Abstract
Armed attacks in the sense of Article 51 and customary international law can be carried out not only by States, but also by non-State actors (NSA). Self-defensive responses to attacks by NSAs, due to their specific characteristics, may contain a combination of elements of anticipatory self-defence and defensive reprisals. If a State, from the territory of which an attack originates, is unable to prevent an NSA attacking a third State, it has to ask for and to accept the assistance of the victim State, other States, or relevant international organisations. The latter cannot use military force in self-defence in such circumstances without having first sought the consent of the territorial State. However, if a State is unwilling to prevent an NSA, operating from its territory, from attacking third States, it becomes an accessory-after-the-fact to armed attacks of the NSA. Self-defensive, either individual or collective, measures can be carried out on the territory of such a State even without its consent.
Self-defence against Armed Attacks by Non-State Actors
