Customary international law

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Customary international law is a form of unwritten international law that arises through general practice, based on the conviction that the norm is legally binding .


According to Article 38 (1) of the Statute of the International Court of Justice (ICJ Statute), customary international law is one of the sources of international law alongside international treaties and the “general legal principles”.

According to the generally accepted definition, customary international law arises from a common legal conviction ( lat. Opinio iuris sive necessitatis ) of legal comrades - here specifically the subjects of international law - and the general practice (lat. Consuetudo ). These two core elements can also be found in the definition of Art. 38 Para. 1 b ICJ Statute.

If there is still broad agreement on the basic definition, the questions that go beyond this are extremely controversial in the literature on international law, especially with regard to the weighting of the two elements. In parts of the literature, for example, the importance of general practice is clearly restricted in relation to the legal conviction that supports it. In contrast, the International Law Commission in its 2018 conclusions on the identification of customary international law emphasized the independent importance of the two elements.

General exercise

In the case law of the International Court of Justice under international law , for example in the Nicaragua decision in 1986, it is clearly expressed that a form of general practice is a constitutive element of customary international law. However, parts of the literature strongly contradict this and also allow the possibility of "spontaneous" emergence of customary international law ( instant customary law ). A continuous, uniform and extensive exercise is necessary, especially with the subjects of international law particularly affected.

Whether a sufficient general exercise is available is measured in individual cases by the distribution in many countries , in representative countries (geographical distribution) or in particularly affected countries. The generality of the legal exercise cannot be determined by counting. Rather, the circumstances of the individual case are to be considered here too. B. customary legal clauses of space law are only shaped by states that operate space travel and are therefore particularly affected. As a rule, therefore, customary international law is given if the behavior of all international law subjects is recorded who can also participate in the respective matter or whose interests are affected - at least according to the ICJ in the North Sea-continental shelf cases 1967-1969. Regional customary international law is also conceivable.

It takes a certain amount of time to practice . In terms of time, the emergence of customary law can already be affirmed after a relatively short period of time; an exercise of the law since "time immemorial" or even over a considerable period of time is by no means necessary. To what extent the Kosovo mission (1999) and the Bush Doctrine (2002) could be based on customary international law is controversial.

Furthermore, no opposing acts may exist. The exercise must be sufficiently uniform, i.e. the parties involved in international law must behave largely in the same way. Individual later deviations from this exercise are then to be qualified as violations of the customary law that has arisen, but do not question the uniformity of the exercise - as long as the deviations are not so numerous and serious that the formation of new, deviating international customary law can be assumed.

Legal belief (opinionio iuris)

The exercise must be based on the conviction that it is legally binding ( opinio iuris , also written opinionio juris ). The act must therefore not only be meant politically or be a national custom ( comitas gentium ) or an expression of arrogance . A subjective element is added to the objective element of the exercise. It is therefore crucial that it becomes apparent to the outside that the actors are aligning their actions with an international exercise because they see this as a right. Here again, however, it is not necessary to have complete uniformity of the conception of all subjects of international law, individual deviations are therefore irrelevant.

Relationship to international treaty law

International treaty law can codify customary law, i.e. in written form that is then also contractually binding for the member states of the treaty. This applies e.g. B. for large parts of the Vienna Convention on Diplomatic Relations , which builds on and writes down customary international law that already existed when the treaty was concluded.

The reverse development is also conceivable: Originally only contractually applicable provisions can become part of customary international law, even if non-contracting parties apply the provision in question and make it clear that they are based on customary validity. Here, too, practice and the opinio iuris are required .

Both developments can ultimately lead to parallel customary international law, i.e. H. a certain rule of international law applies both contractually between the contracting parties and under customary law in the relationship between all states - including non-member states. In the Nicaragua case, for example, the International Court of Justice has expressly stated that the prohibition of interstate violence is not only anchored in the UN Charter and thus applies under contract law, but that it can also claim customary validity.

It is also possible that a contract law norm contradicts a norm of customary international law. In these cases the answer to the question of which norm supersedes the other can prove difficult. Although there is no hierarchy of norms in international law, it can be assumed that contractual norms take precedence over customary international law based on the lex specialis principle. This is justified by the fact that customary international law is optional except for the ius cogens norms. The subjects of international law are therefore free to change, replace or exclude the norms in their relationships with one another. However, this does not mean that customary international law cannot, conversely, supersede international treaty law, as the ICJ confirmed in the North Sea-Continental Shelf judgment of February 20, 1969.

Problems and further questions

Whether states (especially hegemonic powers ) can create new customary international law simply through repeated practice ( consuetudo ) if this behavior is only accepted by other states or these actions are not contradicted ( acquiescence ) is a matter of dispute . According to the definition given at the beginning, the conviction of legal validity required for customary law should be lacking. However, at least particular customary international law can then develop between the legal subjects who recognize such a new legal rule of customary international law.

The UN General Assembly cannot enact international law, but only initiate initiatives for appropriate treaty negotiations between the individual states. Statements by the states and their voting behavior in the UN organs can, however, be an expression of the conviction of the existence of a corresponding customary international law and are therefore an indication of the existence of a opinio iuris .

With the emergence of customary international law, a state cannot prevent the emergence of customary international law, but it can prevent its application. Because a state opposes this from the start, the customary international law that has arisen does not apply to it ( persistent objector ).


See also


Web links


  1. Niels Petersen: The change of unwritten international law in the course of constitutionalization . In: Archives of International Law . tape 46 , no. 4 , 2008, JSTOR : 40800231 : "The extent of the sources of unwritten law ([Art. 38 (1)] lit. b and c [of the ICJ Statute]) is particularly controversial."
  2. ^ IGH statute ( memento of June 29, 2011 in the Internet Archive ).
  3. International Law Commission: Report. Seventieth session (30 April – 1 June and 2 July – 10 August 2018) . Ed .: United Nations . New York 2018, A / 73/10, pp. 12 ff . (English, [accessed January 27, 2020]).
  4. ^ A b North Sea Continental Shelf (Federal Republic of Germany / Denmark) ( Memento from February 6, 2009 in the Internet Archive ).
  5. Carmen Thiele, Fragmentation of International Law as a Challenge for the International Community , in: Archiv des Völkerrechts (AVR), Vol. 46 (2008), pp. 1–41, here p. 6.
  6. Wolff Heintschel von Heinegg , in: Knut Ipsen : Völkerrecht. 5th edition, CH Beck, Munich 2008, § 20 Rn 1–2.
  7. See Federal Constitutional Court , decision of October 21, 1987 (2 BvR 373/83), BVerfGE 77, 137.