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International Criminal Law – What Rules Does the ICC Apply, in What Order, and How Does This Affect the Principle of Legality?
1. Introduction: Jurisdiction vs Applicable Law
Under international criminal law, the ICC’s jurisdiction (what crimes it can hear) is not the same thing as the law it applies to decide guilt, defences, and procedure.
Earlier tribunals like the ICTY, ICTR, and SCSL did not clearly separate:
The Rome Statute does. It gives the ICC:
This is crucial both for legal certainty and for respecting the principle of legality (nullum crimen, nulla poena sine lege).
2. Article 21: Hierarchy of Applicable Law
2.1 The Three-Tier Structure
Article 21(1) sets out what law the ICC may apply and in what order:
Importantly:
Article 21(1)(c) permits the Court to use general principles derived from national laws, not national laws themselves.
National law as such is not directly part of the ICC’s applicable law.
2.2 Article 21(2): Precedent
Article 21(2) allows the ICC to apply principles and rules of law as interpreted in its previous decisions.
2.3 Article 21(3): Human Rights and Non-Discrimination
Article 21(3) adds a crucial cross-cutting requirement:
The application and interpretation of law must be consistent with internationally recognised human rights and free from discrimination based on gender, age, race, colour, language, religion, political opinion, national or social origin, wealth, birth or other status.
So even when applying the Statute:
3. When Can the Court Use Treaties, Custom and General Principles? (Lacunae)
The Appeals Chamber has read Article 21(1) very strictly:
And even then:
So in practice, the ICC is Statute-centred, not a free-roaming international criminal court applying unwritten general international law at will.
4. Applicable Law and the Principle of Legality
A concern sometimes raised is:
“What if the Rome Statute defines crimes more broadly than customary international law or treaties did at the time of the alleged acts?”
Would this violate nullum crimen sine lege (Article 22) or non-retroactivity (Article 24)?
The key points are:
Together, they show that the Rome Statute is itself a source of substantive criminal law for those who fall under its jurisdiction, and its application is strictly prospective (Article 11 & 24).
Example:
Lubanga
and Child Soldiers
In Lubanga, the defence argued that prosecuting the war crime of enlisting, conscripting and using children under 15 violated nullum crimen.
The Pre-Trial Chamber held that:
So, for persons already within the Statute’s jurisdiction (nationals or acts on State Party territory after entry into force), legality concerns are largely addressed.
5. The Harder Problem: Non-States Parties and Retroactive Jurisdiction
More delicate legality questions arise where:
In those cases:
So, unless:
there is a risk that the accused is being convicted for an act that was not, for them, clearly an international crime at that time.
This raises a real legality concern:
Does the combination of Article 21 and expanded jurisdiction through Article 12(3)/Security Council referrals risk de facto ex post facto criminalisation?
Most commentators answer:
6. Ten Case Scenarios – Solutions and Critical Analysis
Scenario 1: Statute vs Customary Law on Command Responsibility
The ICC must decide whether a commander is responsible under a theory of command responsibility. Defence argues that customary international law has a narrower standard than the Rome Statute.
Solution:
Critical analysis:
The Statute-centred approach can mean the ICC applies standards slightly different from custom. This is acceptable as long as the accused had notice and the law is clear, but it raises questions about fragmentation of international criminal law.
Scenario 2: Lacuna on a Defence – Resort to General Principles
The Statute and RPE are silent on whether necessity can fully excuse conduct in a narrow humanitarian context.
Solution:
Critical analysis:
This reflects the creative but constrained role of general principles: they fill genuine gaps but cannot contradict the Rome Statute or international human rights norms.
Scenario 3: Use of Customary IHL to Clarify “Attack Against Civilian Population”
The Statute mentions “attack directed against a civilian population” (Article 7) without fully defining it. The Court needs more precision.
Solution:
Critical analysis:
This shows how Article 21 allows the ICC to stay grounded in the Statute while still drawing on the rich history of IHL to clarify terms, enhancing consistency with previous tribunals.
Scenario 4: Prior ICC Precedent vs Fresh Interpretation
A Trial Chamber wants to interpret “persecution” under crimes against humanity more narrowly than a prior Appeals Chamber ruling.
Solution:
Critical analysis:
This nurtures legal certainty and equality before the law, but also raises the question: how flexible should the ICC be in revising earlier jurisprudence as understandings evolve?
Scenario 5: A Non-State Party Files a Retroactive Article 12(3) Declaration
State X, not a Party to the Statute, submits an Article 12(3) declaration in 2025 accepting ICC jurisdiction for crimes committed in 2019. An accused argues this is retroactive criminalisation.
Solution:
Critical analysis:
This scenario highlights the fine line between retrospective jurisdiction and ex post facto law. Much turns on whether the conduct was already internationally criminal at the time.
