Customary IHL

Both treaty law and customary international law are sources of international law. Treaties, such as the four Geneva Conventions of 1949, are binding only on those states that have expressed their consent to be bound by them (ratified them). In contrast, customary international law binds all states, irrespective of which conventions they have ratified or not. The ICRC notes that '[i]t is generally agreed that the existence of a rule of customary international law requires the presence of two elements, namely state practice (usus) and a belief that such practice is required, prohibited or allowed, depending on the nature of the rule, as a matter of law (opinio juris sive necessitatis)'. As such, customary IHL derives from analysis of multiple sources: military manuals, national legislation, case law, and other official documents. 

As part of their mission to promote IHL, the ICRC maintains a compilation of customary IHL, condensing the established practices into rules accompanied by commentary as well as a collection of related practices. While much debate on the customary status of specific prohibitions and regulations is ongoing, the ICRC Customary IHL database is often referred to as the most authoritative and exhaustive study on customary IHL.

Relevant materials:

• ICRC, Customary International Humanitarian Law Database [English]
• ICRC Customary International Humanitarian Law: Rules, with an introduction by Judge Mykola Gnatovsky 'The Rebirth of Customary International Humanitarian Law' [Ukrainian]
• Melzer, N., International Humanitarian Law: A Comprehensive Introduction, ICRC Reference book [English | Ukrainian]
• ICRC Ukraine, The essence and significance of customary norms of international humanitarian law [English | Ukrainian]
• Global Rights Compliance, 'The Domestic Implementation of International Humanitarian Law in Ukraine (updated)' (February 2021) [English | Ukrainian]