Legal Definitions - customary international law

Customary international law refers to a set of international rules and obligations that arise not from formal written agreements like treaties or conventions, but from the consistent and general practice of states that they follow out of a sense of legal obligation.

Think of it as unwritten international law that develops over time through the repeated actions and shared beliefs of countries. For a practice to become customary international law, two key elements must be present:

• State Practice: This means that a significant number of states consistently engage in a particular conduct over time. It's about what states actually do.
• Opinio Juris: This Latin term means "an opinion of law" or "a belief that an act is legally obligatory." It signifies that states follow the practice not merely out of courtesy, political expediency, or tradition, but because they genuinely believe they are legally bound to do so.

Together, these two elements demonstrate that a particular practice has evolved into a binding legal norm for the international community, even without a specific treaty codifying it.

Examples of Customary International Law:

• Freedom of Navigation on the High Seas: For centuries, nations have consistently allowed ships of all countries to travel freely through international waters without requiring permission or facing interference. This practice is not solely based on specific treaties, but on a long-standing understanding that all states have a legal right to navigate the high seas, and a corresponding legal duty not to impede others. The consistent practice (ships sailing freely, states not interfering) combined with the universal belief that this is a legal right and obligation (opinio juris) makes it a cornerstone of customary international law.

• Prohibition of Piracy: The international community has a long-established practice of treating piracy as a crime against all nations, regardless of where it occurs or the nationality of the pirates or victims. States routinely cooperate to suppress piracy and prosecute individuals engaged in it, even if their own citizens or vessels are not directly involved. This widespread and consistent state practice, coupled with the universal belief that states have a legal duty to combat piracy (opinio juris), demonstrates its status as customary international law. This legal obligation existed long before many modern treaties specifically addressed piracy.

• Immunity of State Property: A fundamental principle is that the property of a sovereign state, such as a warship or an embassy building, is generally immune from seizure or jurisdiction by the courts of another country. For example, if a foreign warship docks in a port, it cannot typically be seized by local authorities to settle a commercial dispute or a private claim against its government. This consistent practice of states respecting the immunity of foreign state property, combined with the shared understanding that this is a legal requirement stemming from sovereign equality (opinio juris), forms a key part of customary international law.