Custom, Power, and the Power of Rules: International Relations and Customary International Law

The previous chapter has explained opinio juris as being those shared understandings which enable States to distinguish between legally relevant and legally irrelevant State practice. This explanation of opinio juris was then developed in terms of its implications for several other, fundamental problems associated with the process of customary international law. This chapter extends the analysis yet further by exploring the insights that might be derived, on the basis of such an explanation, in respect of four other, related issues of importance: (i) the relationship between customary international law and treaties; (ii) the concept of persistent objection; (iii) jus cogens; and (iv) the relationship between jus cogens and erga omnes rules.

It is generally accepted that there are three primary sources of international law, namely, treaties, customary international law and general principles of law.¹ Of these three sources, the first two — treaties and customary international law — are considered much the more important.

Like the process of customary international law, treaties are a kind of regime or institution.² Consequently, to study the relationship between treaties and customary international law is to study the relationship between two different kinds of regimes or institutions. This is something which most international relations scholars have yet to do, having instead focused their attention on the relationship between particular regimes or institutions and States.³ In contrast, international lawyers have deffoted a great deal of attention to the relationship between treaties, rules of customary international law and certain other kinds of institutions, such as, the United Nations and other international organisations.⁴ However, rel

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