Does state behavior contrary to a rule of customary international law undermine the existence of that rule?

Essay, 2020

13 Pages



1.1 Purpose of Essay

2.1 Elements of Customary International Law
2.2 The Traditional and Modern Approach


4.1 Evidence of Acquiescence
4.2 Contrary conduct proves the existence of Customary International Law
4.3 Contrary conduct gives rise to new rules of Customary International Law
4.4 Contrary conduct pleaded as an exception proves the existence of Customary International Law
4.5 Contrary State Conduct v. Jus Cogens (Peremptory Norms)



Customary International Law is one of the major sources of International law and is described in Article 38(1)(b) of the Statute of the International Court of Justice, 1946, as “general practice accepted as law.”

1.1 - Purpose of Essay

The purpose of this essay is to assess the impact of the behaviour of a State, which contrary to a rule of customary international law, on the existence of rule of customary international law, in terms of whether it undermines the existence of that rule or strengthens it. To be able to do this, it is expedient that the following issues are addressed.

1. What is Customary International Law
2. What are rules of customary international law
3. Contrary state behavior


According to the Legal Information Institute of the Cornell Law School, “Customary international law refers to international obligations arising from established international practices, as opposed to obligations arising from formal written conventions and treaties. Customary international law results from a general and consistent practice of states that they follow from a sense of legal obligation.” It was also defined by Judge Read in Fisheries (UK v Norway) (1951), as the generalization of the practice of States.

2.1 - Elements of Customary International Law

According to Lowe (2007) who opined that Article 38(1)(b) “should more properly refer to a general practice as evidence of an international custom accepted as law”, customary international law is made up of two main elements, namely:

- The material element - Which refers to conducts that a state consistently practices for a relevant period of time (Duiturnitas)
- The psychological element (Opinio juris sive necessitates - meaning an opinion of law or necessity), which refers to the belief of a state that it is bound by legal obligation to follow a certain practice and so would act in that certain manner. (Roberts, 2001)

2.2 - The Traditional and Modern Approach

There is a traditional and modern approach to the elements of Customary International Law.

According to Roberts (2001) traditional custom and modern custom are generally assumed to be alternatives because the former emphasizes state practice, whereas the latter emphasizes opinio juris. Lepard (2010) argues however, that the modern approach eliminates the material element which focuses on state practices, thereby destroying that customary feature in customary international law.

While Lowe (2007) holds the view that there are situations of controversy, where one element is highlighted at the expense of the other, thereby making customary international law vague and weak in credibility, Kirgis (1987) recommends that the elements can be viewed and applied interchangeably along a sliding scale, such that in cases where duiturnitas is inadequate, opinio juris would be expected to have a stronger impact and where opinio juris declines, duiturnitas will be stronger.


Customary International Law “emerged from the development of norms” (Biersteker, 2007) and some of these norms as enunciated by Crawford (2012) include (but are not limited to):

- A general prohibition of the use of non-defensive force by one state against another. See Nicaragua v. United States of America (1986). Para 187-190
- The impermissibility of the acquisition of territory by war. See Wall Advisory Opinion, (2004)
- The right of states to defend self against armed attacks by other states. See the Nicaragua case (1986)
- The requirement that any uses of force in self-defense be necessary and proportional. See the Nicaragua case (1986)
- Diplomatic privileges and immunities. See Arrest Warrant Case (2002)
- Diplomatic protection. See Republic of Guinea v. Democratic Republic of Congo (2007) also known as the Diallo Case.
- Principle of resolving disputes between states peacefully. See the Nicaragua Case (1986)

In the Land and Maritime Boundary case of Cameroon v. Nigeria (2002) the ICJ also identified the norms relating to the conclusion and coming into force of treaties as a norm of customary law. See also the Wall Advisory Opinion (2004), where the ICJ held that the interpretation of treaties is a norm of customary international law.

According to Crawford (2012) “customary international law historically developed through the form of liberties and prohibitions, and has remained imprecise with respect to the scope and consequences especially of serious, systemic illegality.”

Crawford (2012) further observes that even though the International Court restated in the case of Argentina v Uruguay (2010) concerning River Uruguay that “customary international law provides for restitution as one form of reparation for injury, restitution being the re-establishment of the situation which existed before occurrence of the wrongful act... (establishing that) this form of redress has a place in the law, it is difficult to state with any certainty the conditions of its application, outside of cases in which it is provided for explicitly.”

