Customary International Law: Formation, Elements and Key ICJ Cases

1.

LECTURE · PUBLIC INTERNATIONAL LAW
Customary
International Law
State practice · Opinio juris · Formation · ICJ jurisprudence
Academic Lecture Series

2.

OUTLINE
Lecture Contents
Five parts
01
03
Concept
What is CIL? Article 38 ICJ Statute. History.
State practice (usus) + opinio juris.
Formation and effects
Landmark cases
Persistent objector, regional CIL, jus cogens.
05
02
Two elements
04
North Sea, Nicaragua, Asylum — ICJ jurisprudence.
Modern issues
ILC Draft Conclusions 2018, cyber, climate.
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PART I
Concept
Article 38 · Definition · Historical development

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CHAPTER 01 · SOURCES
Sources of international law
Article 38(1) of the ICJ Statute — the classic catalog
ARTICLE 38(1) ICJ STATUTE
The Court shall apply: (a) international conventions; (b) international custom; (c) general principles of law; (d) judicial decisions and
teachings.
(a) Conventions
(b) International custom
Treaties — express agreement between states.
«Evidence of general practice accepted as law» — CIL.
(c) General principles
(d) Subsidiary means
Legal principles recognized by civilized nations.
Judicial decisions and teachings — persuasive, not binding.
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CHAPTER 01 · DEFINITION
What is customary international law?
An unwritten, evolving source — binding on all states
ARTICLE 38(1)(b)
«International custom, as evidence of a general practice accepted as law». CIL consists of rules that emerge from
consistent state practice engaged in out of a sense of legal obligation.
Unwritten
Universal in principle
Evolving
Not codified in a treaty — inferred from what states
Once formed, binds all states — including new ones
Norms change over time as state practice and beliefs
do and say.
(subject to persistent objector).
change.
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CHAPTER 01 · HISTORY
Historical development of CIL
From jus gentium to modern doctrine
Antiquity
Ancient Rome
Medieval
Lex mercatoria
1625
Grotius
Jus gentium — «law of nations» — rules common to
Merchant law — international commercial custom
«De Jure Belli ac Pacis» — grounded international law
all peoples, derived from custom.
developed by traders across Europe.
in natural law and custom.
19th c.
1920
2018
Positivism rises
PCIJ Statute
ILC Conclusions
State practice becomes primary source — custom
Art. 38 of the Permanent Court of International Justice
International Law Commission adopts Draft
seen as tacit consent between states.
— formalized CIL as source.
Conclusions on identification of CIL.
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PART II
Two elements
State practice (usus) · Opinio juris

