Definition and significance of the term Casus belli
The term Casus belli originates from Latin and literally means “case of war” or “cause for war.” In international law, Casus belli describes a circumstance or event that is put forward by a state as justification for initiating military force against another state. The Casus belli generally forms the alleged legal or factual basis for the commencement of a military conflict.
Terminological classification
Casus belli is both a political and legal term that plays a central role in modern international law when assessing the permissibility and legitimacy of the use of force by states. What constitutes a Casus belli in each individual case depends on the actual events as well as the legal evaluation by the international community and relevant organs of international law.
Historical development and classification under international law
Development in classical international law
Until well into the 20th century, so-called classical international law distinguished between “just” and “unjust” wars (bellum iustum). States sought to present a Casus belli that would convince the international community of the necessity and legality of their actions. Typical examples of recognized Casus belli included border violations, attacks on nationals, breaches of treaties, or the support of hostile activities by another state.
Modern regulations in international law
With the progressive codification of the prohibition on the use of force within the framework of the United Nations Charter (UN Charter), particularly through Article 2(4), the justification for the use of force in interstate relations has been significantly restricted. Today, a Casus belli can be recognized under international law only in very narrowly defined exceptional cases.
The role of the United Nations Security Council
According to Article 39 et seq. of the UN Charter, only the Security Council is authorized to determine the existence of a threat to peace or an act of aggression and, if necessary, to take action. Unilateral claims of a Casus belli without the Security Council’s mandate run counter to the core principles of modern international law.
Legal basis for recognizing a Casus belli
The prohibition on the use of force and its exceptions
Article 2(4) of the UN Charter sets forth a general prohibition on the use and threat of force in international relations. Exceptions exist only in two narrowly defined scenarios:
Right of self-defense according to Article 51 UN Charter
According to Article 51 of the UN Charter, a state may exercise its right to individual or collective self-defense in the event of an armed attack, until the Security Council has taken the measures necessary to maintain international peace. In this scenario, a Casus belli can be present in the sense of a clearly recognizable armed attack.
Measures taken by the Security Council
On behalf of the Security Council, states may use force to the extent necessary to maintain or restore international peace and security.
Clarification by international jurisprudence
The International Court of Justice (ICJ) has repeatedly established that only an armed attack constitutes a Casus belli for self-defense. Cross-border incidents, economic coercive measures, or political tensions do not currently qualify as a recognized Casus belli according to prevailing jurisprudence.
Examples of Casus belli in the international community
Historically, states have often invoked various rights or obligations to justify acts of war. Classic examples are:
- Protection of own nationals abroad (humanitarian interventions, when covered by the UN)
- Direct armed attack on own national territory
- Breach of a treaty with security-relevant consequences
- International provocations, such as terrorist acts, where state support can be proven
In light of modern international law, however, many reasons asserted in the past are no longer recognized as legitimate Casus belli.
Criticism and misuse of the term Casus belli
Political instrumentalization
The term Casus belli is often also misused to generate domestic approval for military actions or to circumvent international legal constraints. Thus, the pretextual claim of an attack or threat as a Casus belli can be specifically employed to justify interventions that are actually contrary to international law.
Assessment by the international community
The assessment of a Casus belli is now primarily carried out by collective bodies of the international community, particularly the United Nations Security Council. The consent of this body is crucial for the conformity of military measures with international law.
Distinction from other terms
The Casus belli is to be distinguished from such terms as Bellum iustum (just war), Ultimatum or conflict-preventing measures such as sanctions and diplomacy. While an ultimatum still offers the possibility to avoid an armed conflict, Casus belli refers to the actual reason or pretext for entering a war.
Summary
In contemporary international law, the Casus belli describes the justifying reason for the use of military force between states. Through the codification of the prohibition on the use of force in the UN Charter, the recognition of a Casus belli is limited to a few, clearly defined cases—primarily self-defense. The legitimacy of a Casus belli today depends less on the subjective view of the affected state and more on collective standards set by the international community and the relevant institutions of international law. Abusive reliance on a Casus belli is not tenable under international law and may result in international sanctions or countermeasures.
Frequently asked questions
When is a Casus belli considered legitimate in the context of international law?
In the context of international law, a Casus belli is considered legitimate if it conforms to the normative requirements of the international legal order, in particular the Charter of the United Nations. According to Article 2(4) of the UN Charter, the use of force is generally prohibited; exceptions exist only under Chapter VII by decision of the Security Council (collective security) or in the case of individual or collective self-defense pursuant to Article 51. A legitimate Casus belli therefore arises only where there is tangible evidence of an armed attack and the party under attack must immediately resort to military means to maintain its integrity. Preventive strikes, vaguely defined threats, or political motives are not sufficient to constitute a lawful reason for war under international law. The principles of proportionality and necessity are of particular importance.
What role does the United Nations Security Council play in assessing a Casus belli?
The United Nations Security Council has primary responsibility for maintaining international peace and security. In reviewing a Casus belli, the Security Council examines whether a real threat or breach of the peace, or an act of aggression, exists (Article 39 UN Charter). Only upon determination of such a situation can the Security Council authorize mandatory measures against the aggressor, including giving a mandate for the use of force. Without such authorization, the use of force—except in cases of self-defense—remains legally inadmissible and may qualify as an act of aggression.
Can economic or political differences be recognized as a Casus belli?
Economic or political disagreements are generally not recognized as a Casus belli under contemporary international law. The UN Charter prohibits the use of force in disputes that do not constitute a direct armed attack. Such conflicts are to be resolved primarily through diplomatic, peaceful means such as negotiations, mediation, or arbitration (Article 33 UN Charter). Only in extremely rare, well-documented exceptional cases, where economic measures as part of hybrid threats escalate into an armed attack, could a reevaluation take place—but this has not yet been explicitly recognized under international law.
Can a Casus belli be reviewed retrospectively and how is such a review conducted?
A Casus belli can always be subjected to review under international law, in particular by international courts such as the International Court of Justice (ICJ). The review is based on objective criteria, such as the plausibility of the stated facts, compliance with the principle of proportionality, and whether the claimed self-defense actually resulted from an armed attack. The documentation of events retrospectively claimed as grounds for war is critically scrutinized. False, pretextual, or fabricated incidents (so-called “false flag” operations) result in the illegitimacy of the war’s justification under international law and may lead to sanctions against the aggressor state.
What are the legal consequences if a Casus belli is recognized as unlawful?
If a Casus belli is recognized as unlawful, the resulting use of force constitutes an act of aggression. This leads to significant consequences under international law, such as international condemnation, diplomatic measures, sanctions, and possible prosecution of those responsible before international criminal courts, in particular the International Criminal Court (ICC) on the basis of the Rome Statute. In addition, the injured state may, under certain conditions, claim reparations under international law.
Are there special provisions regarding Casus belli in humanitarian international law?
Humanitarian international law, in particular the Geneva Conventions and their Additional Protocols, does not directly regulate the permissibility of a Casus belli, but rather sets out norms for the conduct of parties during armed conflict. However, ius ad bellum, i.e., the right to go to war, is presupposed by these norms and its lawfulness is not assessed by them. This means that even a war that has been unlawfully initiated nevertheless binds the parties to humanitarian international law (“Geneva rights stick to the bayonet”); the legitimacy review of a Casus belli falls under general international law, not humanitarian crisis law.
