Free tool · Public International Law
When Is the Use of Force Lawful Under International Law? An Analyzer
Article 2(4) of the UN Charter prohibits the threat or use of force against the territorial integrity or political independence of any state, and the prohibition is also customary law. The recognised exceptions are narrow: a Security Council authorisation under Chapter VII, the valid consent of the territorial state, and self-defence under Article 51 in response to an armed attack, which must be necessary and proportionate and be reported to the Council. Other claims are contested. Self-defence against non-state actors on another state’s territory, anticipatory self-defence against an imminent attack, and humanitarian intervention without authorisation are debated and are not generally accepted in the same way as the core exceptions.
Tool · Use of Force Analyzer
The force is likely lawful self-defence, subject to continuing necessity and proportionality, and to the Security Council acting to restore peace.
Use of force: Armed force is used against another state’s territory. Article 2(4) prohibits it unless an exception applies.
Armed attack: An armed attack has occurred. Only an armed attack of sufficient gravity (not a minor border incident) triggers the right of self-defence (Nicaragua v United States).
Attack by a state: The attack is attributable to a state, so force against that state may be justified if the other conditions are met.
Necessity: Force is necessary: peaceful means would not have stopped the attack.
Proportionality: The response is proportionate to the attack and aimed at repelling it.
Immediacy: The response is reasonably prompt.
Report to the Security Council: Measures taken in self-defence have been reported to the Security Council, as Article 51 requires.
- Article 2(4) of the UN Charter forbids the threat or use of force against the territorial integrity or political independence of any state. The prohibition is also customary and, in its core, is widely regarded as a peremptory norm. The exceptions are narrow: Security Council authorisation (Chapter VII), self-defence (Article 51), and valid consent. Whether other grounds exist is disputed.
- This tool is a simplified exercise with invented facts. Real cases turn on detailed evidence, the interpretation of contested rules and the practice of states, and the law continues to develop.
Applies the prohibition on the use of force in Article 2(4) of the UN Charter and its exceptions: Security Council authorisation, consent and self-defence (an armed attack, necessity, proportionality, immediacy and reporting), and shows where the law is contested. Invented facts; this is a simplified exercise, and real cases turn on evidence and contested law. A teaching aid, not legal advice. Runs in your browser; nothing is stored or sent.
How to use it
- Say whether armed force is used across a border.
- Tick any Security Council authorisation, consent or armed attack.
- Answer the questions of necessity, proportionality, immediacy and reporting.
- Read whether the force is lawful, contested or unlawful, and why.
Common questions
What counts as an armed attack?
An armed attack of sufficient gravity, not a minor border incident. The International Court has distinguished the most grave forms of the use of force from less grave ones.
Is preventive war lawful?
No, according to the prevailing view. A narrow right to act against an imminent attack is accepted by many states, but force against a possible future threat is widely held to be unlawful.
Does the tool decide a real case?
No. It uses invented facts and simplified rules. Real cases turn on evidence and contested law.
Learn the law behind the tool
This tool is part of our Public International Law course, with lectures, concept maps, flashcards, interactive exercises and a capstone case. Public International Law: Criminal Law and the Use of Force.
More free law tools
General information, simplified, mainly on American law, not legal advice. States and countries differ, and real cases turn on facts that a tool cannot capture.