Customary IHL

Both treaty law and customary international law are sources of international law. Treaties, such as the four Geneva Conventions of 1949, are written conventions in which states formally establish certain rules. Treaties are binding only on those states that have expressed their consent to be bound by them, usually through ratification. Customary international law, on the other hand, derives from "a general practice accepted as law". Such practice can be found in official accounts of military operations but is also reflected in a variety of other official documents, including military manuals, national legislation and case law. The requirement that this practice be "accepted as law" is often referred to as opinio juris. This characteristic sets practices required by law apart from practices followed as a matter of policy, for example.

States recognize that treaties and customary international law are sources of international law and, as such, are binding. This recognition is enshrined, for example, in the Statute of the International Court of Justice. One illustration of the binding nature of customary international law is its application by national and international courts and tribunals.

Generally, the purpose of international law is to regulate the relationship between states; international law is therefore binding on states. This is also true for both treaty-based and customary IHL, as it regulates armed conflicts arising between states. However, a particular feature of IHL is that some of its rules regulate armed conflicts occurring between a state and an armed opposition group or between such groups. The rules that regulate such conflicts are applicable to all parties, whether they are a state or an armed opposition group. The analysis of state practice shows that many rules of customary IHL applicable in non-international armed conflicts are binding on states as well as on armed opposition groups.