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CHAPTER 02 · ELEMENTS
The two-element approach
Both elements are necessary — «objective» + «subjective»
ILC DRAFT CONCLUSION 2 (2018)
«To determine the existence and content of a rule of CIL, it is necessary to ascertain whether there is a general practice accepted as law
(opinio juris)».
State practice (usus)
Opinio juris sive necessitatis
Objective element
Subjective element
What states actually do — actions, omissions, official statements, legislation.
The belief that a practice is required (or permitted) by law — «acceptance as
The «material» element.
law». The «psychological» element.
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CHAPTER 02 · PRACTICE
State practice · What counts?
Wide catalog — all forms of state conduct
Diplomatic acts
Legislation
Court decisions
Notes, declarations, statements at international
National laws implementing or referring to
National courts applying or refusing to apply
conferences.
international rules.
international norms.
Executive conduct
Treaty practice
Votes in IOs
Actions of the executive — military operations,
Ratification, signing, and objections to treaties can
Votes and statements in UN General Assembly, other
arrests, expulsions.
evidence practice.
organs — declarative practice.
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CHAPTER 02 · REQUIREMENTS
Requirements for state practice
North Sea Continental Shelf test — four qualities
01
Extensive
Widespread — must be engaged in by a significant number of states, especially
02
Uniform
Consistent — different states must engage in substantially the same conduct.
specially affected states.
03
Virtually uniform
Not requiring unanimity — some divergence acceptable but not major inconsistency.
04
Representative
Should include states whose interests are specially affected by the rule in question.
→ Duration: no fixed period. Even «short period» may suffice if practice is extensive and virtually uniform (North Sea, para. 74).
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CHAPTER 02 · OPINIO JURIS
Opinio juris sive necessitatis
«Belief that this is required by law»
ICJ IN NORTH SEA (1969)
«Not only must the acts concerned amount to a settled practice, but they must also be such — or be carried out in such a way — as to be
evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it».
Distinguishes law from etiquette
Two versions
Critical function
Bowing to a queen is a courtesy, not a legal duty — no
«Sive necessitatis» — obligation from necessity; or
Without it, state practice is mere convention or comity
opinio juris.
from sense that it is required by law.
— not binding law.
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CHAPTER 02 · PROOF
How to prove opinio juris?
Evidence of the state's legal conviction — often the hardest task
UNGA resolutions
Official statements
Diplomatic correspondence
Especially when adopted with wide majority +
Governments' formal declarations of legal position —
Notes exchanged between states — arguments about
language of legal obligation.
treaty statements, protests.
legal duties.
Court decisions
Domestic legislation
Silence / acquiescence
National courts citing the norm as binding under IL.
Laws implementing what state considers to be an
In some contexts, failure to protest may indicate
international obligation.
acceptance.
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CHAPTER 02 · PARADOX
The «chicken-and-egg» paradox
A classic theoretical problem in the doctrine of CIL
THE PROBLEM
How can states acquire opinio juris before the rule exists? If they believe the practice is already law, but no law exists yet — the belief is
technically mistaken.
Progressive belief
Kelsen's view
As practice grows, belief that it should be law develops — retroactive legitimation.
The first act creating the rule is normatively grounded by the emerging community.
Modern view
Instant custom
Look at process — practice + statements evolve together; no single «moment of
For rapidly emerging rules (space law), Bin Cheng argued for «instant custom»
creation».
without long practice.
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PART III
Formation and effects
Persistent objector · Regional CIL · Jus cogens · Codification

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CHAPTER 03 · OBJECTOR
The persistent objector doctrine
A state can exempt itself from a CIL rule — if it protests early and consistently
DOCTRINE
A state that has consistently objected to a norm during its formation is NOT bound by that norm once it becomes CIL — a limited exception to
the universality of custom.
Objection during formation
Consistent
Must object BEFORE the rule crystallizes — post-hoc protest is too late.
Objection must be maintained continuously — not raised once and dropped.
Clear and public
Does not apply to jus cogens
Objection must be unambiguous, known to the international community.
Persistent objector doctrine does NOT apply to peremptory norms.
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CHAPTER 03 · REGIONAL CIL
Regional and local customary law
CIL between smaller groups of states — the Asylum case doctrine
POSSIBILITY
CIL is not always universal — regional and local (even bilateral) custom can exist between smaller groups of states.
Regional
Bilateral
Latin American diplomatic asylum, European law on state immunity — regional rules.
Rules between two states based on their consistent long-standing practice.
Higher burden
No dissenting states
Party asserting regional CIL must prove practice + opinio juris of ALL states in the
If one state in the region contests the rule, it cannot form as local CIL between them.
group.
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CHAPTER 03 · RELATIONSHIP
CIL and treaties · Relationship
Coexisting sources — complex interplay
TREATIES → CIL
PARALLEL EXISTENCE
Codification
Nicaragua principle
Treaty may codify existing CIL
CIL rule survives treaty
A treaty may declare pre-existing CIL, generate new CIL by widespread
Rules of CIL and treaty rules on the same matter can coexist — even when a
adoption, or crystallize an emerging rule — e.g., VCLT 1969 codifies much of CIL
state cannot be bound by the treaty, it remains bound by the parallel CIL rule.
on treaties.
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CHAPTER 03 · JUS COGENS
Jus cogens · Peremptory norms
The highest form of CIL — cannot be derogated from
VCLT 1969 · ARTICLE 53
«A norm accepted and recognized by the international community of states as a whole as one from which no derogation is
permitted».
Prohibition of aggression
Prohibition of genocide
No state may lawfully wage war of aggression — UN Charter Art. 2(4).
1948 Genocide Convention — universally accepted as jus cogens.
Prohibition of torture
Prohibition of slavery
Absolute prohibition — no exceptions, even in emergency (Convention against
Peremptory rule — recognized since 19th c., accepted by all civilized nations.
Torture).
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CHAPTER 03 · ILC
ILC and the codification of CIL
International Law Commission — guardian of the identification process
ILC MANDATE
Established 1947 by UNGA to promote «progressive development of international law and its codification» — Statute Art. 15.
VCLT 1969
ARSIWA 2001
Codified custom on treaties — widely accepted as reflecting CIL.
Draft Articles on Responsibility of States for Internationally Wrongful Acts.
Draft Conclusions on CIL 2018
Draft on Jus Cogens 2022
Michael Wood as Special Rapporteur — 16 conclusions.
Draft Conclusions on peremptory norms — Special Rapporteur Tladi.
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PART IV
Landmark ICJ cases
North Sea · Nicaragua · Asylum

