What is customary international law?

Unlike treaties, customary international law is a set of rules which bind all states. The International Law Commission an expert legal body with a mandate to undertake the progressive development and codification of international law. It is also responsible for drafting among others, the Vienna Convention on the Law of Treaties and for publishing in 2018 its ‘Draft conclusions on identification of customary international law, with commentaries’.   

As the Commission states in its general commentary, customary international law is ‘unwritten law deriving from practice, accepted by law.’ The two constituent elements of customary international law, which are to be assessed separately, are: a) a general practice (primarily referring to the conduct of States) and b) this practice has to be accepted as law (also referred to as ‘opinio juris’).  

Treaties might also reflect in their provisions rules of customary international law.  

Situations where customary international law plays an important role:   

  • Areas of international law that remain uncodified by a treaty  
  • Situations where even when an area of international law is codified, some States might not be a party to them. For those States, though the treaty does not apply, customary international law does.  
  • Issues might arise in the interpretation of a treaty, where customary international law may be useful.  
  • State practice might influence the interpretation or application of an existing provision of a treaty (e.g. Article 27 of the UN Charter and the practice that the abstention of permanent Security Council does not equal to a veto).  

Customary and treaty law in the laws of armed conflict  

The legal framework governing armed conflict in Ukraine draws from both treaty-based and customary international law. Treaty law includes, among others, the Hague Conventions, the Geneva Conventions, and their Additional Protocols, the Rome Statute, which establish binding obligations for state parties. Complementing these treaties is a vast body of customary rules which bind all states [See, Customary IHL Database by the ICRC] [see further, Judicial Benchbook, paras. 20-21]. These customary rules have crystallised through consistent state practice and include fundamental principles such as, inter alia, the principles of distinction, proportionality and precautions in attack, as well as regulations regarding POWs (Prisoners of War). In ICL, the Nuremberg Principles are considered reflective of customary international law [Judicial Benchbook, para. 141].   

The convergence of treaty and custom is particularly evident in core prohibitions: while grave breaches are defined in the Geneva Conventions, the rule that serious violations of IHL constitute war crimes is also part of customary IHL, along with most serious violations which have the status of war crimes and crimes against humanity, such as the prohibition of torture, the prohibition against mutilation and many others [Judicial Benchbook, paras. 45-50, 85-106, Rule 156, Rule 90, Rule 92, Customary IHL Database]. The ICTY played a major role in identifying and defining existing war crimes and its caselaw is also considered reflective of customary international law. So are Article 7 and 8 (though not in its entirety) and the definitions therein of the Rome Statute [Judicial Benchbook, p. 46 and paras. 1150-1154].  

https://www.asser.nl/media/796504/benchbookplusonplusinternationalpluscrimesplusadjudication.pdfhttps://www.asser.nl/media/796504/benchbookplusonplusinternationalpluscrimesplusadjudication.pdf  

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