Introduction
In the system of public international law, norms are created through various sources, primarily treaties and customary international law (CIL). Within this framework, a hierarchy exists, with a special category of norms known as jus cogens, or peremptory norms, at its apex. The statement, "jus cogens is customary international law but customary international law is not jus cogens", accurately describes the relationship between these two fundamental concepts. This essay will explain this relationship by first defining customary international law and jus cogens separately, and then demonstrating how jus cogens norms form a specific, elevated subset of CIL, while the broader body of CIL does not share this peremptory status.
The Nature of Customary International Law
Customary international law is a primary source of international law, as recognised by Article 38(1)(b) of the Statute of the International Court of Justice, which refers to "international custom, as evidence of a general practice accepted as law". For a rule to become CIL, two elements must be established. The first is state practice (usus), meaning a widespread, consistent, and uniform practice by a significant number of states over time (Shaw, 2017). The second element is opinio juris sive necessitatis, which is the belief held by states that their practice is a legal obligation, not merely a matter of courtesy or habit.
A key feature of ordinary CIL is that it is binding on all states, with one important exception: the 'persistent objector' rule. A state that has clearly and consistently objected to a rule of CIL from its inception is not bound by it. This means that, in principle, states can 'opt-out' of ordinary customary rules. Many rules of CIL, such as those concerning diplomatic immunity or the delineation of maritime boundaries, fall into this category. States are generally free to create treaties between themselves that modify or override these ordinary customary rules in their mutual relations.
The Special Status of Jus Cogens
Jus cogens, or peremptory norms, are a higher form of international law. The concept is formally recognised in Article 53 of the Vienna Convention on the Law of Treaties 1969 (VCLT). This article defines a peremptory norm as one "accepted and recognized by the international community of States as a whole as a norm from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character". This definition highlights the core feature of jus cogens: it is non-derogable.
Unlike ordinary CIL, states cannot 'opt-out' of jus cogens norms. The persistent objector rule does not apply (Shaw, 2017). Any treaty that conflicts with a jus cogens norm at the time of its conclusion is void. Examples of norms widely accepted as having jus cogens status include the prohibitions on the use of force (aggression), genocide, slavery, and torture. These rules are considered so fundamental to the international legal order that they protect the interests of the international community as a whole.
The Hierarchical Relationship
The assertion that "jus cogens is customary international law" is correct because the formation of a jus cogens norm follows the same path as CIL, but with an additional requirement. A norm must first show evidence of extensive state practice and opinio juris to be considered a rule of general international law. However, to achieve jus cogens status, it must also gain a further level of acceptance from the "international community of States as a whole" that it is a fundamental norm from which no derogation is permitted. In this sense, a jus cogens norm is a rule of CIL that has been elevated to a higher status due to its fundamental importance.
Conversely, the statement that "customary international law is not jus cogens" is also correct. The vast majority of CIL rules do not possess this non-derogable character. For instance, customary rules on the extent of the territorial sea or the methods for determining the continental shelf are important but are not considered fundamental to the point of being non-derogable. States frequently conclude bilateral or regional treaties to create different rules that apply between them, thereby derogating from the general CIL rule. This is perfectly permissible for ordinary CIL but would be unlawful if the rule were jus cogens. Therefore, while jus cogens is a type of CIL, it is a very small and specialised category. The larger body of CIL does not share its peremptory nature.
Conclusion
In conclusion, the statement accurately captures the hierarchical relationship between jus cogens and customary international law. All jus cogens norms are, by their nature, rules of customary international law that have been vested with a special legal force by the international community. They are created through state practice and opinio juris, but with the additional qualification that they are recognised as non-derogable. However, the reverse is not true. Most rules of customary international law are not peremptory; they are subject to the persistent objector rule and can be modified by treaty. This distinction is critically important in areas such as national defence, as it clarifies that certain actions, like torture or aggression, are absolutely prohibited under all circumstances, with no legal justification or derogation possible.
References
- Shaw, M. N. (2017) International Law. 8th edn. Cambridge: Cambridge University Press.
- Statute of the International Court of Justice 1945.
- Vienna Convention on the Law of Treaties 1969.
