International Law: The Written and Unwritten Rules That Govern Nations

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When Russia invaded Ukraine in 2022, the world didn’t just witness a military conflict—it saw a massive violation of international law. Countries condemned Russia for breaking rules about territorial sovereignty, the use of force, and diplomatic immunity.

But where do these rules come from? Some exist in written treaties, while others developed over centuries through unwritten customs that nations follow.

International law governs how countries interact on everything from trade and human rights to environmental protection and warfare. For Americans, this matters because these global rules shape U.S. foreign policy, international agreements, and even domestic law in ways that affect daily life.

Two main sources create international law’s foundation. Treaty law consists of formal written agreements between nations, like trade deals or arms control pacts. Customary international law emerges from consistent state practices that countries follow because they believe they’re legally required to do so—the unwritten “rules of the road” for international behavior.

Both types appear in Article 38 of the International Court of Justice Statute as fundamental sources of the global legal order. Understanding how they work helps explain why nations sometimes cooperate and sometimes clash, and how America fits into the complex web of global governance.

Customary International Law: The Unwritten Rules

Customary international law refers to international obligations arising from established, consistent practices that states follow out of legal obligation rather than through formal written agreements. Think of it as the unwritten “rules of the road” that nations observe in their dealings.

The International Court of Justice identifies customary international law in Article 38(1)(b) of its Statute as “international custom, as evidence of a general practice accepted as law.” This isn’t just repeated behavior—it’s behavior states engage in because they believe they’re legally bound to do so.

How Customary Law Forms

For a rule to become customary international law, two essential elements must exist: state practice and opinio juris.

State Practice (What States Do)

State practice refers to consistent and widespread conduct by nations. This encompasses a broad range of official governmental activity, not just physical actions.

Evidence of state practice includes official statements during international conferences, diplomatic communications, formal directives to diplomatic agents, national legislation, court decisions touching international matters, government white papers, legal opinions, military manuals, and documents from intergovernmental organizations.

Both what states actually “do” (like how their navies behave on the high seas) and what they “say” (like official protests against another state’s actions or statements in the UN General Assembly) constitute state practice. The conduct must be attributable to the state, meaning it’s an act of government or officials, and it generally needs to be sufficiently widespread, representative, and consistent across nations.

Opinio Juris (Why States Do It)

The second crucial element is opinio juris sive necessitatis—”an opinion of law or necessity.” This refers to a state’s subjective belief that it follows a particular practice because it’s legally obligated to do so, not merely out of courtesy, convenience, habit, or political expediency.

For example, states might roll out red carpets for visiting heads of state as courtesy, but this isn’t done from believing international law requires it. In contrast, granting immunity from prosecution to visiting heads of state is generally done because states believe international law mandates such immunity.

Opinio juris can sometimes be explicitly stated but often must be inferred from the context of state actions and how states react to each other’s conduct.

Who Is Bound by Customary Law

Generally, once customary international law establishes a rule, it binds all states. This differs fundamentally from treaty law, which only binds states that have agreed to it.

The Persistent Objector Exception

There’s one recognized exception: if a state has clearly and consistently objected to a customary law rule during its formation, that state may not be bound by it. However, this is difficult to achieve. The objection must be made before the rule crystallizes into law and maintained persistently. This exception doesn’t apply to jus cogens norms (discussed later).

New States

The prevailing view in international law holds that newly independent states are generally bound by existing customary international law rules at the time they come into existence. This binding effect is automatic, regardless of whether the new state consented to the rule or participated in its creation—much like new citizens are automatically subject to existing national laws.

Customary Law in U.S. Courts

Yes, customary international law is recognized as part of United States law. The U.S. Supreme Court famously affirmed this principle in The Paquete Habana (1900). In that case, the Court held that under customary international law, small coastal fishing vessels peacefully pursuing their trade were exempt from capture as prizes of war.

