What is International Customary Law? A Guide for MUN

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You’re in committee. A delegate argues that a state’s conduct is illegal, but when you scan your notes, you can’t find a treaty article that says so. For a moment, it sounds like bluffing.

It often isn’t.

International law isn’t made only of signed documents. Some of its most important rules come from what states consistently do, and from the fact that they do it because they believe law requires it. That body of law is called customary international law, and if you’ve ever wondered why strong delegates can make legal arguments even without quoting a treaty, this is usually the answer.

For MUN students, this matters a lot. In many committees, especially crisis, disarmament, humanitarian, and cyber topics, the most persuasive delegates don’t just say an action is “wrong.” They show that the international community has treated it as legally constrained, even when treaty law is incomplete, contested, or silent. If you understand that move, your speeches get sharper, your clauses get stronger, and your position papers stop sounding generic.

Introduction The Unwritten Rules of a MUN Committee

A common MUN problem looks like this. The committee is debating cyber operations, migration, or intervention. One delegate cites a treaty. Another says the issue goes beyond treaty law because states already follow an unwritten legal rule. Half the room nods. The other half looks confused.

That confusion is normal. Most students learn treaties first because treaties are visible. They have names, dates, articles, signatures. Customary international law is harder because you have to infer it from behavior and legal conviction rather than point to one neat text.

Still, if you want to understand how committees work, especially the tougher ones, you need this concept. It shows up when no treaty directly answers the question, when not every state has joined a treaty, or when a delegate wants to argue that a rule applies more broadly than any one convention. That’s part of why learning the broader types of UN committees helps. Different committees rely on legal reasoning in different ways, but customary law can matter in almost all of them.

The Foundations of International Customary Law

Customary international law is easiest to grasp if you start with a simple analogy. In any community, some rules are written down and some are understood through repeated practice. People line up in queues, not because every building posts a legal notice, but because repeated conduct creates an expected rule. International law works similarly, except the stakes are much higher and the actors are states.

States interact constantly. They exchange diplomats, patrol seas, issue protest notes, adopt domestic laws, publish military manuals, and vote in international organizations. Over time, some patterns of behavior begin to harden into legal rules. But repeated action alone isn’t enough. The key point is that states must treat the pattern as something law requires, not just something convenient or polite.

The moment custom became formal law

The modern legal foundation is clear. Article 38(1)(b) of the Statute of the International Court of Justice, adopted in 1945, recognizes international custom as “evidence of a general practice accepted as law” according to Cornell Law School’s explanation of customary international law. That formal recognition matters because it placed customary law among the primary sources of international law.

It also did something more important for students. It gave us a stable test. Since that statute, lawyers and judges have generally identified custom through two elements: a general practice, and acceptance of that practice as law. If you hear a delegate say “this is customary law,” they should be able to prove both.

That formalization is one reason international law is often described as a rules-based system and multilateralism project rather than a pure contest of power. States don’t just negotiate written bargains. They also create common legal expectations through conduct.

Why customary law matters even when no treaty exists

Customary law fills gaps. If there’s no treaty on point, or if a state isn’t party to a treaty, customary law may still provide the legal baseline. That’s why the concept is central in fields like humanitarian law, diplomatic immunity, and refugee protection.

Examples commonly associated with customary status include non-refoulement and immunity for visiting heads of state, as summarized in the Cornell overview above. These aren’t just political preferences. They’re treated as rules with legal force.

For MUN students, that should change how you research. If you only hunt for treaty articles, you’ll miss a huge part of the legal realm. The same habit shows up in constitutional topics too, where written texts don’t explain everything on their own. That’s one reason students who’ve wrestled with concepts like parliamentary sovereignty often adapt well to customary law. In both areas, legal authority isn’t always captured by one document.

A short explainer can help before moving on:

The Two Pillars State Practice and Opinio Juris

If you remember only one framework, remember this one. Customary international law rests on two pillars: state practice and opinio juris. One is about what states do. The other is about why they say they do it.

According to the verified definition, identifying customary law requires “state practice that appears to be sufficiently widespread, representative as well as consistent,” and opinio juris, meaning states act “out of a sense of legal obligation”, with evidence drawn from diplomatic practice, domestic legislation, decisions of intergovernmental organizations, and court decisions, as summarized in the customary international law reference.

