International Customary Law: Principles and Application

Understanding International Customary Law

International custom, as defined by Article 38(1)(b) of the Statute of the International Court of Justice (ICJ), refers to a general practice accepted as law. It represents the ways in which the international community expresses itself, formed by a set of rules revealed not only through the repetition of acts accompanied by a feeling of obligation, but also because these acts are performed in the belief that certain obligations are being met or rights are being exercised.

Essential Elements of Customary Law

The two fundamental elements of international custom are:

• Objective Element (State Practice): This refers to the constant and uniform practice of states. It signifies the consistent use of a particular conduct which, through repetition, becomes established as a norm.
• Subjective Element (Opinio Juris): This is the awareness of states that they are acting as legally required. It means that the practice is considered mandatory, and states believe they must conform their conduct to it, understanding that violating it constitutes a breach of legal obligation.

Categories of International Custom

• Universal Custom: This type of custom has been engaged in by the great majority of states. Such a custom obliges all states, even if they did not participate in its creation or initially support it, unless a state has persistently objected to it from the outset.
• Regional Custom: This involves only a specific group of states, such as the right to asylum, which primarily exists among American states. Its obligation extends only to those participating states, often united by historical, geographical, or economic ties.
• Bilateral Custom: In this form of custom, only two states participate. Its obligation applies exclusively to these two participating states.

New States and Customary Law

New states are given adequate time to analyze existing international practices and determine whether they are bound by them, thus demonstrating their acceptance or objection. A notable example is the case of Yugoslavia, which between 1990 and 1995 dismembered, forming new countries such as Croatia, Bosnia, and a smaller Yugoslavia. These new entities had to navigate their relationship with pre-existing international customary law.

Treaty Reservations in International Law

A country may sign a treaty with reservations. According to the Vienna Convention on the Law of Treaties (1969), a reservation is defined as: "whatever phrased or named, made by a State when signing, ratifying, accepting or approving a treaty or acceding to it, in order to exclude or modify the effects of certain provisions of the treaty in their application to that State."