Customary International Law: How Norms Become Binding

Customary international law is a set of rules that restrict the activities of states and are not written down or codified in any particular source. It arises from established international practices and consistent state behaviour, as opposed to formal written treaties or conventions. Customary international law is considered a primary source of international law by the International Court of Justice, jurists, the United Nations, and its member states. It becomes customary international law when a significant number of states consistently engage in a pattern of behaviour, and a conviction develops among states that this behaviour is required by international law, known as opinio juris.

International practices vs. formal written conventions

Customary international law (CIL) refers to international obligations that arise from established international practices and consistent state behaviour. It is less formal than written conventions and treaties, and often unwritten, but can still be considered a primary source of international law. CIL is based on custom and the principle of opinio juris, which means that states engage in a practice because they believe they are legally compelled to do so.

CIL is distinct from formal written conventions, which are treaties or written agreements that establish certain rules and are binding on the states that consent to them. Treaties are a more formal and explicit way of creating international law, and they are typically ratified by states, which means they have agreed to follow the rules set out in the treaty.

The process of creating CIL is more organic and based on the consistent behaviour and practices of states over time. For a rule to become CIL, there must be a widespread and consistent state practice, and states must engage in this practice with a sense of legal obligation, or opinio juris. This means that a significant number of states must follow the practice, and it must not be rejected by a significant number of states.

CIL is often more flexible and adaptable than formal written conventions, as it can evolve with changing international practices. It is also more accessible, as it is not restricted to those states that have consented to a particular treaty. However, CIL can be more challenging to identify and enforce, as it is not always clearly defined or written down.

While CIL and formal written conventions are distinct, they can also complement each other. For example, CIL can fill gaps in treaty law, as seen in the case of humanitarian law, where CIL provides additional protection to victims of armed conflict. Additionally, some CILs can become formalised through treaties, as seen with the laws of war, which were initially based on CIL before being codified in the Hague and Geneva Conventions.

In conclusion, while international practices leading to CIL and formal written conventions differ in terms of formality and process, both play essential roles in shaping international law and the behaviour of states. CIL provides a flexible and accessible source of international law, while formal written conventions offer more explicit and agreed-upon rules.

State practice

The International Court of Justice (ICJ) recognises state practice as one of the two essential elements of customary international law, alongside opinio juris. This recognition is enshrined in the ICJ Statute, which defines customary international law as "a general practice accepted as law".

The formation of customary international law is an ongoing process, and state practice can evolve over time. For example, the ICRC's study on customary international humanitarian law identified rules that have evolved due to changes in state practice, such as the prohibition of attacks on civilian objects in non-international armed conflicts.

Opinio juris

The International Court of Justice (ICJ) has outlined the standard for customary international law in Article 38(1)(b) of its statute, which states that the custom must be "a general practice accepted as law". Here, the "general practice" refers to state practice, while "accepted as law" refers to opinio juris.

It is important to note that evidence of state practice alone is insufficient to establish customary international law. Opinio juris is crucial in determining whether certain practices have developed into norms of customary international law. States may engage in certain practices for non-legal reasons, such as political expediency. Therefore, opinio juris cannot simply be inferred from state practice and must be clearly expressed by states.

In summary, opinio juris is a critical component of customary international law, representing the subjective belief that a practice is legally binding. It complements state practice by providing the necessary sense of legal obligation, ensuring that practices are followed out of a sense of duty rather than mere courtesy, convenience, or tradition.

International Court of Justice

The International Court of Justice (ICJ) is the principal judicial organ of the United Nations and was established by its Charter. The ICJ is based in The Hague, Netherlands, and is currently presided over by President Joan E. Donoghue. The ICJ's role is to settle legal disputes between states and give advisory opinions on legal questions referred to it by authorised UN bodies and specialised agencies.

The ICJ is composed of 15 independent judges, elected by the UN General Assembly and Security Council. Judges are chosen from people of high moral character who are already qualified for the highest judicial offices in their respective countries, or are recognised as competent in international law. The ICJ remains permanently in session, except during judicial vacations, the dates and duration of which are fixed by the Court.

Cases brought to the ICJ are either contentious cases (legal disputes between states) or advisory proceedings (requests for advisory opinions on legal questions). Contentious cases can be instituted in one of two ways: through the notification of a special agreement, or by means of an application. In the former, a bilateral document is lodged with the Court by either or both of the states involved in the proceedings. In the latter, a unilateral application is submitted by an applicant state against a respondent state.

The ICJ's procedure consists of two parts: written and oral. The written procedure involves the communication of memorials, counter-memorials, and if necessary, replies, as well as supporting documents. The oral procedure involves the hearing of witnesses, experts, agents, counsel, and advocates. The official languages of the Court are French and English, and the Court will deliver its judgment in the language agreed upon by the parties.

For a case to be heard by the ICJ, the states involved must have accepted its jurisdiction in one or more of the following ways:

• By entering into a special agreement to submit the dispute to the Court
• By virtue of a jurisdictional clause, i.e. when they are parties to a treaty containing a provision allowing one of them to refer a dispute to the Court
• Through the reciprocal effect of declarations made under the Statute, where each state accepts the Court's jurisdiction as compulsory in the event of a dispute with another state having made a similar declaration

The ICJ's sources of law are: international treaties and conventions in force, international custom, the general principles of law, judicial decisions, and the teachings of the most highly qualified publicists. The ICJ may also decide a case ex aequo et bono (without confining itself to existing rules of international law) if the parties agree.

Jus cogens

The concept of jus cogens can be traced back to the 18th century, with its roots in natural law principles. The Permanent Court of International Justice recognised the existence of a peremptory norm in the S.S. Wimbledon case in 1923, stating that state sovereignty is not inalienable. Under Article 53 of the Vienna Convention on the Law of Treaties, any treaty that conflicts with a peremptory norm is void.

While there is no universal agreement on which norms are considered jus cogens, or how a norm achieves this status, it is generally accepted that jus cogens bans the aforementioned egregious conduct. Some critics disagree with the hierarchy of international legal norms, and there is also disagreement over how such norms are recognised or established.

Frequently asked questions