Pillar one means watching what states actually do

State practice is the observable side. It includes conduct you can point to.

That can mean:

Diplomatic behavior such as official protests, formal notes, or repeated legal positions in international forums

Domestic law including statutes, regulations, and executive policies

Military materials such as manuals, rules of engagement, or operational guidance

International participation through voting patterns, statements in the UN, and positions taken in multilateral negotiations

Judicial decisions from national and international courts

Students often make the first mistake here. They think one famous speech by one foreign minister proves custom. It doesn’t. Custom requires breadth and consistency. A single state’s behavior might be interesting evidence, but it won’t usually establish a global rule.

A better way to think about state practice is accumulation. You build a file. A military manual here. National legislation there. Repeated diplomatic statements from several regions. A tribunal judgment that treats the rule as already existing. The stronger and more representative that collection becomes, the stronger your argument.

Pillar two asks whether states think law requires the behavior

At this point, many delegates get lost. They understand action. They struggle with legal belief.

Opinio juris doesn’t mean a state privately likes a practice. It means the state presents its conduct as legally required, or objects to another state’s conduct because it sees a legal breach. If states act a certain way only because it’s efficient, strategic, or polite, you may have a habit, not a legal norm.

Take a simple analogy. If everyone in your classroom stands when a teacher enters because they think it’s respectful, that’s social convention. If everyone stands because school rules require it, that’s closer to law. In international affairs, the line is harder to draw, but the logic is similar.

Evidence of opinio juris can come from:

Official legal statements where governments describe conduct as required or prohibited by law

Pleadings before courts where states argue that an international rule exists

Resolutions and declarations when states support language in a way that signals legal commitment rather than mere aspiration

Protests against violations because complaint itself can show belief in a legal rule

Why both pillars must appear together

Think of it this way. State practice is the footprint. Opinio juris is the explanation. You need both to prove the path is legal custom rather than coincidence.

This is especially important in fast-changing topics like cyberspace. States may behave in certain ways, but much of that conduct is classified or ambiguous. Public statements of legal obligation may also be sparse. That makes emerging custom harder to prove. In committee, that means you should be careful. You can argue that a norm is developing without claiming it is already settled.

A useful distinction for delegates:

Question	If the answer is yes	If the answer is no
Do many states behave in a similar way?	You may have evidence of state practice	You probably don’t have custom yet
Do they frame that behavior as legally required?	You may have evidence of opinio juris	You may only have policy preference
Are both present across representative states?	Your CIL argument becomes much stronger	Your claim stays weak or premature

What counts as “sufficiently widespread” in real argument

Students want a magic formula here. There isn’t one neat threshold that works for every issue.

Still, the verified standard helps. Practice must be widespread, representative, and consistent. That means you shouldn’t cherry-pick a few allies and call it global custom. You need evidence across regions, legal systems, and politically important states. In some areas, the conduct of especially relevant states carries special weight.

For MUN, this gives you a smart rhetorical structure:

Start with a pattern of conduct by states.

Add evidence that those states treat the pattern as law.

Then explain why the practice is broad enough to matter.

That’s already more developed than most committee speeches.

What not to do in a position paper

Don’t write, “Customary law says this is illegal,” and move on.

Instead, write something closer to this:

Notice what changed. The claim became reasoned. You didn’t just assert the rule. You showed the method.

How to Identify Unwritten Laws in Practice

Identifying customary law feels less like memorizing a code and more like building a case file. You’re looking for clues, weighing them, and asking whether together they show both practice and legal obligation.

Start with the strongest public evidence

Some forms of evidence are easier to use in MUN because they’re public, quotable, and easier to verify.

Look first at:

International court decisions because they often state whether a rule exists and how it operates

Government statements from foreign ministries, legal advisers, and UN representatives

Domestic legislation and court judgments because they show how states implement legal norms internally

Military manuals and official doctrine especially in humanitarian and use-of-force topics

UN records including voting patterns and debate statements

National courts can matter too. The verified material notes that national court judgments now function as subsidiary evidence alongside public state statements and legal opinions. That’s useful for delegates because it widens your research base beyond treaties and ICJ judgments.

Think like a detective, not a quote collector

A weak delegate hunts for one flashy quote. A strong delegate assembles a pattern.

