The international community has, over time, developed laws to regulate interactions between nation states. Some of these laws have been based on customs, determined largely by precedent as opposed to written codes. These kinds of laws form the body of customary international law. Laws that come from customs but have since been codified pertain to both customary international law as well as to the newer written documents which site them.
Just as individuals might dispute the importance of certain social conventions, governments sometimes differ in their acceptance of particular facets of customary international law. Its existence, however, is acknowledged by all nation states. World organizations such as the United Nations and the International Court of Justice frequently apply customary international law.
Customs are actions that are encouraged by a widely shared sense of obligation or appropriateness. This sense of correct behavior relies strongly on precedent. Precedents are past decisions or actions that are used as guides for proper behavior in related contemporary situations. International legal practitioners can find precedents, and site them as applicable to current cases, in academic documents as well as in the 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 actions contrary to those precedents out of a feeling that they were legally obligated to do so — has defined customary international law. Even if not codified, nation states have felt legally compelled to respect 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 law, this concept is vital. Many argue that the reason many of these customs developed in the first place is that they derive from a universally shared sense of justice.
Customary international law is one of the primary sources of all international law. Some examples of international customary law include the prohibition of genocide, wars of aggression and crimes against humanity. Another important aspect of this body of law is the respect nation states are obliged to show for human rights as well as the unique rights of foreign embassies and diplomats. Treaties or other agreements can incorporate customary laws into a new legal code; these are called codified customary laws. The laws governing conduct in war, for example, were codified in the Geneva Conventions and elsewhere, but originated through custom.