Scenario 6: Security Council Referral of a Non-Party’s Situation
The Security Council refers State Y (a non-party) to the ICC for atrocities committed in 2010. A military leader claims he had no reason to know he was answerable to the ICC.
Solution:
Critical analysis:
Again, the legality concern is managed by anchoring convictions in pre-existing customary and treaty norms, not purely in the Rome Statute’s later application to that person.
Scenario 7: National Law Labels Conduct as “Terrorism,” ICC Calls It “Crimes Against Humanity”
A national court convicts an official for “terrorism” due to widespread attacks on civilians. Later, the ICC considers charging him with crimes against humanity based on the same incidents.
Solution:
Critical analysis:
This shows how the ICC’s conduct-based approach helps respect legality and avoid double jeopardy, even when domestic legal categories are different.
Scenario 8: Interpretation of a Crime in a Way That Might Discriminate
A Chamber considers interpreting the definition of “gender-based persecution” in a restrictive way that effectively excludes persecution of LGBTQ+ persons.
Solution:
Critical analysis:
Article 21(3) functions as a human-rights safety valve, pushing the ICC towards interpretations that reflect modern equality principles and prevent regressive results.
Scenario 9: Defence Argues Child Soldier Crime Not Customary at Time
An accused is charged with conscripting children under 15 in 2003. Defence argues that this specific war crime was not clearly established in customary international law by then.
Solution:
Critical analysis:
This echoes Lubanga: as long as the crime was clearly defined and broadly recognised before the acts, concerns about legality are mitigated.
Scenario 10: Deriving a General Principle on Corporate Criminal Responsibility
The ICC faces a question about whether legal persons (companies) can be directly criminally liable, something the Statute is silent about.
Solution:
Critical analysis:
This underlines the limits of Article 21(1)(c): general principles derived from national laws cannot be used to override clear structural choices in the Rome Statute.
7. Conclusion
Under International Criminal Law, the ICC’s applicable law regime in Article 21:
1. Introduction: Jurisdiction vs Applicable Law
Under international criminal law, the ICC’s jurisdiction (what crimes it can hear) is not the same thing as the law it applies to decide guilt, defences, and procedure.
Earlier tribunals like the ICTY, ICTR, and SCSL did not clearly separate:
- their subject-matter jurisdiction (ratione materiae), and
- their applicable law.
The Rome Statute does. It gives the ICC:
- A defined list of crimes (Articles 5–8 bis), and
- A structured hierarchy of applicable law in Article 21.
This is crucial both for legal certainty and for respecting the principle of legality (nullum crimen, nulla poena sine lege).
2. Article 21: Hierarchy of Applicable Law
2.1 The Three-Tier Structure
Article 21(1) sets out what law the ICC may apply and in what order:
- First level – ICC internal law
- The Rome Statute
- The Elements of Crimes
- The Rules of Procedure and Evidence (RPE)
- Second level – International law
- “Applicable treaties”
- Customary international law (“principles and rules of international law, including established principles of the international law of armed conflict”)
- Third level – General principles of law
- Derived by the Court from national legal systems across the world, including the law of States that would normally exercise jurisdiction.
- These principles must not conflict with the Statute, international law, or internationally recognised norms and standards.
Importantly:
Article 21(1)(c) permits the Court to use general principles derived from national laws, not national laws themselves.
National law as such is not directly part of the ICC’s applicable law.
2.2 Article 21(2): Precedent
Article 21(2) allows the ICC to apply principles and rules of law as interpreted in its previous decisions.
- This creates a kind of de facto precedent system.
- Chambers are not formally bound as in a strict common law hierarchy, but prior decisions carry strong persuasive authority and help ensure consistency.
2.3 Article 21(3): Human Rights and Non-Discrimination
Article 21(3) adds a crucial cross-cutting requirement:
The application and interpretation of law must be consistent with internationally recognised human rights and free from discrimination based on gender, age, race, colour, language, religion, political opinion, national or social origin, wealth, birth or other status.
So even when applying the Statute:
- The ICC must interpret it through a human-rights-consistent lens,
- And cannot interpret provisions in a way that produces discriminatory effects.
3. When Can the Court Use Treaties, Custom and General Principles? (Lacunae)
The Appeals Chamber has read Article 21(1) very strictly:
- The ICC may turn to other treaties, customary international law, or general principles only if there is a lacuna (a genuine gap) in:
- the Statute,
- the Elements of Crimes, or
- the Rules of Procedure and Evidence.
And even then:
- The Court must first try to resolve ambiguity using the Vienna Convention on the Law of Treaties (VCLT) rules of treaty interpretation.
- Only if the Statute still fails to answer the question can it reach out to other sources.
So in practice, the ICC is Statute-centred, not a free-roaming international criminal court applying unwritten general international law at will.
