Customary International Law: Treaties And Their Evolution

Customary international law is a set of international obligations that arise from established international practices, as opposed to formal written treaties or conventions. It is one of the primary sources of international law, alongside treaties and general legal principles. Customary international law is based on the concept of custom and is derived from the consistent conduct of states acting out of a sense of legal obligation. This is often referred to as opinio juris, which means that states follow certain practices because they believe they are legally compelled to do so. Customary international law is important because it fills the gaps left by treaty law and strengthens the protection offered to victims in today's armed conflicts. While treaties are binding only on states that have consented to be bound by them, customary international law is binding on all states, regardless of whether they have codified these laws domestically or through treaties.

Customary international law vs treaty law

Customary international law and treaty law are two distinct sources of international law. While both are considered primary sources of international law, there are key differences between the two.

Customary international law is made up of rules that derive from "a general practice accepted as law". It is comprised of all the written or unwritten rules that form part of the general international concept of justice. Customary international law is not limited to those states that are parties to a particular agreement; instead, it is binding upon all states, regardless of whether they have ratified a treaty. It is important to note that customary international law is not codified in a clear and accessible format, and the content of the rules is generally less specific than what is typically found in a treaty. However, as a source of international humanitarian law (IHL), customary international law plays a fundamental role in armed conflict due to the limited protections afforded by treaty law and the lack of ratification of key treaties.

Treaty law, on the other hand, refers to written agreements that states willingly sign and ratify, and as such, are obliged to follow. Treaties govern the mutual relations between states and are binding only on those states that have signed and ratified the agreement. The Vienna Convention of 1969 sets out the fundamental legal rules relating to treaties, including their definition, who has the capacity to conclude them, and outlines for interpretation, disputes, and reservations. The basis of treaty law is "pacta sunt servanda", which means that agreements must be honoured and adhered to.

The distinction between customary international law and treaty law is necessary and useful in the international legal system. This distinction provides developers of international law with choices regarding the nature and structure of the laws. For example, developers may opt for a treaty when creating a law that confers specific benefits or responsibilities. On the other hand, customary international law may be preferred for its generalist structure and ability to bind all international actors, promoting the creation of laws that protect basic rights and freedoms.

The distinction between the two sources of international law also ensures state accountability and promotes the ability of states to accept specific responsibilities. Customary international law allows for the creation of non-derogable jus cogens norms, which are necessary to hold states accountable for violations of fundamental rights. Treaties, on the other hand, enable states to accept, distribute, and mitigate specific responsibilities.

In summary, customary international law and treaty law are two distinct sources of international law that differ in their formation, scope, and applicability. Customary international law arises from established international practices and is binding upon all states, while treaty law refers to written agreements between states and is binding only on those states that have ratified the treaty. The distinction between the two is necessary and useful in the international legal system, providing developers with choices and ensuring state accountability.

The role of opinio juris

Opinio juris is a Latin phrase that translates to "an opinion of law or necessity". It is the second element necessary to establish a legally binding custom in customary international law.

In customary international law, opinio juris denotes a subjective obligation, meaning a state perceives itself to be bound by the law in question. The International Court of Justice (ICJ) outlined the standard for customary international law in the ICJ Statute, Article 38(1)(b), which requires that the custom be "a general practice accepted as law". The general practice requirement refers to state practice or actions, while the "accepted as law" aspect refers to opinio juris, the belief that these actions are required by law.

Practices that are generally followed by states but which they feel free to legally ignore lack the opinio juris element and thus are not customary international law. Conversely, a practice that was originally to be followed for other reasons may become customary international law if states come to believe they have a legal obligation to the practice.

Opinio juris cannot be inferred from state practice alone. States may engage in behaviours for non-legal reasons, such as political expedience. Therefore, it is essential to understand opinio juris to determine whether certain practices have developed into norms of customary international law. Ideally, states will make clear through official statements or diplomatic exchanges which practices they consider legally obligatory. However, states do not always express their opinio juris, and this absence can create uncertainty about the law.

The International Court of Justice has emphasised the need to prove a "sense of legal duty" as distinct from "acts motivated by considerations of courtesy, convenience or tradition". This was confirmed in the Nicaragua v. United States of America case.

