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Week 1: Reservations and the law of treaties
Module 2: Subjects and sources of IL
Originally, PIL was conceived as a system of rules governing the relationship between states – which is why older
sources refer to public international law as the law of nations. However, especially since the conclusion of WWII,
actors other than states (non-state actors) have increasingly secured a place as subjects within the international
legal system. These actors include international or governmental organisations, individuals, multinational
corporations (MNCs), and non-governmental organisations (NGOs). Unlike states, these subjects do not possess
full legal personality under IL. Instead, they possess certain rights and duties, which vary depending on the actor
concerned – therefore some scholars categorise them as partial subjects under international law.
The fact that contemporary public international law is concerned with both state actors and non-state actors has
also influenced the variety of sources out of which international law is composed. Within the international system,
there is no single body able to create laws which are internationally binding upon everyone: there is no central,
international legislature or parliament. Nor is there a central system of courts with comprehensive and
compulsory jurisdiction to interpret and apply international law. There is, however, a variety of sources of public
international law. These sources provide the procedural legal rules (secondary rules) by way of which substantive
rights and duties come into existence. The substantive rules (containing rights and duties) of international law are
also known as primary rules.
The most authoritative list of sources of public international law can be found in article 38(1) of the Statute ICJ:
● The first three sources – conventions, custom and general principles of law – are mostly
referred to as the principal sources of international law.
● The last two – judicial decisions and teachings of publicists –are mostly referred to as the
subsidiary sources of international law. It is important to note that the Article 38(1) ICJ Statute
does not list all the sources of international law – it is not exhaustive. Other sources, such as
unilateral acts of states and binding decisions of international organisations, are also
well-accepted sources of international law.
As noted above, with regard to the sources of international law, a further distinction can be drawn
between the two different types of rules:
a. Primary rules: these are substantive rules. This is a body of principles and rules (sources) that
lay down the obligations of the subjects of international law. An example would include the
law on the use of force.
b. Secondary rules: these are procedural rules. These rules govern how the primary rules come
into existence, how they can be interpreted and how they can be changed. An example would
include the law of treaties.
• International legal personality: it is primarily an acknowledgement that an entity is capable of
possessing and exercising certain rights and duties under international law. Such entities are
considered subjects of international law and can be referred to as international legal persons. The
capacities of an international legal person may include, amongst other elements, the following:
a. The ability to make claims before international and national tribunals, in respect of breaches
of international law (see, for example, the ICJ Advisory Opinion Reparation for Injuries
Suffered in the Service of the United Nations).
b. Bearing some or all of the obligations under international law (see for example, the prohibition
of the use of force under Article 2(4) UN Charter).
c. The capacity to make valid international agreements (see for example, Article 6 VCLT).
d. The enjoyment of some or all privileges and immunities from national courts (see for example,
the ICJ Arrest Warrant case).
• Treaties (conventions): as referred to in Article 38(1a) of the ICJ Statute, are agreements which are
concluded between subjects of IL. Historically, treaties have most often been concluded between
states. International organisations, however, may also be a party to a treaty (EU to WTO).
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Treaties are based on the principles of consent and pacta sunt servanda. The latter is a customary
rule of international law and has been codified in Article 26 VCLT, which stipulates that every treaty in
force is binding upon the parties to it and must be performed by them in good faith.
The VCLT regulates the law on treaties between states and most provisions therein have obtained the
status of customary international law.
Rules Regulating Treaties
A state becomes bound by a treaty once it signs and ratifies or accedes to the treaty. If a state has
signed but not (yet) ratified or acceded to a treaty, the state is still bound by the object and purpose of
the treaty (Article 18 VCLT). There are only a few limitations on the formulation and conclusion of a
treaty, which are stipulated in Articles 46-53 VCLT. However!
• Peremptory norm of IL (Article 53 VCLT): is a rule of customary international law which cannot be
derogated from [consequence: treaty is void]. It is also known as a norm of jus cogens. Examples of
peremptory norms are the prohibition of genocide, torture and apartheid.
→ They are the highest substantive norms in PIL.
