The meaning of collective self-defence

The phrase ‘individual or collective self-defence’, as used in Article 51 of the Charter of the United Nations (see supra, ch. 7, A (a)), is not easily comprehensible. A close examination of the text, in light of the practice of States, shows that more than a simple dichotomy is involved. It seems necessary to distinguish between no less than four categories of self-defence: (i) individual self-defence individually exercised; (ii) individual self-defence collectively exercised; (iii) collective self-defence individually exercised; and (iv) collective self-defence collectively exercised.

The first category represents the most straightforward implementation of the right of self-defence, and it has been dealt with in ch. 8: Arcadia perpetrates an armed attack against Utopia, and in response Utopia invokes self-defence. This is a one-on-one encounter, and the right of individual self-defence is applied individually.

The second category relates to the situation where an armed attack is launched by the same aggressor (Arcadia), either simultaneously or consecutively, against several States (Utopia, Ruritania, etc.). Both Utopia and Ruritania are entitled to resort to measures of individual self-defence against Arcadia. These measures may still be taken individually, each victim State declining any suggestion of cooperation with the other. Utopia and Ruritania, while resisting armed attacks by the same aggressor (Arcadia), are not obliged to consolidate a united front.