Customary International Law: Treaties' Foundation?

Customary international law is a set of rules that come from a general practice accepted as law and exist independently of treaty law. Customary international law is a primary source of international law, alongside treaties and general principles of law. It is derived from the general practice of states and what states have accepted as law, also known as opinio juris. While treaties are binding only on states that have consented to be bound by them, customary international law can bind states that have not ratified the relevant treaty, as long as they observe its provisions in good faith. Customary international law is dynamic and continually evolving, with rules that can be codified through treaties and domestic laws or recognized solely as customary law. Treaties and customary international law are not interchangeable, and states can deviate from customary international law by enacting treaties and conflicting laws. However, customary international law is crucial as it fills gaps left by treaty law and strengthens the protection offered to victims in today's armed conflicts.

Customary international law and treaty law as sources of international law

Customary international law and treaty law are both recognised as sources of international law. Customary international law is derived from "a general practice accepted as law", whereas treaty law is based on written conventions that are formally established by states. Treaties, such as the Geneva Conventions, are binding only on states that have consented to be bound by them, typically through ratification. On the other hand, customary international law is based on unwritten rules and practices that are widely accepted as legal obligations. These practices can be found in various official documents, including military manuals, national legislation, and case law.

Customary international law plays a crucial role in filling gaps left by treaty law, particularly in the context of armed conflicts. For example, while treaty law may not expressly prohibit attacks on civilian objects in non-international armed conflicts, such a prohibition has been established under customary international law. Additionally, customary international law can be useful in coalition warfare, providing rules that govern legal matters not covered by other agreements during armed conflicts.

The recognition of customary international law as a source of international law is enshrined in the Statute of the International Court of Justice. It is defined in Article 38(1)(b) as "a general practice accepted as law", determined by two factors: the general practice of states and what states have accepted as law (opinio juris). This means that a significant number of states engage in the practice, and they do so out of a sense of legal obligation.

Customary international law can be identified through various forms of evidence, including treaties, decisions of national and international courts, national legislation, opinions of national legal advisors, diplomatic correspondence, and the practice of international organisations. It is worth noting that not all customary international laws carry the same weight. Some rise to the level of jus cogens, which are peremptory norms accepted by the international community as non-derogable rights, while others may only be followed by a small group of states.

Customary international law and treaties: the binding nature

Customary international law and treaties are both sources of international law. Treaties, such as the Geneva Conventions, are written conventions in which states formally establish certain rules. Treaties are binding only on those states that have expressed their consent to be bound by them, usually through ratification.

Customary international law, on the other hand, is derived from "a general practice accepted as law". This means that the practice is widespread and consistent among states, and that states engage in the practice out of a sense of legal obligation, or opinio juris. Customary international law is often unwritten, and research into state practice is required to determine its existence.

Customary international law and treaties are not mutually exclusive. In fact, customary international law can underpin treaties. For example, the laws of war, or jus in bello, were a matter of customary law before they were codified in the Hague Conventions and the Geneva Conventions. Additionally, if any treaty or law has been called customary international law, then parties that have not ratified the treaty will still be bound to observe its provisions in good faith.

Customary international law is also important because it fills in gaps left by treaty law. For example, while treaty law does not expressly prohibit attacks on civilian objects in non-international armed conflicts, such a prohibition has developed under customary international law. Customary international law can also be useful in cases of coalition warfare.

In summary, customary international law and treaties are both sources of international law, and customary international law can underpin treaties. Customary international law is important because it fills in gaps left by treaty law and provides a binding framework for states to regulate their relationships, even when they have not expressly consented to be bound by a treaty.

Customary international law and treaties: the relationship

Customary international law and treaties are both sources of international law. Treaties are formal, written conventions in which states establish certain rules and give their consent to be bound by them, usually through ratification. Customary international law, on the other hand, consists of rules that arise from "a general practice accepted as law", often referred to as opinio juris. These practices can be found in official accounts, such as military manuals, national legislation, and case law.

