Customary international law (CIL) is one of the primary sources of international law, alongside treaties and general principles of law, as recognized by Article 38(1)(b) of the Statute of the International Court of Justice (ICJ). It arises from the general and consistent practice of states followed by them from a sense of legal obligation. For a rule to be considered customary international law, it must possess two essential constituent elements:

1. State Practice (Objective Element): This refers to the actual conduct of states.
2. Opinio Juris Sive Necessitatis (Subjective Element): This refers to the belief by states that a practice is rendered obligatory by the existence of a rule of law requiring it.

Let's discuss each element with the help of relevant case law:

1. State Practice (Objective Element): State practice encompasses any act or statement by a state that indicates its position on a particular matter of international law. It is the 'material' or 'behavioral' element of custom.

• Forms of Practice: State practice can take various forms, including:

  • Physical acts: Such as the actions of naval vessels, military operations, or the treatment of foreign nationals.
  • Verbal acts: Statements made in international forums (e.g., UN General Assembly resolutions, diplomatic correspondence, speeches by state representatives), official publications, national legislation, and judicial decisions.
  • Treaty practice: The signing, ratification, or accession to treaties can indicate state practice, especially if a treaty provision is widely adopted and seen as reflecting existing or emerging custom.
  • Omissions: Deliberate abstention from acting can also constitute practice, if done out of a sense of legal obligation.

• Characteristics of Practice:

  • Generality: The practice must be sufficiently widespread and representative, not necessarily universal. The ICJ in the North Sea Continental Shelf cases (1969) stated that "extensive and virtually uniform" practice is required. It's not about the sheer number of states, but the participation of states whose interests are specially affected by the rule in question.
  • Consistency/Uniformity: The practice must be reasonably consistent. Minor inconsistencies do not necessarily prevent the formation of a custom, especially if they are treated as breaches of the rule rather than as indications of a new rule. In the Nicaragua v. United States (1986) case, the ICJ noted that instances of state conduct inconsistent with a given rule should generally be treated as breaches of that rule, not as evidence of the formation of a new customary rule.
  • Duration: While duration is a factor, there is no precise time element. The ICJ in the North Sea Continental Shelf cases noted that "a short period of time is not necessarily, or of itself, a bar to the formation of a new rule of customary international law... provided that within the period in question, short though it might be, State practice, including that of States whose interests are specially affected, had been both extensive and virtually uniform."

2. Opinio Juris Sive Necessitatis (Subjective Element): This is the psychological or subjective element, meaning states must engage in the practice because they believe they are legally obligated to do so, not merely out of courtesy, convenience, political expediency, or tradition. It's the 'sense of legal obligation'.

• Distinction from Usage: Without opinio juris, a consistent practice remains mere 'usage' or 'habit' (e.g., diplomatic protocol, saluting flags) and does not become CIL.

• Proof of Opinio Juris: It is often inferred from state practice itself, but courts look for explicit statements, resolutions, or justifications for actions that demonstrate a belief in legal obligation.

• Case Examples for Opinio Juris:

  • North Sea Continental Shelf cases (1969): The ICJ examined whether the equidistance principle for continental shelf delimitation had become a rule of CIL. While many states had used the equidistance method, the Court found that they had not done so out of a belief that they were legally bound to, but rather for practical reasons or as a matter of treaty obligation. Thus, the equidistance principle had not become CIL.
  • Nicaragua v. United States (1986): The ICJ extensively discussed opinio juris regarding the prohibition on the use of force and the principle of non-intervention. It found that despite some instances of states using force, the widespread condemnation of such acts by the international community and the consistent invocation of the prohibition in international forums demonstrated a strong opinio juris that these prohibitions were indeed rules of CIL. The Court stated that "for a new customary rule to be formed, not only must the acts concerned amount to a settled practice, but they must also be such, or be carried out in such a manner, as to be evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it."
  • Lotus case (France v. Turkey, 1927 - PCIJ): The Permanent Court of International Justice (PCIJ) held that abstention from acting could only give rise to CIL if it was based on a conscious duty to abstain, i.e., accompanied by opinio juris.

Conclusion: Both elements – widespread and consistent state practice and the accompanying opinio juris – are indispensable for the formation of a rule of customary international law. The absence of either element means that a practice, no matter how common, does not attain the status of binding international law. The interplay between these two elements is crucial for the dynamic development of international law, reflecting both the actual behavior of states and their underlying legal convictions.