Having dealt in Chapter 7 with diverse problems pertaining to the interpretation of the expression ‘armed attack’ (which constitutes the foundation of the right of self-defence under Article 51 of the Charter of the United Nations), it is now necessary to sketch the optional modes of self-defence available to a State facing an armed attack. At the outset of the discussion it should be pointed out that, for an armed attack to justify counter-measures of 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. Yet, in exceptional circumstances, an armed attack – although mounted from the territory of a foreign State – is not launched by that State. 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

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 legitimate use of counter-force. Like its corollary (armed attack), self-defence assumes more than one concrete form. The cardinal division, here as elsewhere when the use of force by States is at issue, is between war and measures ‘short of war’.