w. of rules established by custom or treaty which

w. Introduction
International Law

rule is a set of rules established by custom or treaty which are recognised by
states and are considered as binding. Without such rules, the international arena
would be chaotic since there would be no governing law between states. These
rules serve as a framework which define states’
responsibilities towards other states and within the boundaries of other
states. Such rules comprise human rights, security and self-defence, crime,
migration, war, sustainable development, world trade and other issues which
effect the world. These rules also serve as the basis of bilateral agreements
between one state and the other.

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 Treaties are written as new needs arise on the
international arena where states are party to such treaties on agreement and
once they agree to give the necessary consent to the contents of the agreement.
A state binds itself to a treaty by signature, ratification, acceptance,
approval and accession. The most important source of legal international
obligations are treaties, having a direct influence on content of general
international law. Treaties are generally defined by the Vienna Convention as
being “an international agreement
concluded between States in written form and governed by international law,
whether embodied in a single instrument or in two or more related instruments
and whatever its particular designation”1.


 Customary International law, on the
other hand, is also an important characteristic of international law which involves the principle of custom. It involves the long
established practices or customs in any given country which are considered to
be legal and therefore considered as part of the law.  Customary law is focused on the
actual practice and opino juris of the states, i.e. the belief by states that
their practice is legally required by the norm. Many scholars argue the
requirement of opino juris since the ICJ never requests proof of opino juris to
ensure an established form of practice in any given state but it rather focuses
on actual practice in a particular state which is part of the norm and which
norm is accepted by the states, as per the 1985 Continental Shelf Case between
Libya and Malta.


Article 39 (1)(B) of the
Statute of the International Court of Justice ( ICJ)  affirms that the court shall apply in addition
to conventions ‘international custom, as evidence of a general practice
accepted as law2.
 Nevertheless, states must believe that
they must conform with the norm for customary law to be regarded as required by
law. What this means is that for customary law to be considered as part of a
requirement by law, states need to believe that the practice in itself is
legal, part of the norm and a requirement by law.

International law, as a general rule, is set to
establish relations between states and to regulate their behaviour. It is thus pertinent
that customary law is addressed and acknowledged and given its due importance.  Customary law, in simple terms, is any given
behaviour by a state regarded as part of the norm. Although States are expected
to conform to international law and conventions, whether and how customary law
is applied in a country should also be given its due importance. The consent of
states who are party to conventions and international law should be obtained.
It should be observed that states bind themselves to their own actions and
practices as regard this part of their customary law. In actual fact, customary
law which consists of a set of unwritten rules, mostly emerge in cases of
lacunae where treaties do not manage to regulate all aspects of international

The principle and importance of customary law is
difficult to explain yet it is a vital part of international law where together
with treaties and conventions complete international law in its entirety. The
ICJ is tasked to consider customary international law not only by the general
practice accepted by law but also by what states have accepted as law. Therefore,
opinio juris is subject to dispute but is also a legally binding practice. A
general practice, referred to as the objective element, because it is visible
and accepted by law. These two elements have been juridically approved by the
International Court of Justice.

In fact, apart from the 1985 Libya Malta case, the
International Court of Justice also considered these two elements, general
practice and opinion juris, in the 1986 Nicaragua Judgement as the basis of the
formation of customary law.

Similarly, treaties are
considered the strongest and most binding type of source of international law
because the countries that agree to and sign it are represented by it. Treaties
form international law by creating standards, agreements, and diplomatic ties
between states. Treaties are similar to contracts between countries. States may
debate the interpretation or implementation of a treaty, but the written
provisions of a treaty are binding.3  The ‘Vienna Convention on the law of Treaties
of 1969’ is the main regulator which monitors treaties and is considered as the
“treaty of treaties”.4 This convention gives a
clear definition of a treaty and describes how treaties are made, amended,
interpreted, how they operate and are terminated. The rights and obligations of
parties are described and outlines in specific treaties.5 Article 2 (1)(a) of the
VCLT defines a treaty as “treaty means an international agreement concluded between States in written
form and governed by international law, whether embodied in a single instrument or in
two or more related instruments and whatever its particular designation;”6 VCLT only relates to
treaties that are governed by international law. Existing customary law is
codified by treaties which in return is binding on non- party states as well.7
Treaties could be a direct source of international law or reflective of a
customary or general principles of law as evidence. International treaties
could be law making treaties or treaty contracts.8