4. Applicable Law and the Principle of Legality
A concern sometimes raised is:
“What if the Rome Statute defines crimes more broadly than customary international law or treaties did at the time of the alleged acts?”
Would this violate nullum crimen sine lege (Article 22) or non-retroactivity (Article 24)?
The key points are:
- Articles 6, 7, 8, and 8 bis look like jurisdictional provisions, but in practice they define crimes and modes of responsibility. They have substantive criminal law effect.
- Several Statute provisions underline this:
- Article 22(1) – No one is criminally responsible under the Statute unless their conduct, at the time it occurred, constituted a crime within the jurisdiction of the Court.
- Article 24(1) – No one is criminally responsible under the Statute for conduct before its entry into force.
- Article 20(3) – Refers to conduct “proscribed” under Articles 6, 7, and 8.
Together, they show that the Rome Statute is itself a source of substantive criminal law for those who fall under its jurisdiction, and its application is strictly prospective (Article 11 & 24).
Example:
Lubanga
and Child Soldiers
In Lubanga, the defence argued that prosecuting the war crime of enlisting, conscripting and using children under 15 violated nullum crimen.
The Pre-Trial Chamber held that:
- The crimes of using child soldiers were clearly defined in Articles 8(2)(b)(xxvi), 8(2)(e)(vii), 22–24, and 77.
- These provisions entered into force before the acts in question.
- Therefore, the accused had notice, and the offences were properly criminalised.
So, for persons already within the Statute’s jurisdiction (nationals or acts on State Party territory after entry into force), legality concerns are largely addressed.
5. The Harder Problem: Non-States Parties and Retroactive Jurisdiction
More delicate legality questions arise where:
- A non-State Party accepts ICC jurisdiction under Article 12(3) by declaration, possibly covering past crimes, or
- The UN Security Council refers a situation involving crimes committed:
- on the territory of a non-State Party, and/or
- by nationals of a non-State Party.
In those cases:
- At the time of the alleged conduct, the accused was not yet within the jurisdiction of the ICC (no State Party nationality or territorial link under Article 12(2)).
- The ICC’s jurisdiction is effectively extended after the fact by declaration or SC referral.
So, unless:
- The conduct and mode of liability were already criminalised under customary international law or treaty law at the time of commission,
there is a risk that the accused is being convicted for an act that was not, for them, clearly an international crime at that time.
This raises a real legality concern:
Does the combination of Article 21 and expanded jurisdiction through Article 12(3)/Security Council referrals risk de facto ex post facto criminalisation?
Most commentators answer:
- If the conduct was already a customary international crime (e.g., genocide, torture, grave war crimes), legality is usually satisfied.
- Problems arise mostly if the ICC applies novel or expanded interpretations beyond what custom or treaty law established at the time.
6. Ten Case Scenarios – Solutions and Critical Analysis
Scenario 1: Statute vs Customary Law on Command Responsibility
The ICC must decide whether a commander is responsible under a theory of command responsibility. Defence argues that customary international law has a narrower standard than the Rome Statute.
Solution:
- Under Article 21(1)(a), the Court must apply the Statute first, including Article 28 on command responsibility.
- Only if the Statute is ambiguous can it look at customary law under Article 21(1)(b).
Critical analysis:
The Statute-centred approach can mean the ICC applies standards slightly different from custom. This is acceptable as long as the accused had notice and the law is clear, but it raises questions about fragmentation of international criminal law.
Scenario 2: Lacuna on a Defence – Resort to General Principles
The Statute and RPE are silent on whether necessity can fully excuse conduct in a narrow humanitarian context.
Solution:
- The Court first checks whether Articles 31–33 (defences) and the RPE cover the issue.
- If not, there is a lacuna.
- It may then derive general principles from comparative national laws (Article 21(1)(c)), ensuring compatibility with the Statute and human rights.
Critical analysis:
This reflects the creative but constrained role of general principles: they fill genuine gaps but cannot contradict the Rome Statute or international human rights norms.
Scenario 3: Use of Customary IHL to Clarify “Attack Against Civilian Population”
The Statute mentions “attack directed against a civilian population” (Article 7) without fully defining it. The Court needs more precision.
Solution:
- The Court interprets the Statute using VCLT rules first.
- If ambiguity remains, it turns to customary IHL and relevant treaties (like Additional Protocols) under Article 21(1)(b).
Critical analysis:
This shows how Article 21 allows the ICC to stay grounded in the Statute while still drawing on the rich history of IHL to clarify terms, enhancing consistency with previous tribunals.
Scenario 4: Prior ICC Precedent vs Fresh Interpretation
A Trial Chamber wants to interpret “persecution” under crimes against humanity more narrowly than a prior Appeals Chamber ruling.