The Court stated that international law “is part of our law, and must be ascertained and administered by the courts of justice of appropriate jurisdiction as often as questions of right depending upon it are duly presented for their determination.”

U.S. courts can therefore apply customary international law. In doing so, they may look to various sources, including decisions of international tribunals, legal scholars’ writings, and sometimes U.S. Department of State views, particularly when evidence of customary law is complex or not easily accessible.

The Alien Tort Statute

A significant U.S. statute interacting with customary international law is the Alien Tort Statute (ATS), codified at 28 U.S.C. § 1350. Enacted by the First Congress in 1789, the ATS grants U.S. federal district courts jurisdiction over civil actions brought by aliens for torts “committed in violation of the law of nations or a treaty of the United States.”

For nearly two centuries, the ATS lay dormant. However, the 1980 case of Filártiga v. Peña-Irala revived it. The U.S. Court of Appeals for the Second Circuit held that deliberate torture under official authority violated universally accepted international law norms, allowing a suit under the ATS.

Subsequent Supreme Court cases have clarified and narrowed the ATS scope. In Sosa v. Alvarez-Machain (2004), the Supreme Court ruled that the ATS primarily serves as a jurisdictional statute but allows federal courts to recognize a “narrow set” of causes of action for customary international law violations that are as “specific, universal, and obligatory” as those understood in the 18th century—like piracy, violations of safe conducts, and infringements of ambassadors’ rights.

Later cases including Kiobel v. Royal Dutch Petroleum (2013), Jesner v. Arab Bank, PLC (2018), and Nestlé USA, Inc. v. Doe (2021) have further constrained the ATS, particularly regarding extraterritorial application and foreign corporation liability.

Examples of Customary International Law

Several well-established customary international law principles include:

Non-refoulement: This principle prohibits states from returning refugees or asylum seekers to countries where they have well-founded fears of persecution.

Diplomatic and Head of State Immunity: Foreign leaders and accredited diplomats are generally immune from host country court jurisdiction for official acts and, to a large extent, personal acts, ensuring they can perform duties without interference. This is extensively codified in the Vienna Convention on Diplomatic Relations, 1961.

Prohibition of Genocide: The intentional destruction, in whole or in part, of national, ethnic, racial, or religious groups is prohibited under customary law and considered a jus cogens norm. This is also codified in the Genocide Convention.

Fundamental Prohibitions: Norms against slavery, torture, piracy, wars of aggression, and crimes against humanity are widely recognized as customary international law, with many also having jus cogens status.

Treaty Law: The Written Agreements

Treaty law, contrasting with customary law’s unwritten nature, consists of formal written agreements that states willingly enter into. These agreements create specific legal obligations for countries that are parties to them.

What Is a Treaty?

According to Article 2(1)(a) of the Vienna Convention on the Law of Treaties, a treaty is “an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation.”

This means treaties can go by many names—convention, charter, protocol, pact, accord, covenant, or agreement—but if they meet this definition, they’re considered treaties under international law. The Vienna Convention itself is widely regarded as the authoritative source on treaty law, codifying many pre-existing customary international law rules about how treaties are made, interpreted, and function.

How Treaties Are Made

The creation of a treaty involves several distinct steps, particularly for the United States:

Negotiation

The process begins with negotiation. Authorized representatives of involved states (often diplomats or subject-matter experts) meet to discuss and agree upon proposed treaty terms. In the U.S. system, the Executive Branch, under the President, is responsible for negotiating treaties.

Signature

Once a text is agreed upon, representatives of negotiating states typically sign it. Signature can have different legal effects depending on treaty terms. It may simply authenticate the agreed-upon text, or it might signify a state’s consent to be bound if the treaty specifies that signature alone is sufficient for entry into force.

More commonly, especially for major multilateral treaties, signature indicates preliminary endorsement and intention to proceed toward ratification, creating an obligation to refrain from acts that would defeat the treaty’s object and purpose pending ratification.