In the case of New Zealand v. France (1990), also known as the Rainbow Warrior case, New Zealand demanded that two people released from custody by France be returned to custody as their release by the latter was a violation of a previous settlement. The tribunal reckoned that the case did not revolve around restitution, but on cessation, and therefore found that “cessation could not be granted on the implausible ground that the unfulfilled obligation to detain had expired in the meantime.” (Crawford, 2012)


There are situations where States digress from known or existing rules of customary International Law. It is unacceptable policy for a conduct or pattern of behaviour to be embraced as right or permissible over a period of time, only for it to be rejected later as wrong, improper or illegal, as this will be very confusing for those engaged in that conduct or pattern of behaviour. This standard applies in all the leading legal societies, thus prohibiting any conduct or pattern of behaviour inconsistent with that which has already been accepted. Those engaged in the activity would not know where they stood. This position is applicable in International Law as well. (Lowe, 2007)

To be able to show that a particular practice is accompanied by the required opinio juris, Judge Abraham of the ICJ suggested while giving his separate opinion in the case of Belgium v Senegal (2013) involving Obligation to Prosecute or Extradite, it is important to prove that the practice is observed with a different motive or as justifiable under international law (even though not necessarily required by it), by some or most of the states engaged in that practice. (Thirlway, 2019)

It must be noted however, that the mere fact that it cannot be proved that it is believed that a particular rule of Customary International Law exists, does not necessarily mean that the particular rule in question does not exist, but if it can be shown that such a belief exists, then it is taken as sufficient proof of the existence of that rule. (Thirlway, 2019).

4.1 - Evidence of Acquiescence

Evidence of acquiescence among other things, is fact indicating that something, whether an event, conduct or pattern of behaviour, was acceptance without protest, albeit reluctantly. According Lowe (2007) evidence of acquiescence can be used to establish the fact that a State sees a particular activity or status quo as “consistent with international law.” In this regard and to this extent, the state acquiescing, might be added to the list of the States who clearly and openly support the particular rule which is capable of emerging from that conduct “along with any States whose positive actions have contributed to the general practice accepted as law”

4.2 - Contrary conduct proves the existence of Customary International Law

The situation where a State conducts itself in a particular way and other states protest against that conduct or activity is capable of and actually does prove the existence of a particular rule of Customary International Law, thereby “implicitly affirm the rule in question.” (Lowe, 2007) The ICJ affirmed this position in the case of Nicaragua v. United States of America (1986), in its advisory opinions and orders concerning military and paramilitary activities of the United States of America (USA) in and against the territory of Nicaragua. Lowe (2007) summarizes this position well, in saying that “breaches of international law may, paradoxically, strengthen the law rather than weaken it, if the offending State is condemned and isolated.”

4.3 - Contrary conduct gives rise to new rules of Customary International Law

Understanding that rules are an integral part of a set, which work together to derive the desired results, it is important to note that when a contrary state conduct gives rise to a new rule, it would mean among other things, that the old rule has evolved into a new one. As Lowe (2007) pointed out “Protests against breaches of international law are particularly significant... (and) if other States remain silent in the face of an apparent violation of the law, it may be that the first steps towards a change in the law are being taken,” hence ultimately, the development and emergence of a new rule of Customary International Law, clearly showing again, that even when there is contrary state conduct, it will merely give rise to a mutation of the old rule which brings about another rule of Customary International Law, but does not weaken the rule.

4.4 - Contrary conduct pleaded as an exception proves the existence of Customary International Law

In the Nicaragua v. USA case, the USA did not directly contest the rule relating to the use of armed force in Nicaragua as they were also clear that it was a violation of a rule of Customary International Law. They however, objected on the basis of the fact that they had gotten involved in the spirit of collective self-defence as provided for by Article 51 of the United Nations Charter, 1945, recognising the rights of a state which has been attacked by another state to defend itself individually or collectively (in collaboration with other state members of the UN), thus claiming that their military activities in Nicaragua was in response to the attack of the latter against El-Salvador. This objection does not undermine a rule or rules of Customary International Law, because as an exception, it actually recognises the existence of the rule, therefore affirming it.

According to Lowe (2007) “there is also a category of rules or principles of customary international law that admit of no derogation: that is to say, States may not escape their binding force either by persistent objection or by making agreements to disregard the rule.” They are referred to as Jus Cogens.


Excerpt out of 13 pages


Does state behavior contrary to a rule of customary international law undermine the existence of that rule?
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ISBN (eBook)
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Law, International Law, International Customary Law, Treaties, Sources of International Law
Quote paper
Ogochukwu C. Nweke (Author), 2020, Does state behavior contrary to a rule of customary international law undermine the existence of that rule?, Munich, GRIN Verlag,


  • guest on 3/19/2022

    beautifully written

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