Solution:
- Article 21(2) allows application of law as interpreted in previous decisions, especially higher-level rulings.
- The Trial Chamber should generally follow Appeals Chamber precedent unless there are compelling reasons to depart.
Critical analysis:
This nurtures legal certainty and equality before the law, but also raises the question: how flexible should the ICC be in revising earlier jurisprudence as understandings evolve?
Scenario 5: A Non-State Party Files a Retroactive Article 12(3) Declaration
State X, not a Party to the Statute, submits an Article 12(3) declaration in 2025 accepting ICC jurisdiction for crimes committed in 2019. An accused argues this is retroactive criminalisation.
Solution:
- The Court notes that the ICC Statute itself entered into force long before 2019 and defined the crimes clearly.
- It then examines whether, in 2019, the relevant conduct (e.g., genocide, torture, serious war crimes) was already criminal under customary international law or applicable treaties.
- If yes, nullum crimen is respected; the declaration merely provides a forum for prosecution, not new crimes.
Critical analysis:
This scenario highlights the fine line between retrospective jurisdiction and ex post facto law. Much turns on whether the conduct was already internationally criminal at the time.
Scenario 6: Security Council Referral of a Non-Party’s Situation
The Security Council refers State Y (a non-party) to the ICC for atrocities committed in 2010. A military leader claims he had no reason to know he was answerable to the ICC.
Solution:
- The Court assesses whether the alleged acts (e.g., deliberate attacks on civilians) were already recognised as war crimes under customary IHL and treaties like the Geneva Conventions.
- If they were, the leader had objective notice that his acts were criminal internationally.
- The SC referral creates jurisdiction, but does not invent new crimes.
Critical analysis:
Again, the legality concern is managed by anchoring convictions in pre-existing customary and treaty norms, not purely in the Rome Statute’s later application to that person.
Scenario 7: National Law Labels Conduct as “Terrorism,” ICC Calls It “Crimes Against Humanity”
A national court convicts an official for “terrorism” due to widespread attacks on civilians. Later, the ICC considers charging him with crimes against humanity based on the same incidents.
Solution:
- For admissibility and ne bis in idem, the ICC looks at conduct, not labels (Articles 17 & 20(3)).
- If the national trial was genuine, covering the same conduct, the ICC may have to decline to prosecute again, regardless of the label “terrorism” vs “crimes against humanity”.
Critical analysis:
This shows how the ICC’s conduct-based approach helps respect legality and avoid double jeopardy, even when domestic legal categories are different.
Scenario 8: Interpretation of a Crime in a Way That Might Discriminate
A Chamber considers interpreting the definition of “gender-based persecution” in a restrictive way that effectively excludes persecution of LGBTQ+ persons.
Solution:
- Article 21(3) requires the Court’s interpretation to be consistent with internationally recognised human rights and free from adverse distinction based on gender or other status.
- An interpretation that excludes LGBTQ+ persons from protection would likely be incompatible with evolving international human rights norms.
- The Court should adopt an inclusive interpretation.
Critical analysis:
Article 21(3) functions as a human-rights safety valve, pushing the ICC towards interpretations that reflect modern equality principles and prevent regressive results.
Scenario 9: Defence Argues Child Soldier Crime Not Customary at Time
An accused is charged with conscripting children under 15 in 2003. Defence argues that this specific war crime was not clearly established in customary international law by then.
Solution:
- The Court notes that by 2003, prohibitions on child soldiering were reflected in multiple treaties and widespread State practice, strongly suggesting customary status.
- It also emphasises that the conduct was defined with sufficient precision in the Statute before 2003.
- Therefore, legality is satisfied.
Critical analysis:
This echoes Lubanga: as long as the crime was clearly defined and broadly recognised before the acts, concerns about legality are mitigated.
Scenario 10: Deriving a General Principle on Corporate Criminal Responsibility
The ICC faces a question about whether legal persons (companies) can be directly criminally liable, something the Statute is silent about.
Solution:
- The Court examines Article 25, which speaks of “persons” but is framed in terms of natural persons.
- It checks whether there is a genuine lacuna: did the drafters intentionally exclude corporate liability?
- Given the drafting history and text, it’s likely not a lacuna but a deliberate choice.
- The Court should not import a general principle from domestic systems to create corporate criminal liability at the ICC.
Critical analysis:
This underlines the limits of Article 21(1)(c): general principles derived from national laws cannot be used to override clear structural choices in the Rome Statute.
7. Conclusion
Under International Criminal Law, the ICC’s applicable law regime in Article 21:
- Puts the Rome Statute at the centre,
- Allows carefully controlled use of treaties, custom, and general principles only where there are real gaps,
- Is constrained by human rights and non-discrimination, and
- Interacts closely with the principle of legality, especially when jurisdiction is extended to non-State Parties.
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