Legal Definitions - opinio juris (international law)

Opinio juris is a Latin term used in international law that refers to the belief by states that a particular practice is legally obligatory, rather than simply a matter of custom, courtesy, or convenience. It is a crucial element in establishing customary international law, which consists of general practices accepted as law.

For a practice to become a binding rule of customary international law, states must not only consistently follow the practice (the objective element) but also do so because they feel legally compelled to, believing it is required by law (the subjective element of opinio juris). Without this sense of legal obligation, a widespread practice remains just a habit or a matter of policy, not a rule of international law.

• Example 1: The Prohibition of Genocide

  The prohibition of genocide is a universally accepted principle. While it is codified in treaties like the Genocide Convention, it is also considered a rule of customary international law. This is because states not only refrain from committing genocide (the consistent practice) but also do so out of a profound belief that they are legally bound by international law to prevent and punish such acts. The global condemnation of genocide and the expectation that all states will uphold this prohibition demonstrates the strong opinio juris surrounding this rule.

• Example 2: Freedom of Navigation in International Waters

  Naval vessels and commercial ships from all nations routinely traverse international waters without seeking permission from coastal states. This is a widespread and long-standing practice. This practice is considered customary international law because states universally believe they have a legal right to do so, and that other states are legally obligated to respect this freedom. If a state were to attempt to block innocent passage through international waters, it would be seen as violating a fundamental principle of international law, demonstrating the presence of opinio juris regarding this practice.

• Example 3: Providing Humanitarian Aid

  It is common for states to provide humanitarian aid to other countries experiencing natural disasters or conflicts. This is a frequent and often generous practice. However, while providing aid is widespread and commendable, it is generally not considered a legal obligation under customary international law. States typically provide aid out of generosity, moral duty, or political interest, not because they believe they are legally compelled to do so. Therefore, despite the consistent practice, the absence of opinio juris means that providing humanitarian aid, in most contexts, does not constitute a binding rule of customary international law. This illustrates why both elements (practice and opinio juris) are necessary.