The 1955 Nottebohm case which was decided by the
International Court of Justice was an 
important and significant case where in the decision the judge declared
that he would not be referring to the Bancroft treaties. The reason was that
the judge considered these treaties as incorrect and “as constituting a
precedent”. According to the judge the Bancroft treaties were only binding on
the contracting parties.9
It was concluded that there was no international law pertinent to the Nottebohm
It would unsafe to conclude conclusion that treaties are can cause precedents
which are law binding on non-signatories.11
This case is by no means an isolated example of such use of treaties. The
reports of international tribunals often have reference to citations of
treaties introduced by parties as support for their contentions although the
parties were not signatories of the treaties. For example Colombia cited a
number of extradition treaties in the Asylum case, which the ICJ found
inapposite for the question of diplomatic asylum without stating whether such treaties
were irrelevant as precedent.12
Some courts have found that similar provisions in many extradition treaties
have been applicable to states which have not expressly accepted those

Treaties are expected to be executed in good faith,
in keeping with the principle of pacta sunt servanda arguably the oldest
principle of international law, and so treaties are sometimes described as
sources of obligation under law. Without this principle, which is explicitly
mentioned in many agreements, treaties would be neither binding nor
The use of multilateral treaties increased dramatically as they have proved to
be an effective way to establish new rules of international law, particularly
where there is a need to respond quickly to changing circumstances.14

Article 38 of the Statute
of the International Court of Justice recognises a definitive statement of the
sources of international law where it states; 1) the Court, whose function is
to decide in accordance with international law such disputes as are submitted
to it, shall apply: a) international conventions establishing rules recognized
by the contesting states; b) international custom; c) the general principles of
law; d) subject to the provisions of article 59, judicial decisions and the
writing of most highly qualified publicists as subsidiary means for the
determination of the rule of law; 2) the provision shall not prejudice the
power of the court to decide a case ex aequo et bono, if the parties agree

Thus it can be stated that treaties and custom are
both equally important and contribute to the making of international law. All
branches of international law incorporate customary law and treaties. Treaties,
protocols, conventions together with customary law which bind all states parties,
make up international law.16 The
effectiveness of international law is much based on how treaties and customary
law complement each other. It is important to note that treaties are an explicit source of law which
gains legitimacy through signatures while customary law is implicit through
generally accepted practises of nation states thus the importance of giving
both equal weight and to understand the importance of the basis of the formation
of international law.17

It can therefore be stated that customary
international law and treaties are both law making processes
however the way treaties are formed differ to the way customary law is created
in that customary law is not supported by dispute resolution mechanisms and
does not contain flexibility clauses as found in treaties. Furthermore,
Customary law applies to all states whilst treaties apply to those states who
consent to it.18
However the basic difference is always practice versus negotiation since the
way treaties are created is always based on negotiation processes between
states whilst custom is formed through practice.

customary law is formed by practice, it is important to note that in the case
of territorial sea disputes, states argue their position based on custom whilst
other states respond by written agreements. This is a clear picture of how
custom and treaties contribute to the coordination and position of states in
agreement with the rest of the states. On one hand we have territorial sea
claims as ‘custom’ and on the other hand we have written disputes which are
open to negotiation and which eventually establish treaties, custom and
practice in a written solidified manner

Nicaragua vs United States Case (1986)

The Nicaragua vs United States Case (1986)
concerned issues relating to military and paramilitary activities carried out
by the US against Nicaragua between 1981 and 1984. Due to a multilateral treaty
reservation, the court could not refer to the United Nations Charter .
Therefore the court based its findings on customary law and the general principles
of indternal law since the issue concerned the use of force and self-defence.19

In this particular case the court considered the
fact that although custom may not be codified it still exists side by side to
treaties and so both customary and treaty law should be applied.

In a similar case, the North Sea Continental Shelf
Cases, the same argument was used in that the principles of customary
international should be considered as existing side by side treaty law and that
treaties do not necessarily or always supervene where customary law ceases to

Reference was also made to Article 51 of the UN
Charter which states that a treaty itself can recognise the existence of
customary international law with respect to the same subject matter. For
example, the Court said that the term “inherent” in Article 51 recognised
that customary law rights of self-defence existed alongside treaty provisions.20 In
this case it was argued that when a state exercises the right of self-defence
under Article 51, it also has an obligation to report the use of force to the
Security Council. Thus it was declared by the court that this was a treaty
requirement which did not exist under customary law and so the United States
was not breaching customary law but was rather failing to proof that self-defence
was required since no report was submitted to the security council about the
necessity to use force as a means of self-defence.

interesting argument in the Nicaragua vs United States case is that Article 51
of the UN Charter does not define the term ‘armed attack’ and the concept of
necessity and proportionality, whilst customary law does. The Court highlighted
this as proof that customary law does exist alongside treaty law and that the
sources of law do not always overlap or have the same content. Furthermore Article 31 of the Vienna
Convention on the Law of Treaties21
specifically states that treaty law can be interpreted with the assistance of
customary law or general principles of international law.