Ratification

Ratification is the formal act by which a state confirms its consent to be bound by a treaty. The specific ratification process varies according to each state’s domestic constitutional requirements.

The U.S. Constitutional Process

The U.S. Constitution, Article II, Section 2 states that the President “shall have Power, by and with the Advice and Consent of the Senate, to make Treaties, provided two-thirds of the Senators present concur.” This establishes shared responsibility:

1. The President, through the Department of State or other agencies, negotiates and signs the treaty
2. The President submits the treaty to the U.S. Senate for “advice and consent”
3. The treaty is typically referred to the Senate Foreign Relations Committee, which may hold hearings, debate provisions, and recommend whether the full Senate should approve it
4. The full Senate considers the treaty. For consent, a “resolution of ratification” must be approved by a two-thirds majority of Senators present and voting
5. If the Senate grants consent, the treaty returns to the President, who formally ratifies it on behalf of the United States
6. The final step to make the treaty internationally binding is exchanging ratification instruments with the other party (bilateral treaty) or depositing them with a designated depositary (multilateral treaties, often the UN Secretary-General)

The two-thirds requirement for Senate approval is a significant hurdle, reflecting the framers’ intent to ensure broad support for the nation’s most solemn international commitments. This means treaty-making isn’t solely an executive function but requires substantial legislative buy-in.

However, it also means that treaties negotiated and signed by the President may fail to gain Senate approval, as happened with the Vienna Convention on the Law of Treaties itself (which the U.S. signed but never ratified) or the League of Nations Covenant after World War I.

Entry into Force

A treaty enters into force and becomes legally binding on states that have consented to be bound in such manner and upon such date as the treaty provides or as negotiating states agree. For multilateral treaties, this often occurs after a specified number of states have ratified or acceded to it.

Who Is Bound by Treaty Law

A fundamental principle of treaty law is that treaties bind only states that have formally consented to be bound (typically through signature, ratification, or accession). This is often summarized by the Latin maxim pacta sunt servanda (“agreements must be kept”), which is a cornerstone of international law and is affirmed in Article 26 of the Vienna Convention on the Law of Treaties.

Conversely, treaties generally don’t create obligations or rights for third states (states not party to the treaty) without their consent. Once a state is bound by a treaty, it’s obligated to perform its provisions in good faith. Generally, a state cannot invoke provisions of its own domestic law as justification for failing to perform a treaty.

Reservations, Understandings, and Declarations

When becoming a party to a multilateral treaty, a state may sometimes seek to make reservations, understandings, and declarations (RUDs).

A reservation is a unilateral statement made by a state when signing, ratifying, accepting, approving, or acceding to a treaty, whereby it purports to exclude or modify the legal effect of certain treaty provisions in their application to that state. Reservations are generally permitted unless the treaty prohibits them, allows only specified reservations, or the reservation is incompatible with the treaty’s object and purpose.

An understanding or declaration is a statement by which a state clarifies its interpretation of a particular provision or the treaty as a whole. Unlike a reservation, it doesn’t aim to modify the treaty’s legal obligations.

RUDs represent a balancing act. They can encourage wider treaty participation by allowing states to join even if they have concerns about specific parts. However, if reservations are too numerous or significant, they can undermine treaty integrity and lead to fragmented obligations.

For the U.S., the Senate may insist on certain RUDs as a condition of its advice and consent to a treaty, allowing the U.S. to tailor its international commitments to its domestic legal framework or policy objectives. This can sometimes lead to complex interactions if other treaty parties object to U.S. reservations.

Key Steps in the U.S. Treaty-Making Process (Article II Treaties)

Treaties in U.S. Law

Treaties play a significant role in the U.S. legal framework.

“Supreme Law of the Land”

The Supremacy Clause of the U.S. Constitution (Article VI) states: “This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land.”