International Court of Justice recognition

The International Court of Justice (ICJ) is the main judicial body of the United Nations, and it settles disagreements between member states. The Statute of the ICJ, in Article 38(1)(b), defines customary international law as "a general practice accepted as law". This is determined through two factors: the general practice of states, and what states have accepted as law (opinio juris).

The ICJ has recognised that customary international law consists of rules derived from the consistent conduct of states acting out of a belief that the law required them to act in such a way. This is confirmed by the two essential elements of customary international law: state practice and opinio juris.

The ICJ has also acknowledged that customary international law can be established by showing state practice and opinio juris. In the Legality of the Threat or Use of Nuclear Weapons, the ICJ confirmed these two elements as essential.

In the North Sea Continental Shelf case, the ICJ emphasised the need to prove a "sense of legal duty", which is distinct from "acts motivated by considerations of courtesy, convenience, or tradition". This was further confirmed in Nicaragua v. United States of America.

The recognition of customary international law by the ICJ ranges from simple bilateral recognition to worldwide multilateral recognition. An example of the former is the Right of Passage Over Indian Territory case between Portugal and India, where the ICJ found that the long-continued practice between the two states accepted as regulating their relations should form the basis of mutual rights and obligations.

Other examples of customary international law recognised by the ICJ include the immunity of visiting foreign heads of state and the principle of non-refoulement.

Customary international law and consent

Customary international law (CIL) is derived from "a general practice accepted as law". It is a less formal set of expectations than written treaties or conventions, and it is one of the primary sources of international law. While treaties are binding only on states that have consented to be bound by them, CIL does not require consent to be binding.

The International Court of Justice (ICJ) Statute, incorporated into the United Nations Charter, acknowledges the existence of CIL in Article 38(1)(b). This article defines CIL as "a general practice accepted as law", with two determining factors: the general practice of states, and what states have accepted as law (opinio juris sive necessitatis). This means that a large number of states must engage in the practice, and they must do so out of a sense of legal obligation rather than custom or habit.

The ICJ has confirmed that the two essential elements of CIL are state practice and opinio juris. The opinio juris element was further clarified in the North Sea Continental Shelf case, where the ICJ held that acts must be carried out in such a way as to demonstrate a belief that this practice is rendered obligatory by the existence of a rule of law requiring it. In other words, states must feel they are conforming to a legal obligation.

While CIL does not require consent to be binding, the concept of consent does come into play in some situations. For example, in the Libya/Malta Continental Shelf and Gulf of Maine cases, the ICJ examined whether the state party to the proceedings had individually recognized the existence of the relevant customary rule. Additionally, some authors have put forward consent theories of CIL, arguing that consent is a requirement for the existence of CIL. However, these theories face challenges in reconciling the concept of consent with opinio juris.

Customary international law in practice

Customary international law is a crucial aspect of international law, recognised by the International Court of Justice, jurists, the United Nations, and its member states. It involves the principle of custom, which is distinct from formal written treaties or conventions. Customary international law arises from established or usual international practices, which are less formal and often unwritten.

In practice, customary international law is identified through two key elements: state practice and opinio juris. State practice refers to the widespread and consistent repetition of similar international acts over time by a significant number of states. This practice must be accepted as law and followed out of a sense of legal obligation, rather than mere custom or habit.

The opinio juris element requires that states engage in the practice because they feel legally compelled to do so. In other words, they believe that a particular rule is legitimate and obligatory, not just following it out of courtesy, convenience, or tradition. This belief in the rule's legitimacy is what distinguishes opinio juris from state practice.

The International Court of Justice (ICJ) has clarified that there are two types of customary international law. The first type includes legal rules that are inherent and self-evident consequences of fundamental international legal principles. For example, the sovereignty of a state within its borders arises from the fundamental principle of state sovereignty.

The second type, which is the focus of customary international law discussions, is opinio juris itself. The ICJ has emphasised that evidence of opinio juris is primarily found in how states act, but it can also be supported by the treaties they adopt and other governmental actions.

Some examples of customary international law include the doctrine of non-refoulement, which prohibits the forced return of refugees to a country where they face danger, and the granting of immunity to visiting foreign heads of state.

Customary international law is particularly significant when it rises to the level of jus cogens, or peremptory norms. These are fundamental principles of international law that are universally accepted and non-derogable. Examples of jus cogens include the prohibition of slavery, torture, genocide, war of aggression, and crimes against humanity. Any laws or treaties that conflict with jus cogens are considered null and void.

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