Customary international law is developed from what?

Customary international law is a fundamental aspect of how international law evolves, and it is mainly developed from the practices of nations over time. This type of law arises when certain practices and behaviors are consistently followed by states out of a sense of legal obligation, also known as opinio juris. As nations interact and engage with one another, specific norms and customs develop and are gradually recognized as binding.

This recognition often occurs through widespread acceptance and usage of a practice that becomes viewed as necessary for the governance of international relations. For example, the principle of non-refoulement in refugee law emerged from consistent state practice and is now considered an essential rule of customary international law, reflecting how states should treat individuals seeking asylum.

The other choices may contribute to international law but do not constitute its primary source. Legal treaties and agreements are formal written contracts between states but are distinct from customary law, which is less formal and often unwritten. Decisions of international courts can provide clarification and interpretation of existing laws but do not create customary law on their own. Military operations and rules, while they can influence international law, primarily pertain to specific legal frameworks such as humanitarian law rather than the broader context of customary law development.