Casus Belli

Doctrine Peace and Conflict Preferred

Act or event cited by a State as justification for going to war.

Independent referenceNot an official or legal definition
Sources citedSee the sources at the end of this page
Status Under reviewLast updated 11 October 2026
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Definition

In plain language

A casus belli is the event or action that a country says gives it the right to start a war, such as an attack on its territory or a blockade.

Academic definition

In classical international law, States relied on just causes for war developed in just war theory, and a casus belli was the specific injury invoked to justify hostilities. Since Article 2(4) of the UN Charter prohibits the threat or use of force, only self-defence under Article 51 or Security Council authorisation provides lawful grounds, so the term now describes a political claim rather than a legal entitlement. Historical examples include the 1914 assassination at Sarajevo and the alleged Gulf of Tonkin attacks of 1964, both used to justify escalation.

Classification

Term type
Doctrine
Category
Peace and Conflict
Subject field
Law on the Use of Force
Subfield
Justification of War
Difficulty level
Intermediate
Grammatical category
Latin noun phrase

Language variants

Alternative terms
  • cause of war
  • occasion for war
Non-preferred terms
  • not_applicable
Abbreviations
  • not_applicable
Acronyms
  • not_applicable
Native terms
  • casus belli (Latin, 'occasion of war')
Transliteration
not_applicable
Pronunciation
KAH-sus BEL-eye

Context

Legal effect: Political

Scope

Covers claimed justifications for resort to war; the legal rules on when force is lawful are found in jus ad bellum, and the rules on conduct of hostilities are in jus in bello.

Legal context

UN Charter Articles 2(4), 39 and 51 now regulate the use of force. A State that cites a casus belli must still show that its action is self-defence or authorised by the Council; declaring a casus belli does not make force lawful.

Diplomatic context

Governments sometimes warn that an action by another State would constitute a casus belli to signal a red line, as with threats over blockades or closure of straits.

Institutional context

Statements appear in government declarations, parliamentary debates and Security Council speeches, and are assessed against the Charter by the Council and the ICJ.

Historical context

The concept comes from Roman and medieval just war thinking, and nineteenth-century diplomacy treated declared grievances as formal justifications.

Why it matters

Whether an event is claimed as a casus belli shapes escalation, alliance obligations and later legal arguments about the legality of a war.

Usage

Register: Academic

Usage note

Italicise the Latin phrase in formal writing; the plural is 'casus belli' without change.

Example

The minister warned that a blockade of the strait would be regarded as a casus belli.

Example scenario

State A announces that any attempt by State B to close a key shipping lane will be treated as a casus belli. State B then asks the Security Council to examine the statement as a threat of force under the Charter.

Common misuse

Treating a declared casus belli as a legal justification for war; modern law requires self-defence or Council authorisation.

Not to be confused with

  • Jus ad bellum (the legal framework)
  • Declaration of war
  • Armed attack under Article 51

Sources

  1. Charter of the United Nations (1945), United Nations, Article 2(4). un.org
  2. Official definition. not_found_in_source
  3. Treaty reference. un.org
  4. Gray, International Law and the Use of Force. not_found_in_source
  5. not_found_in_source. not_found_in_source

Source type: Treaty. Source reliability: Primary.

This entry is an independent reference definition, not an official or legally binding text. Where an official definition exists, it is linked above and is authoritative. Read our methodology.

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