If you’re researching a possible customary rule, ask:

Who is acting? One state or many states?

How are they acting? Consistently or sporadically?

How are they describing their conduct? As policy, morality, or law?

Who objects? Persistent objections can weaken claims of universality.

That method pairs well with good source evaluation habits. If you want a practical research routine for separating strong evidence from weak material, this guide on how to find credible sources and evaluate information is useful for MUN prep.

Landmark Examples and Influential Case Law

Customary law becomes much easier to remember when you attach it to disputes. Cases show you that these aren’t abstract classroom ideas. States argue about them because they have practical implications.

Nicaragua v United States made the point impossible to ignore

One of the cases every MUN delegate should know is Nicaragua v. United States before the ICJ in 1986, discussed in the verified ECCHR summary on customary international law and the Nicaragua case. The issue included whether the United States had violated customary rules on the use of force through its support for contra rebels.

Why does this case matter so much? Because it shows that customary law can operate independently of treaty law. Even when treaty arguments become contested, the Court can still ask whether a customary rule exists and whether it has been breached. For MUN, that is gold. It gives you a clean answer when another delegate says, “But my state didn’t sign that treaty.”

You can respond: treaty consent matters, but it isn’t the whole story.

Persistent objectors complicate the picture

Customary law is broad, but it isn’t mechanically identical for every state in every situation. The verified material highlights the idea of the persistent objector, meaning a state that consistently opposes an emerging rule may avoid being bound by that specific custom.

That’s an advanced point, and it impresses chairs when used carefully. But don’t overuse it. You need evidence that the objection was clear, consistent, and maintained from the norm’s formation. A state can’t usually ignore a rule late in the process and call itself a persistent objector after the fact.

Examples delegates can use without overclaiming

Some customary rules are commonly treated as well established in practice, including non-refoulement and immunity for visiting heads of state, both mentioned in the verified Cornell material cited earlier. In committee, these examples help because they show the range of customary law. It doesn’t only govern war. It also shapes migration, diplomacy, and state responsibility.

If your committee discusses territory, intervention, or occupation, you may find it helpful to connect legal status arguments with clear conceptual language. Students often confuse sovereignty questions, annexation, and control on the ground, so a plain-language explainer on what annexing land means can help sharpen that side of your argument.

Customary Law vs Treaty Law Understanding the Difference

Clarifying this distinction helps many students. They’ve heard both words. They know both are “international law.” But they’re not the same thing.

A treaty is a written agreement. States consent to it. Usually, it binds the parties that sign and ratify it.

Customary international law is different. It arises from general practice accepted as law, and the verified material states that it binds all states except proven persistent objectors. That universal quality is why it matters so much in MUN. It can reach beyond formal treaty membership.

A side by side comparison

Feature	Customary International Law (CIL)	Treaty Law
How it forms	Through state practice plus opinio juris	Through negotiated written agreement
Who is bound	All states except proven persistent objectors	States that consent to be bound
Main proof method	Evidence from practice, legal statements, judgments, legislation	Text of the treaty and ratification status
Best use in MUN	When treaty law is missing, incomplete, or not universal	When a treaty directly governs the issue
Common student mistake	Treating repeated behavior alone as enough	Assuming a treaty binds everyone

Sometimes treaties write down existing custom

A treaty can codify a customary rule. In other words, states may already follow a norm as law, and then later write it into a treaty text. Even after codification, the customary rule can continue to exist independently.

That matters in debate. If a state isn’t party to the treaty, you shouldn’t stop there. Ask whether the same rule also exists as custom.

Sometimes treaties help create new custom

The relationship also runs the other way. Widespread treaty participation and consistent implementation can help generate or confirm customary rules over time. If many states adopt the same treaty obligation and then behave as though the obligation reflects law more broadly, the treaty’s influence may spill beyond its own membership.

A practical benchmark appears in the verified material from Asser. The ICRC’s 2005 Customary IHL Study identified 161 rules applicable globally in armed conflict, and 136 of those rules, or 85%, were considered binding on all states regardless of Geneva Conventions ratification, as summarized in the Asser explanation of customary law. That’s a powerful illustration of how custom creates a baseline for the entire international community.

