Customary International Law

Customary international law is an important concept in the field of international law. It consists of rules and principles that have developed over time through the consistent and widespread practice of states, accompanied by a belief that such practice is legally required (opinio juris).

State Practice
Customary international law arises from the repeated actions and behaviour of states. These actions can include legislative measures, executive decisions, diplomatic statements, and judicial rulings. The practice must be consistent and widespread, meaning that it is not merely sporadic or limited to a few states. The more states engage in the same behaviour, the stronger the evidence of state practice.

For example, the principle of diplomatic immunity is a customary international law norm. It has developed through the consistent practice of states granting immunity to diplomats and their families. This practice can be seen in the widespread establishment of diplomatic missions, the granting of diplomatic passports, and the practice of refraining from arresting or prosecuting diplomats for certain offences.

Opinio Juris
State practice alone is not enough to establish a customary international law norm. There must also be a belief among states that the practice is legally required or obligatory. This belief is known as opinio juris. It distinguishes customary international law from mere habit, convenience, or voluntary arrangements.

Opinio juris can be inferred from the consistent and long-standing practice of states, as well as from official statements, treaty provisions, domestic legislation, and legal opinions of states. States' expressions of their understanding that a particular behaviour is legally required contribute to the formation of customary international law.

For instance, the prohibition of torture is widely recognised as a customary international law norm. It has evolved through the consistent condemnation of torture by states, the inclusion of anti-torture provisions in international and domestic laws, and the ratification of treaties like the Convention Against Torture. States' expressions of outrage, their adoption of legislation criminalising torture, and their support for the prosecution of perpetrators reflect the opinio juris.

Persistent Objector
While customary international law binds states that have engaged in the practice and share the opinio juris, there is an exception known as the persistent objector rule. According to this rule, a state that consistently and unequivocally objects to a customary norm during its formation cannot be bound by that norm.

To be considered a persistent objector, a state must express its objection to the norm with clarity and consistency. It must be persistent in its objection throughout the development of the norm and must not later acquiesce to or benefit from the norm. However, once a state becomes aware of a customary norm and its widespread acceptance, it cannot raise a valid objection.

In summary, customary international law arises from the consistent and widespread practice of states coupled with a belief that such practice is legally required (opinio juris). It is an important source of international law, binding states even in the absence of explicit consent. Customary international law evolves through state practice and opinio juris, and it can be influenced by changes in societal values, international events, and emerging challenges in the global arena.