Article 51 of the UN Charter recognizes the inherent right of individual or collective self-defense if an armed attack occurs, until the Security Council takes measures necessary to maintain international peace and security. A state invoking self-defense must notify the Council. The Charter’s proportionality principle concerns the scale and scope of force needed for the defensive purpose; it is distinct from proportionality in an individual attack under IHL.
Self-defense is limited by purpose and necessity
Jus ad bellum proportionality asks whether the overall force used is limited to what is necessary to halt or repel the armed attack and address the threat. It is not a casualty-for-casualty calculation. Necessity does not mean that every conceivable non-military option must first be exhausted; it requires that force respond to the armed attack and be needed for the defensive aim.
These principles do not provide automatic authority to dismantle an adversary’s governance structure. The ICRC’s customary rule on proportionality in attack applies to individual attacks under IHL. The defensive objective and the circumstances determine whether particular force is necessary and proportionate.
IHL still governs each attack
Even when a state acts in self-defense, IHL governs how it fights. Each attack must distinguish military objectives from civilians and civilian objects, avoid expected incidental harm excessive in relation to the concrete and direct military advantage anticipated, and take feasible precautions. Article 57 of Additional Protocol I sets out key precautions. Compliance with jus ad bellum does not establish compliance with IHL, and vice versa.
For the separate attack-level rule, see proportionality in an attack. For Israel’s review and legal limits, see how Israel reviews targeting decisions.