Sometimes states limit their acceptance of a treaty by submitting a unilateral statement purporting to
exclude or to modify the legal obligations following from the treaty: a reservation. Nowadays, the
general rule is that reservations are allowed upon signature, ratification or accession, unless the treaty
concerned provides otherwise. Furthermore, a reservation may never be incompatible with the object
and purpose of the treaty (Articles 19-23 VCLT). In the ICJ Advisory Opinion on Reservations to the
Convention on the Prevention and Punishment of the Crime of Genocide, in which the Court
held that a reservation may not be incompatible with the object and purpose of the treaty in question.
This ruling served as a source of inspiration for the articles on reservation of the VCLT.
• Interpretation (Article 31 VCLT): in case of a dispute regarding the implications of a treaty provision
for a particular situation, the treaty needs to be interpreted (and thereafter applied). Unless the treaty
in question itself contains provisions concerning its interpretation, the provisions of the VCLT on treaty
interpretation apply. Article 32 VCLT provides further guidance on the interpretation of treaties.
Customary international law
Article 38(1b) ICJ Statute stipulates that, in order for a rule to become customary international law,
two elements need to be in place:
a. State practice [objective element]: refers to the repeated behaviour of states in a certain
manner. Relevant case law:
● North Sea Continental Shelf: the ICJ stated that state practice should be extensive
and virtually uniform: including the practice of those states specially affected.
● Nicaragua: the Court reiterated that state practice does not have to be absolutely
uniform, as long as any inconsistent practice by individual states is regarded as a
breach of the rule in question or, if the state concerned claims that its inconsistent
conduct was justified by an applicable exception or justification.
b. Opinio iuris sive necessitates [subjective element]: refers to the belief on the part of states
that they acted in such a manner based on their conviction that international law required
them to so act. Because opinio iuris relates to the mental state of the state actor thus, asking
'why the state behaved as it did', it can be difficult to identify and prove.
● In practice, a variety of sources have been used to prove the existence of opinio iuris, such as:
official government press releases and other government statements of policy or intent; a
pattern of treaties ratified by a state that all include similar obligation(s); national legislation;
national and international judicial decisions and resolutions or declarations adopted by organs
of the United Nations.
● In past cases, the ICJ has used resolutions of the UN General Assembly to identify
the existence of opinio iuris. In both the Nuclear Weapons Advisory Opinion and
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the Nicaragua case, the Court explained when and how a resolution of the General
Assembly can be used to identify opinio iuris.
General principles of law
• General principles of law (article 38(1) ICJ Statute): as a principal source of international law,
together with treaties and customary law. They can be defined as legal principles recognized in the
domestic law of nations worldwide, which have been transposed into international law (res judicata, equity
and estoppel or pacta sunt servanda). In addition to the general principles of law derived from the domestic
state practice, there are also general principles of international law. These are principles that have a
distinct international character because of the context in which they apply (sovereign equality of nations).
In principle, no hierarchy exists among the three primary sources of international law. However, if we
look at the practice of international courts and tribunals there is a general tendency of relying on
general principles of law whenever a sufficient answer to a legal question is not provided by
international custom or treaties. In other words, in case of conflicting rules, general principles of law
are recognized as lex generalis, whereas customary law and treaties are recognised as lex specialis.
The same approach towards general principles has been applied by the ICJ. Moreover, to date, no
ICJ ruling has been decided solely on the basis of general principles of law. The ICJ has relied on
general principles of law in order to fill legal gaps, interpret legal rules and to confirm a decision based
on other legal rules to reinforce the legal reasoning of the court.
Subsidiary sources
Article 38(1d) ICJ Statute mentions judicial decisions and the teachings of the most highly qualified
publicists as sources that can be used as a subsidiary means for the determination of rules of
international law.
● Decisions of the International Court of Justice: the decisions of the ICJ do not have binding
force except in the circumstances stipulated in Article 59 ICJ Statute. However, the Court
regularly refers to its earlier rulings, even as the Court does not formally work with a system of
precedent. In practice, the Court takes its previous judgments into account in order to
maintain judicial consistency. See for example the Nuclear Weapons Advisory Opinion, in
which the Court restated the criteria for self-defence, which it had previously determined in
the Nicaragua case.
● Decisions of other International Courts and Tribunals: the ICJ has also referred to decisions of
other international courts and tribunals; either in support of its own legal arguments or in
adopting an opposing view. See for example the Genocide Convention case, in which the
ICJ took a different view than the International Criminal Tribunal for the Former Yugoslavia
(ICTY) in the Tadic case on the degree of control needed to hold a state responsible for the
conduct of an armed group. International courts and tribunals similarly rely on judgments of
the ICJ in order to substantiate their arguments or take an opposite view on the matter.
