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 and Conventions are written agreements that states willingly sign and ratify and as such are obliged to follow. Such agreements, which are also called statutes or protocols, govern the mutual relations between states. They are, however, only binding on those states that have signed and also ratified the particular treaty.
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, disputes, 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 includes stark disagreement on the scope and content of the agreement. In order to increase the number of signatories and ratifications of a treaty, and hence global order, international law does allow for 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 specified reservations are permitted and they cannot undermine the object and purpose of the Treaty.
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 treaties, especially human rights treaties, provide for a derogations system, which allow for a state party to temporally suspend or limit their legal obligations in exceptional circumstances, for example during 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, 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, as it is a body of law specifically designed to provide minimum protection during armed conflict.
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 or unwritten rules that form part of the general international concept of justice.
Unlike treaty law, which is only applicable to those states that are parties to the particular agreement, customary law is binding upon all states, regardless of whether they have ratified a treaty.
Unlike treaty law, customary international law is limited in that it is not codified in a clear and accessible format and the content of the rules is generally less specific that what you may find in a treaty. However, as a source of IHL, customary international law is of fundamental importance in armed conflict due to the limited protections afforded to internal conflicts by treaty law and the lack of ratification of key treaties. Customary international law exists independently from treaty law and in 2006 the Independent Commission of the Red Cross (ICRC) published a collection of the rules of IHL considered to be customary in nature. They identified 161 Rules of customary international law.
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 for identifying a rule as part of customary international law: state practice (usus) and legal nature of that practice (opinion-uris)
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 (Opino Juris) is the expressed opinion of states, individually or collectively, that their actions have a legal and not a mere policy 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.