Customary international law is a body of laws based on customs and precedents that regulate interactions between nation states. It is recognized by all nation states and applied by global organizations. Opinio juris sive necessitatis is the Latin term for the sense of legal obligation to abide by customary laws. Customary international law includes the prohibition of genocide, wars of aggression, and crimes against humanity, as well as respect for human rights and foreign embassies and diplomats. Codified customary laws are incorporated into new legal codes through treaties or agreements.
The international community has, over time, developed laws to regulate the interactions between nation states. Some of these laws have been based on custom, determined largely by precedent as opposed to written codes. These types of laws constitute the body of customary international law. Laws that come from customs, but have since been codified, concern both customary international law and the more recent written documents that place them.
Just as individuals might dispute the importance of certain social conventions, governments sometimes differ in their acceptance of particular aspects of customary international law. Its existence, however, is recognized by all nation states. Global organizations such as the United Nations and the International Court of Justice frequently apply customary international law.
Customs are actions encouraged by a widely shared sense of duty or appropriateness. This sense of proper behavior builds heavily on the previous one. Precedents are past decisions or actions that are used as guides to correct behavior in related contemporary situations. International law professionals can find precedents and place them as applicable to current cases, in academic papers, and in past verdicts of international organizations. Appeals to common knowledge of frequent international behavior are also relevant in demonstrating customary international law.
Centuries of nation states creating common precedents – and condemning any action contrary to those precedents out of a sense of legal obligation to do so – have defined customary international law. Even if not codified, nation states have felt legally obligated to abide by customary laws. The Latin term for this sense of legal obligation is opinio juris sive necessitatis. When establishing the validity of an international custom as a law, this concept is fundamental. Many argue that the reason many of these customs developed in the first place is that they stem from a universally shared sense of justice.
Customary international law is one of the primary sources of all international law. Some examples of customary international law include the prohibition of genocide, wars of aggression and crimes against humanity. Another important aspect of this body of legislation is the respect that nation states are obliged to show for human rights, as well as for the unique rights of foreign embassies and diplomats. Treaties or other agreements may incorporate customary laws into a new legal code; these are called codified customary laws. Laws governing conduct in warfare, for example, were codified in the Geneva Conventions and elsewhere, but they originated out of custom.
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