● Decisions of domestic courts: can be used by the ICJ and other international courts and
tribunals to identify customary international law and/or general principles of law. Thus, parties
before the ICJ or other international courts and tribunals can refer to national case law in
order to substantiate their argument(s). In the Arrest Warrant case, Belgium relied, amongst
other cases, on the Pinochet case that was heard before the UK's House of Lords - as
supporting argument for not violating the immunity of Minister of Foreign Affairs of the
Democratic Republic of the Congo. Also see the reasoning of the ICJ in the Arrest Warrant
case on whether a customary rule of international law regarding immunities could be deduced
from the Pinochet case.
● Teachings of the most highly qualified publicists: can also play a subsidiary role in determining
or interpreting the applicable law. The scholarly works of prominent jurists are therefore not
primary sources of international law as such, but may contribute to development of treaty
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rules, customary law and general principles of law. The ICJ does not generally refer to
scholarly work in its rulings; however, other courts and tribunals, notably, the Appellate Body
of the WTO, do so at times.
Additional sources of IL
In addition to the sources mentioned in article 38(1) ICJ Statute, there are also other sources of
international law:
● Unilateral acts of states, e.g. statements made by relevant high-ranking state officials (such
as President, Minister for Foreign Affairs): can give rise to a legal obligation under certain
circumstances. Examples of such acts could include official protests to the conduct of other
states or the recognition of another state (recognition of statehood). The ICJ dealt with the
issue of state declarations by way of unilateral acts in the Nuclear Tests.
Unilateral acts of states may constitute a source of international legal obligation when two
conditions are met:
a. Firstly, the state should have the intention to be bound by its statement;
b. and secondly, the statement should be public.
● Decisions of international organisations: are also recognized as sources of international law.
Think for example of the binding resolutions of the UN Security Council adopted under
Chapter VII of the UN Charter. However, it remains unclear to what extent decisions of
international organisations form a separate category of the sources of international law. One
could also argue that the authority for international organisations to adopt binding decisions
derives from their constituent treaties, in which case the source of law would be the treaty.
• Soft law: refers to quasi-legal instruments which, formally speaking, are not legally binding, but
which might nevertheless be of great legal relevance and political importance. Soft law instruments
may also influence the development of legally binding (hard) international law, especially to the extent
that such instruments might be indicative of emerging international customary law – both of
demonstrating the existence of state practice and of opinio iuris.
→ Examples of prominent soft law instruments include the non-binding resolutions of international organisations, such as the
recommendations or resolutions adopted by the UN General Assembly. A well-known example is the Universal Declaration of
Human Rights (UDHR), adopted through a non-legally binding resolution by the GA, which have proven to be highly influential
on the development of international human rights law and on the conduct of states.
Module 3: The relationship between treaty law and customary international law
The relationship between treaties and customary international law was set-out by the International
Court of Justice (ICJ) in its 1969 decision in the North Sea Continental Shelf cases (joined cases
Germany/Denmark and Germany/Netherlands). The case involved Denmark and the Netherlands, on the one
hand, and Germany on the other hand. The dispute concerned the delimitation of the continental shelf
in the North Sea. The term continental shelf, for geographers and geologists, refers to a natural phenomenon; international
lawyers use the term to refer to a distinct area of the seabed and subsoil over which coastal states exercise jurisdiction.
Central question: Was Germany under a legal obligation to apply the equidistance-special
circumstances method? "[para 21] does the equidistance-special circumstances principle constitute a
mandatory rule, either on a conventional or on a customary international law basis, in such a way as to govern
any delimitation of the North Sea continental shelf areas between the Federal Republic and the Kingdoms of
Denmark and the Netherlands respectively? Another and shorter way of formulating the question would be to ask
whether, in any delimitation of these areas, the Federal Republic is under a legal obligation to accept the
application of the equidistance-special circumstances principle."
The issue at stake in this case concerned the delimitation of the continental shelves of the three states (Denmark,
Germany and the Netherlands). Germany and Denmark, and Germany and the Netherlands had concluded
treaties delimiting their continental shelves close to the coast (the lines A-B and D-C on the map below). On the
other hand, Denmark and the Netherlands had concluded a treaty delimiting their continental shelves in the
