How does customary international law become part of the law of war?

Customary international law becomes part of the law of war through the convergence of two fundamental elements: state practice and opinio juris.12

• State Practice: This refers to the actual, consistent, and uniform conduct of states over time. For a practice to form the basis of a binding custom, it must be extensive and generally accepted by the international community.1
• Opinio Juris: This is the belief by states that their practice is required by, or in conformity with, international law. It is the conviction that an act is necessary by rule of law, which converts a mere usage into a binding legal obligation.12

Once these elements are established, the resulting customary rule binds all states, regardless of whether they have expressly consented to it, unless a state has persistently objected from the rule's inception.1

In the context of the laws of war, customary principles often become codified in treaties. When a treaty reflects widespread state practice, its provisions may bind even non-party states as customary law. Additionally, the Martens Clause ensures that in areas not covered by specific treaties, belligerents remain protected by the principles of the law of nations, as derived from established usages, the laws of humanity, and the dictates of public conscience.2