This means U.S. treaties, once ratified by the President with Senate advice and consent, have federal law status. They’re superior to state laws—if a state law conflicts with a U.S. treaty, the treaty prevails. In relation to federal statutes, the general rule is “the later in time controls.” However, all treaties (like federal statutes) are subordinate to the U.S. Constitution.

A landmark Supreme Court case illustrating treaty power is Missouri v. Holland (1920). Missouri challenged the Migratory Bird Treaty Act of 1918, which implemented a treaty between the U.S. and Great Britain (acting for Canada) to protect migratory birds. Missouri argued that regulating migratory birds was a power reserved to states under the Tenth Amendment.

The Supreme Court upheld the Act, reasoning that treaty power is an independent grant of federal authority. Justice Oliver Wendell Holmes Jr. famously stated that while acts of Congress are supreme law only when made “in pursuance of the Constitution,” treaties are supreme when made “under the authority of the United States.”

This decision underscored that the federal government can use its treaty power to address matters of national and international concern, even if those matters might otherwise fall within areas traditionally regulated by states.

Treaties vs. Executive Agreements

While “Article II treaties” (requiring Senate advice and consent) are well-known forms of international agreement for the U.S., they’re not the only type.

Treaties (Article II Treaties) are made by the President with advice and consent of a two-thirds Senate majority. Executive Agreements are international agreements entered into by the President or Executive Branch that don’t receive Senate advice and consent as treaties. Despite this difference in domestic procedure, executive agreements are still considered binding on the United States under international law.

Presidents have increasingly used executive agreements, especially in recent decades. There are several types:

• Congressional-Executive Agreements: Authorized or approved by simple majority vote in both House and Senate, either before or after negotiation. Many trade agreements are handled this way.
• Agreements Pursuant to a Treaty: Made by the President under authority of an existing, Senate-approved Article II treaty.
• Sole Executive Agreements: Made by the President based solely on independent constitutional powers, such as Commander-in-Chief power or general executive power over foreign affairs.

The frequent use of executive agreements reflects modern diplomacy’s practical demands, which often require quicker and more flexible arrangements than the formal treaty process allows. However, this practice generates ongoing debate about power balance between President and Congress in conducting foreign relations.

Self-Executing vs. Non-Self-Executing Treaties

Even if the U.S. ratifies a treaty and it becomes “supreme Law of the Land,” it doesn’t automatically mean private individuals can go to U.S. courts and directly enforce its provisions.

Self-Executing Treaties have provisions considered directly applicable as domestic law without need for further congressional legislation. Their terms are usually specific and clear enough for courts to apply directly.

Non-Self-Executing Treaties require Congress to pass implementing legislation before their provisions can be enforced as domestic law by U.S. courts. Such treaties may create binding obligations for the U.S. government internationally, but individuals generally cannot rely on them in court until Congress acts to translate those international obligations into domestic law.

The Supreme Court addressed this distinction in Medellin v. Texas (2008). The Court held that an International Court of Justice judgment was not directly enforceable in U.S. state courts as domestic law. The Supreme Court stated: “while treaties ‘may comprise international commitments… they are not domestic law unless Congress has either enacted implementing statutes or the treaty itself conveys an intention that it be ‘self-executing’ and is ratified on these terms’.”

Example: The U.S.-Mexico-Canada Agreement

A prominent example of a modern international agreement with significant U.S. impact is the United States-Mexico-Canada Agreement (USMCA). This agreement, which replaced NAFTA, governs vast economic relations between the three North American countries, covering agriculture, manufacturing, digital trade, labor, and environmental standards.

The USMCA was approved by U.S. Congress through implementing legislation (Public Law 116-113) rather than as an Article II treaty. This illustrates how many significant international economic agreements are brought into domestic legal effect through the congressional-executive agreement route.

The Vienna Convention on the Law of Treaties

The Vienna Convention on the Law of Treaties, adopted in 1969 and entering into force in 1980, is the foundational international agreement codifying rules for how treaties between states are made, interpreted, amended, and terminated. It’s often called “the treaty on treaties.”

