Customary International Humanitarian Law
Rules derived from general practice accepted as law that regulate conduct during armed conflicts.
Updated April 23, 2026
How It Works in Practice
Customary International Humanitarian Law (IHL) consists of unwritten rules that have evolved over time through the consistent and general practices of states, which are accepted as legally binding during armed conflicts. These rules regulate how parties to a conflict must behave, aiming to limit the effects of war on combatants and civilians alike. Unlike treaty law, which is written and explicitly agreed upon, customary IHL emerges from the actual behavior of states and their recognition that such behavior is obligatory. This means that even states that have not ratified specific treaties may still be bound by these customary rules if they reflect widely accepted practices.
For example, customary IHL includes principles such as the humane treatment of prisoners of war, the prohibition of torture, and the distinction between combatants and civilians. These principles apply universally in all types of armed conflicts, whether international or non-international.
Why It Matters
Customary IHL plays a crucial role in the international legal system because it fills gaps where treaty law may not exist or be universally ratified. It ensures that there are minimum standards of conduct that all parties in a conflict must follow, thus protecting human dignity during war. This universality is vital in situations where states have different treaty commitments or where non-state armed groups are involved.
Moreover, customary IHL forms the basis for many international criminal prosecutions and helps guide the work of humanitarian organizations such as the International Committee of the Red Cross (ICRC). It also influences the drafting of new treaties and the interpretation of existing ones.
Customary International Humanitarian Law vs Treaty Law
While both customary IHL and treaty law regulate conduct in armed conflict, they differ primarily in their sources and formation. Treaty law arises from explicit agreements between states documented in written form, such as the Geneva Conventions. Customary IHL, on the other hand, develops from the consistent practice of states accompanied by a belief that such practice is legally obligatory (opinio juris).
This distinction means that treaty law applies only to states that have ratified the treaty, whereas customary IHL applies universally, regardless of treaty ratification. In practice, many treaty provisions reflect customary law, and vice versa, but the two are distinct legal sources.
Common Misconceptions
One common misconception is that customary IHL is less authoritative or less enforceable than treaty law. In reality, customary IHL carries the same legal weight and is binding on all states and parties to a conflict. Another misunderstanding is that customary rules are vague or uncertain. While some areas of customary IHL may be debated, many core principles have been clearly established through consistent state practice and judicial decisions.
Real-World Examples
A well-known example of customary IHL is the rule prohibiting the use of poison or poisoned weapons in armed conflict. This prohibition is widely accepted and practiced, even by states that may not have ratified specific treaties banning such weapons. Similarly, the principle of distinction—requiring parties to distinguish between combatants and civilians—is a customary norm that guides military operations worldwide.
The International Criminal Tribunal for the former Yugoslavia (ICTY) and the International Criminal Court (ICC) have both relied on customary IHL norms to prosecute war crimes, demonstrating its practical application in international justice.
Example
The principle of distinction, a cornerstone of customary international humanitarian law, requires armed forces to differentiate between combatants and civilians during military operations.