Definition
Rules of international law governing when a state may lawfully resort to armed force against another state.
TERMINOLOGY NOTE
Distinction: jus ad bellum governs resort to force; jus in bello governs conduct in armed conflict regardless of that resort's legality.
Theoretical Context
The UN Charter generally prohibits threats or uses of force between states, with recognised exceptions including self-defence under Article 51 and Security Council authorisation. Necessity and proportionality shape self-defence analysis. The legality of resorting to force must not be conflated with the rules governing hostilities.