The United States signed the VCLT in 1970 but has never formally ratified it, meaning it’s not a party to the Convention. Despite this, the VCLT holds considerable influence in U.S. practice. The U.S. government, including the Department of State and U.S. courts, generally regards many VCLT provisions as reflecting customary international law.

This is particularly true for interpretation rules found in Articles 31 and 32. This acceptance of VCLT principles as customary international law means its rules often guide how the U.S. approaches treaty obligations and interprets treaty texts, even without formal VCLT membership.

Customary International Law vs. Treaty Law: A Quick Comparison

How Customary Law and Treaties Interact

Customary international law and treaty law aren’t isolated concepts—they frequently interact and influence each other.

When Unwritten Rules Become Written

A common interaction occurs when rules developed over time through state practice and opinio juris are later formalized into written treaties. This process is called codification. The aim is often to clarify the customary law rule, make it more precise, and provide a definitive written source.

Several important treaties are largely codifications of pre-existing customary international law:

The Vienna Convention on Diplomatic Relations (1961) put into written form many long-standing customary rules regarding diplomatic immunity, embassy inviolability, and diplomatic mission conduct.

The Vienna Convention on the Law of Treaties itself is considered largely a codification of customary international law concerning treaty making, interpretation, and termination.

Many laws of war principles, such as those in Hague Conventions and Geneva Conventions, originated as customary law governing armed conflict conduct.

When customary international law is codified into a treaty, the customary rule doesn’t cease to exist. It continues binding all states as customary law, independently of the treaty. The treaty binds only states that become parties to it.

This dual existence is important because a state cannot avoid a customary international law obligation simply by choosing not to join a treaty that codifies that same obligation. For instance, even if a state isn’t party to the Vienna Convention on Diplomatic Relations, it’s still bound by fundamental customary law rules regarding diplomatic immunity.

When Written Rules Shape Behavior

Treaties can also play a role in forming new customary international law. Provisions in treaties, especially those in widely ratified multilateral conventions, can crystallize emerging customary law rules or generate new state practice and opinio juris that eventually solidifies into customary law binding on all states, including those not party to the original treaty.

The International Court of Justice addressed this dynamic in the North Sea Continental Shelf Cases (1969). The Court considered whether the “equidistance principle” for delimiting continental shelf, contained in the 1958 Geneva Convention on the Continental Shelf, had become customary international law binding on Germany (which had signed but not ratified the Convention).

The ICJ outlined several conditions for a treaty provision to generate customary international law:

• The provision should be of a “fundamentally norm-creating character”
• There should be widespread and representative participation in the convention, including by states whose interests were specially affected
• Even without such participation, there must be extensive and virtually uniform state practice, undertaken in a way demonstrating general recognition that a rule of law or legal obligation is involved

In the North Sea cases, the Court found that the equidistance principle had not, at that time, met these criteria to become customary international law. This illustrates that treaties aren’t merely static legal documents—they can be catalysts in unwritten international law evolution.

Interpreting Treaties

The rules for interpreting treaties are themselves part of customary international law and have been codified in Articles 31-33 of the VCLT. These rules provide a common framework for all states to understand their treaty obligations.

VCLT Article 31 states that a “treaty shall be interpreted in good faith in accordance with the ordinary meaning to be given to the terms of the treaty in their context and in the light of its object and purpose.”

Even though the United States isn’t party to the VCLT, U.S. courts and the Executive Branch frequently refer to VCLT Articles 31 and 32 as reflecting customary international law rules of treaty interpretation. This shared understanding of how to “read” treaties is vital for international relations stability and predictability.

When Customary Law and Treaties Conflict

Occasionally, a customary international law rule and treaty provision may appear to conflict. International law provides principles to resolve such situations:

Lex Specialis (Specific Law Prevails): If a treaty provision addresses a specific situation in more detail than a general customary law rule, the treaty provision will typically govern relations between treaty parties concerning that specific situation.

