639. Having dealt in Chapter 7 with multiple problems pertaining to the interpretation of the expression ‘armed attack’, which constitutes the mainstay of the right of self-defence under Article 51 of the Charter of the United Nations (quoted supra 498), it is now necessary to sketch the optional modalities of self-defence available to a State subjected to an armed attack. At the outset of the discussion it should be recalled (see supra 594 et seq.) that, for an armed attack to open the way for self-defence under Article 51, it need not be committed by another State. Ordinarily, the perpetrator of the armed attack is indeed a foreign State as such. Still, in exceptional circumstances, an armed attack – although mounted from the territory of a foreign State – is not launched by that (or any other) State, but by non-State actors. Whether an armed attack is initiated by or only from a foreign country, the target State is allowed to resort to self-defence by responding to unlawful force with lawful counter-force. Given, however, the different features of the two types of armed attack, they will be addressed separately.

Self-defence in response to an armed attack by a State

640. The expression ‘self-defence’, as used in Article 51 or in customary international law, is by no means self-explanatory. It is a tag attached to the lawful use of counter-force. Like its corollary (armed attack), self-defence may assume more than one concrete form. The principal division, here as elsewhere when the use of force by States is at issue, is between war and measures ‘short of war’.