Which one wins if they conflict

Students often ask for a simple hierarchy. Usually, the better answer is that the relationship must be interpreted carefully. Treaty and custom can exist in parallel. They aren’t automatically in conflict just because they address the same subject.

Where there is tension, lawyers ask narrower questions. Is the treaty meant to replace the custom for its parties? Does the custom continue for non-parties? Is there a more fundamental norm at stake?

Jus cogens comes into play. These are peremptory norms from which states can’t derogate. The verified material gives the ban on slavery as an example of a customary rule with that higher status. For MUN, you don’t need to become a specialist in jus cogens overnight. Just remember that some norms sit at the very top of the system and can’t be contracted away.

A MUN Delegate's Guide to Using Customary Law

Knowing what is international customary law is useful. Using it well is what wins points.

Most delegates mention custom vaguely. They say, “International customary law supports this position,” then move on. That rarely lands because it sounds borrowed. Chairs and competitors notice when you haven’t done the legal work.

Build your argument in three moves

Use this structure in speeches and position papers.

Identify the rule you’re claimingBe precise. Don’t say “customary law exists on this issue.” Say what the rule is.

Prove the rule with evidenceShow state practice and opinio juris through public material.

Apply the rule to the committee problemExplain why the conduct being debated falls within that rule.

Here’s a clean speech template:

That formula works because it sounds disciplined, not theatrical.

What evidence should a delegate actually cite

The verified material gives especially practical guidance here. For MUN delegates, proving customary law can involve UNGA voting records or the ICRC database of 161 customary IHL rules, and a March 2026 UN Cyber Consultations process reported 70% state endorsement of a “no-first-use” cyber norm, which could be used to argue for an emerging rule, according to the verified summary linked to Duke Law’s customary law material.

That gives you two useful lessons.

First, not every committee issue has settled customary law. Sometimes you’re arguing for an emerging norm.

Second, you need evidence types that fit the topic. For armed conflict, the ICRC database is useful. For cyber, state consultations and official positions may matter more.

If you’re sorting your evidence base, this guide to primary vs secondary sources in MUN research can help you decide what belongs in your strongest citations.

Copyable sentence starters for committee use

Try these in moderated caucus, unmods, and papers:

“This position is not only policy-based but supported by customary international law, as shown by state practice and legal statements.”

“Even in the absence of a universally ratified treaty, states have increasingly treated this rule as legally binding.”

“The committee shouldn’t confuse political preference with legal custom. Both practice and opinio juris must be demonstrated.”

“Our draft clause reflects an emerging customary norm rather than a merely aspirational declaration.”

And for rebuttal:

“The opposing argument cites conduct, but it hasn’t shown opinio juris.”

“Treaty non-membership doesn’t end the analysis if the underlying rule also exists in customary form.”

“One state’s policy doesn’t establish general and representative practice.”

How to avoid sounding fake when using legal language

Students sometimes overcompensate. They pack speeches with Latin and legal jargon. That usually weakens the point.

Instead:

Name the rule clearly rather than hiding it in vague legalese

Use fewer claims, but support them better

Distinguish settled law from emerging law so you don’t overstate your case

Concede uncertainty where it exists because that makes the rest of your argument more credible

One model paragraph for a position paper

Here’s a practical example you can adapt:

That paragraph does three things well. It defines the method. It signals seriousness. It stays careful enough to be believable.

Conclusion The Living Law of Nations

Customary international law is often called unwritten law, but that phrase can mislead students. It isn’t vague folklore. It’s a disciplined way of identifying legal rules from what states repeatedly do and from the fact that they treat those actions as law.

For MUN delegates, that makes it one of the most useful tools in the room. It helps when treaties don’t fully answer the question. It helps when a state isn’t party to an agreement. It helps when your committee is dealing with emerging issues like cyber operations or broader environmental obligations, where legal development is still unfolding.

If you can explain the two pillars, spot the evidence, and apply the rule without exaggerating, you’ll sound different from most delegates. More precise. More credible. More prepared for the kind of debate where legal sophistication changes how the room sees your position.

That’s the value of learning what is international customary law. You stop treating international law as a stack of documents and start seeing it as a living system built through state behavior, legal conviction, and argument.

If you want faster, clearer MUN research support, Model Diplomat helps students turn complex international law and IR topics into usable committee arguments, sourced answers, and stronger position papers.