Customary International Law

Introduction

Customary international law (CIL) refers to the set of binding legal obligations and rights that emerge from the practice of states and opinio juris, the belief that such practice is carried out because it is required by law. CIL is an important and authoritative source of international law that is considered universal, applying to all states, but also controversial due to its unclear origins and content.

Unlike treaties which are explicitly negotiated and codified, the norms of CIL are not written down in any single document. Instead, CIL arises from the general and consistent practice of states, along with the subjective belief that such conduct is legally obligated. This makes the content of CIL norms often difficult to clearly define. There is debate regarding what kind of state practice contributes to the formation of CIL rules, and the subjective determination of opinio juris adds further complexity.

Nonetheless, CIL holds an influential position within the international legal system. According to the Statute of the International Court of Justice, international custom is considered “evidence of a general practice accepted as law.” The norms and principles of CIL are binding on all states, whether or not they have codified or agreed to them in treaties. Thus CIL provides a universal framework for evaluating the legality of state conduct and relations between states. However, the process for determining these norms remains controversial.

Constituent Elements of CIL

Customary international law (CIL) is composed of two key elements: state practice and opinio juris.

State practice refers to general and consistent conduct by states that develops over time and becomes accepted as law. This conduct can include diplomatic acts, public measures, official government statements, and the enactment of domestic laws and policies. For a practice to contribute to CIL, it must be extensive, representative (reflect the practice of states most interested in the norm), and consistent over time. Not all state practice necessarily contributes to CIL; the practice must reflect a legal obligation rather than simply habit or political expediency.

Opinio juris refers to the subjective belief by states that certain conduct is carried out because it is a legal obligation. In other words, states must believe that they are conforming to what is already law when engaging in a general practice. This distinguishes CIL norms from mere customs or habits. When states consistently engage in a practice with a sense of legal obligation over time, that practice hardens into binding CIL.

Both consistent state practice and opinio juris are required for a CIL norm to emerge and become binding on states. Neither element alone is sufficient. CIL thus depends both on what states do (practice) and what states believe about those practices (opinio juris).

Theories on the Formation of CIL

There are two main competing theories on how customary international law is formed:

Moral Theory

The moral theory looks primarily at the content and substance of the purported norm of CIL. It asks - is this an objectively moral norm that states should follow? Advocates of this theory argue that for a principle to become CIL, it must be based on fundamental moral truths. The goal is to identify norms that are universal and that all ethical states have a duty to follow.

Political Process Theory

In contrast, the political process theory looks more at the process by which states come to accept a norm as CIL. It focuses less on the moral status of the norm itself. This theory argues that a norm becomes CIL through explicit or implicit agreement among states, based on political factors like power dynamics, national interests, and bargaining. CIL emerges through states pursuing political goals and agendas, not from an objective moral order.

Not All State Conduct Counts as CIL

Merely looking at what states do is not enough to determine the existence of a CIL norm. The key is that the conduct must be accompanied by opinio juris - the belief that the conduct is carried out because it is a legal obligation. In other words, not all state conduct amounts to CIL. Only the portion of state practice that is accepted by states as law, and undertaken with a sense of legal obligation, contributes to the formation of CIL.

For a CIL norm to emerge, state practice needs to be extensive as well as virtually uniform. The acts must occur out of a sense of obligation. There should be no significant objection to the norm by other states. Inconsistent conduct would call into question whether a practice is accepted as law. CIL can only crystallize from conduct that is consistent and has occurred over a considerable period of time across states. Essentially, CIL norms evolve from the widespread conduct and beliefs shared by the international community as a whole.

Material Sources of Customary International Law

Customary international law draws its content from material sources - that is, the evidence that shows what states actually do and believe about the legality of certain conduct. There are three main material sources that contribute to the formation of customary norms:

Treaties

Treaties, even when not universally ratified, can provide evidence of emerging customary norms. Widespread participation in a treaty, even without full ratification, can demonstrate state practice and legal beliefs. Provisions in treaties that are widely accepted often parallel customary rules. For example, key principles of the law of the sea were codified in the UN Convention on the Law of the Sea, translating many existing customs into treaty language.

Resolutions of International Organizations

Resolutions passed by international organizations, especially the United Nations General Assembly, provide insights into global perspectives on what customary practices exist or are emerging. While not legally binding themselves, UNGA resolutions can crystallize global consensus and advance new norms. The cumulative impact of resolutions over time can demonstrate the direction of customary international law.

Unilateral State Actions

The actions, statements, and policies that states adopt unilaterally can also provide evidence of international custom. State practices and expressions of legal beliefs within national legislation, official statements, military manuals, and other unilateral acts can all contribute to customary norms. Even the protests and objections states make against other states’ actions provide insights into their stances on customary law.

Formal Sources of CIL

Formal sources refer to what makes a norm legally binding as international law. The key formal source establishing customary international law is Article 38 of the Statute of the International Court of Justice. Article 38 lists “international custom” as one of the sources of law to be applied by the ICJ. This enshrines customary international law as a primary source of binding international law equivalent to treaties and general principles.