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ICJ · CASE 51/52
Case 1 · North Sea Continental Shelf (1969)
Germany v. Denmark and Netherlands · the foundational CIL case
FACTS
Delimitation of North Sea continental shelf between Germany and its neighbors. Whether the «equidistance principle» in Art. 6 of the 1958
Geneva Convention on the Continental Shelf bound Germany as CIL — Germany was not a party.
Applicants
Respondent
Denmark and Netherlands — argued equidistance principle applies as CIL.
Federal Republic of Germany — argued no CIL rule existed; principle inequitable.
Question
Citation
Had the equidistance principle in the Geneva Convention passed into CIL binding
ICJ Reports 1969, p. 3 · https://www.icj-cij.org/case/51
Germany?
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ICJ · HOLDINGS
North Sea · Holdings and legacy
ICJ's landmark articulation of the two-element test
PARA. 77 — TWO-ELEMENT TEST
«Not only must the acts amount to a settled practice, but they must be carried out in such a way as to be evidence of a belief that this practice
is rendered obligatory by a rule of law requiring it».
Equidistance NOT CIL
Duration flexible
The equidistance principle from Art. 6 had not crystallized into CIL binding Germany.
«Short period» may suffice if practice extensive + virtually uniform + includes
affected states.
Opinio juris required
Delimitation by agreement
State practice alone is insufficient — must be accompanied by legal belief.
Continental shelf boundaries to be established by agreement based on equitable
principles.
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ICJ · CASE 70
Case 2 · Nicaragua v. United States (1986)
Military and Paramilitary Activities in and against Nicaragua
FACTS
US support for Contra rebels + mining of Nicaraguan harbors. USA argued that under the Vandenberg reservation, the Court could only
apply treaty law, not CIL. Court applied parallel CIL rules on use of force.
Applicant
Respondent
Nicaragua — alleged US violations of sovereignty, non-intervention, use of force.
United States — refused to participate after preliminary phase.
Question
Citation
Do CIL rules on use of force and non-intervention parallel UN Charter provisions?
ICJ Reports 1986, p. 14 · https://www.icj-cij.org/case/70
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ICJ · FINDINGS
Nicaragua · Findings and legacy
ICJ's methodology on identifying CIL — highly influential
Parallel CIL exists
UNGA resolutions as evidence
Rules on non-use of force, non-intervention, sovereignty exist as CIL — parallel to UN
GA Resolution 2625 (1970) on Friendly Relations = strong evidence of opinio juris.
Charter.
Even against major powers
Contrary practice doesn't defeat CIL
USA held bound by CIL despite Vandenberg reservation on multilateral treaties.
State conduct inconsistent with rule + treated by that state as breach = CONFIRMS
the rule.
Self-defense (Art. 51)
Collective self-defense
Right of self-defense exists in CIL — but requires prior «armed attack» +
Requires (a) armed attack occurred, (b) victim state declares itself attacked, (c)
proportionality + necessity.
requests help.
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ICJ · CASE 7
Case 3 · Asylum (Colombia v. Peru, 1950)
The foundational case on regional CIL
FACTS
Peruvian politician Haya de la Torre took refuge in Colombian Embassy in Lima after failed uprising. Colombia claimed unilateral right to
«qualify» the offense as political — under alleged Latin American CIL.
Applicant
Respondent
Colombia — asserted regional CIL right of diplomatic asylum in Latin America.
Peru — denied existence of such a regional custom.
Question
Citation
Does the Colombian claim reflect a regional CIL rule of Latin American states?
ICJ Reports 1950, p. 266 · https://www.icj-cij.org/case/7
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ICJ · LEGACY
Asylum · Implications and legacy
First articulation of regional CIL — high burden of proof
COURT'S DICTUM
«The party which relies on a custom must prove that this custom is established in such a manner that it has become binding on the other
party». — ICJ Reports 1950, p. 276.
Regional CIL possible
Higher burden of proof
Court confirmed regional/local CIL can exist between smaller groups of states.
Party asserting regional CIL must prove practice + opinio juris of ALL states in group.
Colombia failed the test
Dissenting states matter
Practice on asylum «uncertain, contradictory, fluctuating» — no CIL rule found.
Peru had actively opposed the alleged rule — cannot form as CIL between them.
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PART V
Modern issues
ILC 2018 · Cyber · Climate · Domestic law