Lex Posterior (Later Law Prevails): If a customary law rule and treaty rule on the same subject matter are of equal standing, the one that came into existence later generally prevails.

Treaties Can Modify Customary Law: States are generally free to agree by treaty to rules that differ from existing customary international law, as between themselves. The treaty will govern their mutual relations on that subject, but the underlying customary law rule continues applying to states not party to that specific treaty.

Jus Cogens: The “Super Norms” of International Law

A critical concept in understanding international law hierarchy is jus cogens, which refers to peremptory norms of general international law. These are fundamental principles of customary international law that are “accepted and recognized by the international community of States as a whole as a norm from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character.”

Examples of Jus Cogens Norms

Widely accepted examples include the prohibition of aggression, prohibition of genocide, prohibition of crimes against humanity, basic rules of international humanitarian law, prohibition of racial discrimination and apartheid, prohibition of slavery, and prohibition of torture. The right of self-determination is also frequently cited as having jus cogens status.

Overriding Power

Jus cogens norms sit at the apex of the international legal hierarchy:

• A treaty is void if, at the time of its conclusion, it conflicts with a jus cogens norm
• If a new jus cogens norm emerges, any existing treaty conflicting with that norm becomes void and terminates
• Similarly, a customary international law rule doesn’t come into existence if it would conflict with a jus cogens norm
• The persistent objector rule doesn’t apply to jus cogens norms—all states are bound by them

The concept of jus cogens introduces explicit hierarchy and moral foundation into international law, signifying that there are certain fundamental values from which the international community will not permit deviation, regardless of individual state consent.

Why International Law Matters to Americans

International law isn’t an abstract concept relevant only to diplomats and international lawyers. It has tangible impacts on the United States and daily lives of its citizens.

How Global Rules Touch Everyday Life

Trade and Economy: International trade agreements like the USMCA govern billions of dollars in commerce, affecting American jobs, consumer goods prices, and U.S. industry competitiveness.

Human Rights: While the U.S. has a complex history with international human rights treaties, customary international law norms relating to fundamental human rights can influence U.S. law and policy. The U.S. is party to the Convention Against Torture and the International Covenant on Civil and Political Rights, which protects individual freedoms.

Environment: International environmental agreements and customary law principles address transboundary issues like climate change, air and water pollution, and endangered species conservation, all with direct and indirect consequences for the U.S. environment and economy.

Travel and Communication: A vast web of treaties and customary law rules governs international aviation (safety standards, air routes), shipping (navigation rights, maritime safety), postal services, and telecommunications (satellite orbits, radio frequencies). These rules make international travel, commerce, and communication possible, predictable, and relatively safe.

National Security and Diplomacy: International law concerning use of force (found in the UN Charter and customary law), arms control, and diplomatic and consular relations are central to U.S. national security strategy and foreign policy conduct.

Understanding America’s Role in the World

A basic understanding of customary international law and treaty law provides citizens with essential tools to comprehend how the United States engages with the rest of the world. It helps in evaluating the nation’s international commitments, its adherence to global norms, and the legal frameworks that underpin its foreign relations.

This knowledge empowers citizens to participate more meaningfully in discussions about U.S. foreign policy and its global responsibilities.

Where to Learn More

For those interested in deeper exploration:

• U.S. Department of State (Office of Treaty Affairs) offers information on treaties to which the U.S. is party, including texts and status updates
• Congress.gov provides legislative information for treaties submitted to the U.S. Senate, including committee reports and Senate actions
• United Nations Treaty Collection is a comprehensive database of multilateral treaties deposited with the UN Secretary-General

International law may seem distant from daily American life, but it forms the invisible foundation for much of what makes modern global society function. From the morning coffee imported under trade agreements to the diplomatic immunity protecting American diplomats abroad, these written and unwritten rules shape the world we navigate every day.

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