Sources of International Law

The main sources of international law are treaty law, international customary law, and general principles of law recognised by civilised nations.

Treaties - thus, written law - is an important source of international law.

Treaty law

Treaties and conventions are written agreements that states willingly sign and ratify, and therefore are obliged to follow. Such agreements, which are also called statutes or protocols, govern the mutual relations between states. A particular treaty, however, is only binding on those states that have signed and ratified it.

The Vienna Convention of the Law of Treaties of 1969 sets out the fundamental legal rules relating to treaties. The Vienna Convention defines a treaty, identifies who has the capacity to conclude a treaty, and outlines treaty interpretation, dispute settlement mechanisms, and reservations.

The basis of treaty law is ‘pacta sunt servanda’, which means that agreements must be honoured and adhered to.

Reservations, declarations and derogations

Many states are involved in the process of drafting a treaty, which often means that there is strong disagreement about the scope and content of an agreement. In order to increase the number of signatories and ratifications of a treaty, international law allows states to limit the full application of a treaty, or clarify their specific understanding of the legal content. This is done through reservations, declarations and derogations.

Reservations are defined by the Vienna Convention as:
A unilateral statement, however phrased or named, made by a State, when signing, ratifying, accepting, approving or acceding to a treaty, whereby it purports to exclude or to modify the legal effect of certain provisions of the treaty in their application to that State. (Article 2 (1)(d))

Only reservations that are clearly specified and that do not undermine the object and purpose of the treaty, are permitted. For more information on treaty reservations see the website of the International Law Commission.

Declarations, unlike reservations, do not affect legal obligations, but are often made when a state expresses its consent to be bound by a specific treaty. The state uses the declaration to explain or clarify its understanding of particular aspects of the treaty text. For examples see the reservations and declarations made to the Fourth Geneva Convention.

Some legal instruments, especially human rights treaties, provide for a derogations system, which allows a state to temporally suspend or limit its legal obligations in exceptional circumstances, for instance during an armed conflict or national emergency. For example, the freedom of assembly may be limited during times of armed conflict. However, some rights can never be derogated from under any circumstances, notably the prohibition on torture, and inhumane and degrading treatment. For more on derogations and human rights law see the website of the Rule of Law in Armed Conflict Project.

It is important to note that international humanitarian law (IHL) does not have a system of derogations, since this body of law is specifically designed to provide a minimum level of protection during armed conflict which must be upheld under any circumstances. 

Customary international law

Customary international law is made up of rules that derive from a general practice accepted as law. Customary international law is comprised of all the written and unwritten rules that form part of the general international concept of justice.

Unlike treaty law, which is only applicable to those states that have signed and ratified a particular agreement, customary law is binding upon all states, regardless of whether they are party to a treaty.

Unlike treaty law, customary international law is limited in that it is not codified in a clear and accessible format, and the rules are generally less specific than those written down in a treaty.

As a source of IHL, customary international law is of fundamental importance since treaty law offers only limited protection and key treaties are not universally ratified. In 2006, the International Committee of the Red Cross (ICRC) published a collection of the IHL rules that are considered to be customary international law. They ICRC identified 161 such rules.

How does a rule become customary international law?

When states respect certain rules consistently in their international and internal relations, with legal intentions, these practices become accepted by the international community as applicable rules of customary international law. 

There are two criteria that determine if a rule is part of customary international law: state practice (usus) and the legal nature of that practice (opinio juris)

State practice (usus): Customary law is confirmed through the behaviour of states (objective criteria), manifested through their official statements and actions.

Legal nature of practice (opinio juris) is the expressed opinion of states, individually or collectively, that their actions have a legal and not a mere political basis.

In short, customary international law is based on consistent actions by the majority of the international community. Examples of customary international law are the prohibition on the arbitrary deprivation of life, the prohibition on torture, and the rule that civilians and civilian objects cannot be the subject of direct attacks during armed conflict.