When does customary international law become 'jus cogens'?

Question

The Statute of the International Criminal Court acknowledges the existence of international criminal law. Customary International Law "...... consists of rules of law derived from the consistent conduct of States acting out of the belief that the law required them to act that way."

So, customary international law is something:

1. That is repeatedly resorted to by a large number of Nation-States as a method of widespread conduct.
2. On which there is consensus, exhibited by the above and some discernable sense of obligation (whether moral/ethical, or legal).

Customary International Law is binding on all States unless a State specifically objects to it (silence is treated as consent).

However, jus cogens law (a subset of customary international law) is binding on all countries, regardless of consent. It is a fundamental principle of law accepted by the international community as a norm to which no derogation is ever permitted.

So, what makes a non-binding customary international law attain the status of a binding jus cogens customary international law?

Answer 1

The simplest answer is that it is decisions of international courts such as the International Criminal Triubunal for the Former Yugoslavia, or the International Criminal Court, that elevate principles to the status of jus cogens, by stating that they should be considered as such. It may take a series of consistent decisions, that are also consistent with treaties ratified by a large number of countries and widespread practice by states. It is not always perfectly clear cut what principles are considered peremptory norms.