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International custom

Customary international law requires two elements: a consistent practice of states and the conviction of those states that the consistent practice is required by a legal obligation, referred to as opinio juris. Custom distinguishes itself from treaty law as it is binding on all states, regardless of whether they have participated in the practice, with the exception of states who have been persistent objectors during the process of the custom being formed and special or local forms of customary law. The requirement for state practice relates to the practice, either through action or failure to act, of states in relation to other states or international organisations. There is no legal requirement for state practice to be uniform or for the practice to be long-running, although the ICJ has set a high bar for enforcement in the cases of Anglo-Norwegian Fisheries and North Sea Continental Shelf. There has been legal debate on this topic with the only prominent view on the length of time necessary to establish custom explained by Humphrey Waldock as varying "according to the nature of the case". The practice is not required to be followed universally by states, but there must be a "general recognition" by states "whose interests are specially affected".

The second element of the test, opinio juris, the belief of a party that a particular action is required by the law is referred to as the subjective element. The ICJ has stated in dictum in North Sea Continental Shelf that, "Not only must the acts concerned amount to a settled practice, but they must also be such, or be carried out in such a way, as to be evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it". A committee of the International Law Association has argued that there is a general presumption of an opinio juris where state practice is proven but it may be necessary if the practice suggests that the states did not believe it was creating a precedent. The test in these circumstances is whether opinio juris can be proven by the states' failure to protest. Other academics believe that intention to create customary law can be shown by states including the principle in multiple bilateral and multilateral treaties, so that treaty law is necessary to form customs.

The adoption of the VCLT in 1969 established the concept of jus cogens, or peremptory norms, which are "a norm 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". Where customary or treaty law conflicts with a peremptory norm, it will be considered invalid, but there is no agreed definition of jus cogens. Academics have debated what principles are considered peremptory norms but the mostly widely agreed is the principle of non-use of force. The next year, the ICJ defined erga omnes obligations as those owed to "the international community as a whole", which included the illegality of genocide and human rights.