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CHAPTER 05 · ILC 2018
ILC Draft Conclusions on CIL (2018)
Michael Wood as Special Rapporteur — 16 conclusions in five parts
AUTHORITATIVE RESTATEMENT
Adopted by ILC in 2018 — «taken note of» by UNGA. Not binding but highly authoritative on identification of CIL.
Part I · Introduction
Part II · Two elements
Scope + basic approach — two-element test confirmed.
General practice (Conclusion 4-8) + opinio juris (Conclusion 9-10).
Part III · Manifestations
Part IV · Exceptions
Forms of state practice; forms of evidence of opinio juris.
Persistent objector (13); particular custom (16); relationship with treaties.
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CHAPTER 05 · CHALLENGES
Modern challenges for CIL
Emerging norms in unprecedented domains
Cyber operations
Climate change
Tallinn Manual 2.0 (2017) — but is there yet CIL on cyber attacks? Attribution issue.
«No harm rule» + due diligence in transboundary environmental protection —
emerging CIL.
Autonomous weapons (LAWS)
Outer space
State practice divided — Group of Governmental Experts negotiating; no CIL yet.
«Common heritage of mankind» principle — status as CIL disputed since 1979 Moon
Treaty.
Human rights
New states
Which rights bind states as CIL? — HRC's General Comments, universal instruments.
«Colonial-era» CIL binds newly independent states? — CIL «inherited» debate.
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CHAPTER 05 · DOMESTIC
CIL in domestic legal systems
Two approaches — monism and dualism
MONIST APPROACH
DUALIST APPROACH
Direct incorporation
Transformation required
Germany, Netherlands, USA (partly)
UK, most Commonwealth countries
CIL automatically part of domestic law — «international law is part of the law of
CIL needs to be transformed by act of Parliament into domestic law. But
the land». USA: «The Paquete Habana» (1900) — CIL is part of US law.
courts often apply CIL directly in absence of contrary statute.
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CASE · KAZAKHSTAN
Kazakhstan and CIL
Constitutional and doctrinal position
Constitution Art. 4
01
CIL role
02
«International treaties ratified by KZ have priority over its laws». Silent on CIL.
CIL applied in courts through referenceto general principles + treaties reflecting CIL.
Jus cogens
VCLT 1969
03
04
Peremptory norms binding on KZ — genocide, torture, aggression prohibitions.
KZ acceded 1994 — codifies CIL on treaties; binding through both custom and treaty.
ICJ jurisdiction
Practice contribution
05
06
KZ has not made an Art. 36(2) declaration — jurisdiction only by compromis or treaty
KZ voted for key GA resolutions on non-use of force, self-determination, HR —
clause.
practice + opinio juris.
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KEY TAKEAWAYS
Summary
CIL — «evidence of a general practice accepted as law» (Art. 38(1)(b) ICJ
01
Statute). Unwritten, universal, evolving.
Two elements required: state practice (usus) + opinio juris. Both are
02
North Sea Continental Shelf (1969) — established two-element test;
03
short period may suffice.
Nicaragua v. USA (1986) — parallel CIL exists alongside treaties; UNGA
04
Asylum (1950) — regional/local CIL possible but requires proving practice
05
+ opinio juris of ALL states in group.
necessary; neither alone is sufficient.
resolutions evidence opinio juris.
ILC Draft Conclusions on CIL (2018) — authoritative restatement; jus
06
cogens is highest form, no derogation.