What is customary international law?
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 bind only those States which 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 is set forth, 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.
Who is bound by customary international humanitarian law?
Generally, the purpose of international law is to regulate the relationship between States and, therefo re, it is binding upon States. This is also true for international humanitarian law, whether treaty or customary, as it regulates armed conflicts arising between States.
However, a particular feature of international humanitarian law 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 a State or an armed opposition group. The analysis of State practice shows that many rules of customary international humanitarian law applicable in non-international armed conflicts bind States as well as armed opposition groups.