1.3.1 Criteria for self-defence
In order to lawfully exercise the right to self-defence, a state must be able to demonstrate that it has been a victim of an armed attack. The burden of proof in such a case lies with the state seeking to justify the use of force in self-defence. Nevertheless, not all attacks will constitute an armed attack for the purposes of Article 51: only the most grave forms of attack will qualify (Nicaragua Case, para.191).
Furthermore, the ICJ held in the Nicaragua Case (Merits) that ‘self-defence would warrant only measures which are proportional to the armed attack and necessary to respond to it’ (para. 176). This statement sets out two important principles in international law concerning the use of force: the principle of proportionality and the principle of necessity. In this context, proportionality means that the response to an armed attack must be reflective of the scope, nature and gravity of the attack itself. On the other hand, the principle of necessity guards against the use of measures which are excessive and not necessary in response to an armed attack.
The meaning of ‘armed attack’ causes significant controversy in international law. In the Nicaragua Case and in Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory Advisory Opinion ICJ Rep 2004, the ICJ rejected the idea that an armed attack may include ‘not only acts by armed bands where such acts occur on a significant scale but also assistance to rebels in the form of the provision of weapons or logistical or other support’(Nicaragua Case, para.195). In other words, it is necessary to show that an armed attack is attributable to a state.
In the Nicaragua Case, Judge Higgins strongly opposed this view and argued that the act involving the use of force from actors other than a state, such as groups of insurgents or terrorist groups, may give rise to the exercise of the right of self-defence by the attacked state. This statement highlights a very contentious issue in modern international relations, namely the use of force in self-defence against non-state actors.