The relationship between customary international law and treaties is complex and intertwined. Customary international law can underpin treaties in the sense that it serves as a basis or foundation for the creation and interpretation of treaties. Treaties may codify existing customary international law, elevating it from unwritten practice to formal, written conventions. For example, the laws of war, or jus in bello, were customary before being codified in the Hague Conventions and the Geneva Conventions.

Customary international law and treaties can coexist and complement each other. Customary international law can fill gaps left by treaty law, strengthening the protection offered, as seen in the context of armed conflicts. For instance, while treaty law may not expressly prohibit attacks on civilian objects in non-international armed conflicts, such a prohibition exists under customary international law. Customary rules can define the obligations of parties in greater detail than treaty law, and they can apply to both international and non-international armed conflicts.

However, it is important to note that states can deviate from customary international law by enacting treaties and conflicting laws. Treaties may also shape the development of customary international law. If a treaty is recognised as customary international law, even states that have not ratified it will be bound to observe its provisions in good faith. This dynamic illustrates the reciprocal influence between customary international law and treaties, where treaties can contribute to the evolution of customary law.

Customary international law and treaties: the differences

Customary international law and treaties are both sources of international law. However, they differ in several key ways.

Firstly, customary international law is based on "general practice accepted as law", which is often referred to as opinio juris. This means that it arises from established or usual international practices that are widely followed by states. On the other hand, treaties are written conventions in which states formally establish certain rules. Treaties, such as the Geneva Conventions, are binding only on those states that have consented to be bound by them, typically through ratification. Customary international law, on the other hand, is generally binding on all states, regardless of whether they have codified it domestically or through treaties.

Secondly, customary international law is often unwritten, and research into state practice is required to determine its existence. It can be found in a variety of official documents, including military manuals, national legislation, and case law. Treaties, on the other hand, are formal written agreements between states.

Thirdly, customary international law can fill gaps left by treaty law and strengthen the protection offered to victims. For example, while treaty law may not expressly prohibit attacks on civilian objects in non-international armed conflicts, such a prohibition has developed under customary international law.

Finally, customary international law can be useful in the case of coalition warfare, as it applies to all armed conflicts, regardless of whether they are international or non-international. Treaties, on the other hand, may only apply to specific situations or relationships between states.

Customary international law and treaties: the applicability

Customary international law and treaties are both sources of international law. Treaties are written conventions in which states formally establish certain rules and are binding only on those states that have expressed their consent to be bound by them, usually through ratification. On the other hand, customary international law derives from "a general practice accepted as law", which can be found in official accounts of military operations and other official documents such as military manuals, national legislation, and case law.

Customary international law and treaties can interact in several ways. Firstly, customary international law can fill in gaps left by treaty law, strengthening the protection offered to victims in today's armed conflicts. For example, while treaty law does not expressly prohibit attacks on civilian objects in non-international armed conflicts, such a prohibition has developed under customary international law. Secondly, customary international law can be useful in the case of coalition warfare. Thirdly, some customary international laws may rise to the level of jus cogens, which are peremptory norms or compelling laws accepted by the international community of states as non-derogable. Examples of jus cogens include the prohibition of slavery, torture, genocide, war of aggression, and crimes against humanity. Fourthly, customary international law can be codified through treaties and domestic laws, as was the case with the laws of war, which were long a matter of customary law before being codified in the Hague Conventions and Geneva Conventions.

It is important to note that states can deviate from customary international law by enacting treaties and conflicting laws. However, if any treaty or law has been called customary international law, then parties that have not ratified said treaty will still be bound to observe its provisions in good faith. The recognition of customary international law and treaties as binding sources of international law is enshrined in the Statute of the International Court of Justice.

In summary, customary international law and treaties have a complex relationship. Customary international law can underpin treaties by filling in gaps and providing additional applicable rules, especially in the context of armed conflicts. Treaties can also codify customary international law, giving it a more formal status. Ultimately, both customary international law and treaties contribute to the body of international law that regulates the relationship between states.

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