Another formal source of customary international law is its inclusion in treaties. When norms of customary international law are codified or referenced in treaties, this can reinforce their binding status. For example, key principles of customary international law have been incorporated into the UN Convention on the Law of the Sea and the Vienna Convention on the Law of Treaties. The inclusion of customary norms in widely ratified treaties helps confirm their acceptance by states as legally binding international law.

Examples of CIL Norms

Two notable examples of customary international law norms are the law of the sea and the legality of nuclear weapons.

Law of the Sea

The law of the sea is one of the oldest and most fundamental areas of international law. Over centuries, rules governing territorial waters, fishing rights, definitions of the continental shelf, rights of passage, and other maritime rights and duties of states emerged from customary state practice and opinio juris. For example, the 3 nautical mile limit on territorial waters adjacent to a coastal state emerged as a customary norm from the 18th-19th centuries, based on the range of cannon at the time.

More recently, the United Nations Convention on the Law of the Sea (UNCLOS) codified many existing customary rules on maritime boundaries and rights. However, even states that are not parties to UNCLOS, like the United States, recognize most of its provisions as representing customary international law. Areas like the 12 nautical mile limit on territorial waters are now accepted as custom by most states.

Legality of Nuclear Weapons

In the 1990s, the International Court of Justice considered whether the threat or use of nuclear weapons is permitted under international law. After reviewing treaties, UN resolutions, and state practice, the ICJ concluded there was no comprehensive prohibition on nuclear weapons under customary law. However, it found the threat or use of such weapons must comply with general international humanitarian law principles, like proportionality and necessity. This advisory opinion remains controversial but influential in delineating the current customary international rules on nuclear weapons.

The analysis shows how customary norms on issues like nuclear weapons and the law of the sea can emerge from state practice and opinio juris over time, even without comprehensive treaty codification. CIL thus allows international law to adapt and evolve based on changing state conduct and views.

Why Do States Comply with Customary International Law?

There are several factors that motivate states to comply with customary international law norms:

Regime Type

Some scholars argue that liberal democratic states have an inherent tendency to comply with international law, including CIL, due to shared liberal values and principles. The transparency and accountability of democratic governments creates domestic pressure to comply with established international rules and norms. However, critics point out that democracies do not always comply and authoritarian states sometimes do comply with CIL. Regime type alone does not definitively predict compliance.

Systemic Benefits

CIL allows states to manage expectations and facilitate cooperation through shared understandings. Even without formal treaties, CIL establishes baseline standards of conduct that enable states to predict how other states will likely behave. This mutual reassurance promotes order andstability. Compliance with CIL norms can thus produce systemic benefits for all states by reducing uncertainties.

Clarity of Norms

The clarity and specificity of a CIL norm affects the likelihood of compliance. Clear and well-defined CIL principles that reflect concrete state practice and a discernible opinio juris are more likely to garner compliance than abstract, contested norms with little detectable state practice or legal belief underpinning them. Compliance partly depends on CIL norms signaling clear expectations to states.

Criticisms of CIL

Critics have raised concerns about customary international law’s lack of clarity and slow process of formation:

• Lack of clarity - Since there is no definitive list of CIL norms, it can be unclear exactly what is included in CIL at any given time. The requirements of consistent state practice and opinio juris are open to interpretation. This ambiguity leads to disagreements over whether certain norms have actually crystallized into CIL. Some scholars argue this uncertainty weakens CIL and causes inconsistent applications.

• Slow process of formation - Developing a new CIL norm is a gradual process requiring extensive, uniform state practice over time. This means CIL may lag behind modern developments or fail to address new issues. Critics point out this lengthy process prevents CIL from adapting quickly to changing realities in the international system. For example, norms around cyber operations are still developing at a slow pace through state practice.

The decentralized, organic process of CIL formation has tradeoffs. While it depends on consensus and widespread acceptance, CIL can also lack clear definitions and be slow to form in response to new issues. Critics argue this makes CIL deficient compared to codified treaties in some areas of international law.

Conclusion

Customary international law (CIL) plays an important role in regulating state conduct and establishing norms of behavior that are widely accepted and followed by the international community. Though CIL has no definitive list of rules, it arises out of long-standing practices of states accompanied by opinio juris, the belief that the conduct is carried out of legal obligation.

The two main elements of CIL are state practice and opinio juris. For a norm to become CIL, it must reflect consistent conduct and legal beliefs among states over time. Not all state actions constitute CIL; the focus is on widespread conduct that states intend to fulfill a legal duty. CIL’s norms evolve as an expression of the collective will of states.

CIL’s sources are more material than formal - what states actually do, along with treaties, UN resolutions, and other texts that shape international norms. States comply with CIL for reasons including regime type, systemic benefits, moral duty, and self-interest. While CIL has drawbacks like lack of codification, it remains an authoritative source of international law and continues to provide a framework for evaluating state behavior and expectations.

Going forward, CIL will likely continue to adapt to new challenges and technologies. CIL provides means to regulate emerging domains like cybersecurity and outer space. Though CIL has weaknesses, its flexibility and grounding in state practice give it an important role in the international legal order. Clarifying norms and their basis in consistent state conduct and beliefs will strengthen CIL’s legitimacy and utility. Overall, CIL represents an evolving product of state behavior and shared understandings that shapes global order.