily governed by the 1969 Vienna Convention on the Law of Treaties (VCLT), which codifies many aspects of customary international law regarding treaties. Treaties can be international conventions, covenants, protocols, or agreements, and they establish legally binding obligations for states that are parties to them. The lifecycle of a treaty typically involves the following stages:

  1. Negotiation of Text: This is the initial drafting process where state representatives discuss, propose, and agree upon the terms and content of the treaty. Multilateral treaties often necessitate extensive conferences.

  2. Adoption of Text: Once negotiations are complete, the text of the treaty is formally approved. For multilateral treaties, this often requires a vote by participating states, usually by a two-thirds majority at an international conference, as stipulated by the VCLT, or by consensus.

  3. Signing: Countries sign the treaty to authenticate its text and express their preliminary intention to be bound by its provisions in the future. Signing itself does not usually create a binding legal obligation but does oblige states to refrain from acts that would defeat the object and purpose of the treaty (Article 18 VCLT).

  4. Ratification: This is the formal act by which a state confirms its consent to be legally bound by a treaty. It typically involves an internal legislative or executive process, such as parliamentary approval, before the state deposits its instrument of ratification with the treaty depositary. Ratification transforms the political act of signing into a legal commitment.

  5. Coming into Force: A treaty enters into full legal effect after specific conditions are met, as stipulated within the treaty itself. This commonly requires a minimum number of ratifications to be deposited (e.g., 60 states for the Rome Statute of the ICC).

  6. Accession: This process allows a state that did not participate in the negotiation and signing of a treaty to become a party to it after it has been created and has often already entered into force.

Bilateral vs. Multilateral Treaties

  • Bilateral Treaties: These are agreements between two states. They often come into force relatively quickly, sometimes upon signature, but more commonly upon the exchange of instruments of ratification between the two parties, as explicitly agreed in the treaty.

  • Multilateral Treaties: These involve three or more states. They almost always require completion of the ratification process by a specified number of states before they come into force, reflecting the complexity of gaining widespread agreement.

    • For states that join a multilateral treaty after it has already become effective (via ratification or accession), the treaty typically enters into force for that specific state from the date of their own ratification or accession.

Important Concepts in Treaty Law

  1. Pacta Sunt Servanda: This is the foundational principle of treaty law, codified in Article 26 of the VCLT. It means that agreements must be kept. Every treaty in force is binding upon the parties to it and must be performed by them in good faith. This principle is indispensable for the stability and credibility of international relations.

  2. Reservations and Their Effects: Reservations are unilateral statements made by a state when signing, ratifying, accepting, approving, or acceding to a treaty, whereby it purports to exclude or modify the legal effect of certain provisions of the treaty in their application to that state. Reservations are governed by Article 19-23 of the VCLT and are generally permissible unless the treaty prohibits them, specifies only certain reservations are allowed, or the reservation is incompatible with the object and purpose of the treaty.

  3. Jus Cogens: These are peremptory norms of general international law, meaning they are fundamental principles of international law accepted and recognized by the international community of states as a whole as norms from which no derogation is permitted. Any treaty provision that violates a jus cogens norm is considered void. Examples include the prohibitions against genocide, slavery, torture, aggression, and apartheid.

Customary International Law
Definition and Characteristics

International custom serves as evidence of a general practice accepted as law. The formation of customary international law requires two essential elements, often referred to as the dual elements doctrine:

  1. General Practice of States (Objective element): This refers to the consistent and widespread behavior of states. It encompasses not only physical acts but also includes diplomatic correspondence, policy statements, legislative acts concerning international relations, and voting patterns in international organizations.

    • Uniformity: While states must adopt the practice in a substantially consistent manner, absolute uniformity is not required. Minor inconsistencies do not necessarily preclude the formation of a custom, especially if the general trend of state practice supports the rule. The key is that instances of non-compliance are generally treated as breaches of the rule, not as evidence of the emergence of a new rule.

    • Consistency: The practice must be adopted consistently by states over time and should not fluctuate capriciously. It implies a certain duration and repetition of acts.

    • Generality of Practice: A sufficient number of states must have adopted the practice, but it does not require universal acceptance. The exact number is not mathematically determined and often depends on the circumstances, including the participation of "Specially Affected States" whose interests are particularly impacted by the proposed customary rule (e.g., maritime states for laws of the sea).

  2. Opinio Juris (Subjective element):

    • Accepted as Law (Opinio Juris): States may engage in practices for various reasons, such as political expediency, economic interests, or mere courtesy (comity). However, a general practice only becomes a norm of customary international law if it is accompanied by a belief among states that adherence to the practice is a legal obligation. This psychological element distinguishes legally binding custom from mere habit or politeness.

Criteria for New Customary Rule Formation

For a new customary rule to emerge, both the objective and subjective elements must be present and interlinked:

  1. There must be a discernible, settled practice among states, characterized by repeated and consistent acts or omissions (state practice).

  2. This practice must be coupled with opinio juris sive necessitatis, meaning that the conduct of states reflects a belief that the practice is obligatory under law, and not merely out of comity or political aspiration. Essentially, states must act in a certain way because they feel legally compelled to do so.

Relevant Cases

  • Nicaragua v. USA (1986): The ICJ emphasized that for a rule to be established as customary international law, the conduct of states must, in general, be consistent with such a rule, and instances of state conduct inconsistent with a given rule should generally have been treated as breaches of that rule, not as indications of the recognition of a new rule. The ICJ also highlighted that "complete consistency" is not required.

  • Lotus Case (France v. Turkey, 1927): This case is crucial for understanding opinio juris. The Permanent Court of International Justice (PCIJ) found that mere abstention from prosecution by states did not indicate a legal obligation to abstain. For abstention to become a custom, states must have been aware of a duty to abstain. This case illustrates that an omission (inactivity) can only form part of state practice if it is based on a conscious duty to refrain from acting.

Additional Legal Precedents

  1. French v. Turkey, Lotus Case (1927): As noted above, this seminal case established the high bar for proving opinio juris. It highlighted that the lack of prosecution by states in certain circumstances was not sufficient to prove an international custom requiring such abstention; there was no positive proof of a legal duty motivating the abstention.

  2. Barcelona Traction Case (Belgium v. Spain, ICJ 1970): This case extensively addressed the rights of corporate entities and shareholders in international law, particularly concerning diplomatic protection. The ICJ stated that, where international law has not established specific rules regarding the treatment of corporate entities (which are creations of municipal law), municipal laws would be referenced. This is because international law must take cognizance of the increasing complexity of corporate regulations and legal personality within national systems that affect the rights of both the corporate entity itself and its shareholders, acknowledging the interplay between national and international legal systems.