As regards the issue of opinio juris and state practice, the court did
consider these as essential and contributory factors to the formation of
customary practices. With regards to state practice, the court claimed that
custom does not have to be consistent as inconsistency does not affect the
existence of the customary rule principle as long as it is not breached. However,
for a customary law rule to be formed state practice is always required. In
customary international law, opinio juris is the second element necessary to
establish a legally binding custom. It denotes a subjective obligation, a sense
on behalf of a state that it is bound to the law in question.22
Article 38(1)(b) states that the custom to be applied must be “accepted as law”

Australia v Japan:
Whaling in the Antarctic (2014)

treaty concerning this case dated back to 1946 by the International Convention
for the Regulation of Whaling. This was case where Australia accused Japan was
in breach of obligations under the 1946 International Convention for the
Regulation of Whaling and of other international obligations for the
preservation of marine mammals and the marine environment23

this dispute the Court looked closely at the contents of the agreement and the
interpretation of provisions, particularly Paragraph 1 which states that states:
“may grant to any of their nationals a special permit authorizing that
national to kill, take and treat whales for purposes of scientific research”.

Thus although treaties bind states by means of agreement in an attempt
to bring about international order, disputes also arise and cases are presented
to the ICJ to assist with the correct interpretation of the contents of the
agreement. ICJ is then tasked with delving into the text, interpreting it and
adapting it to countries on a case by case basis.

As per
the above two examples the relationship between treaty and custom is strong and
is directly related to the importance of international law particularly with regards
to theory and practice. Both exert an equal amount of influence on each other
and on legislators so that any issue rising on the international arena is
addressed using both theory and practice, and therefore both custom and
agreements reached in treaties. Thus regulations presented by both equally
contribute to the body of international law and at times may also conflict each
other. The Vienna Convention makes reference of customary law which will
continue to be regarded as equally important to treaties. Yet the contents of
the Vienna Convention advocates the importance attributed to customary law.


activity can constitute state practice for the formation of a customary norm.
One has to see whether states are only acting under their treaty obligations or
whether adherence to treaty obligations has grown to include opinio juris and
broader state practice.24
An example is the UN Convention on law of high seas as a lot of UNCLOS
provisions have become customary international law. States which are not part
of the UNCLOS are bound by some of the provisions of the convention.25
Therefore it is safe to say that for a treaty to create custom, you need non
treaty parties acting as if they are bound by the treaty. A treaty can be used
to contract out of a customary international law. There are however certain
obligations that no state can contract out of such as the jus cogens norms
which are certain fundamental, overriding principles of international law, from
which no derogation is ever permitted.26
Such examples include the prohibition on the use of force, the law of genocide,
principle of racial non- discrimination, crimes against humanity and the rules
prohibiting trade in slaves or human trafficking.27

disputes, custom and treaties are looked at first and formerly to evaluate
rules in a given situation.  As per the
humanitarian law case in former Yugoslavia, the ICJ identified the importance
of applying customary law which included the consideration of treaties such as the
Geneva Conventions of 12 August 1949 for the Protection of War Victims,  the Hague Convention (IV) Respecting the Laws
and Customs of War on Land and the Regulations , the Convention on the
Prevention and Punishment of the Crime of Genocide of 9 December 1948 and the
Charter of the International Military Tribunal of 8 August 1945. In this case
the court found it important to refer to customary law incorporated in the
aforementioned treaties so as to reach a fair and amicable agreement.

 As a final note on hierarchy and what should
come first, whether it is treaty or customary law it can be stated that although
a treaty can prevail over customary rule since it is a form of legal agreement
between states, such treaty should not affect the rights of States not party to
that treaty. Therefore, there is no set hierarchy of norms in international
law. States can agree to deviate from customary law and enact treaties to
enforce changes. In cases where a treaty conflicts with customary law, treaties
will prevail over customary rules which are not consistent. In cases of
genocide, torture and slavery treaties will prevail and enable other states to
intervene and defend people. Therefore, there exists no strict hierarchy
between treaty and customary law.


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