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to successfully mount a defence is context-dependent; more force may be necessary, or less force may be sufficient, to repel the attack or defeat one that is imminent. In addition, there is no requirement that the defensive force be of the same nature as that constituting the armed attack. Therefore, a cyber use of force may be resorted to in response to a kinetic armed attack, and vice versa. 6. The proportionality requirement should not be overstated. It may be that the originator of the cyber armed attack is relatively invulnerable to cyber operations. This would not preclude kinetic operations in an effort to compel the attacker to desist, although they must be scaled to that purpose.
Rule 15 – Imminence and immediacy The right to use force in self-defence arises if a cyber armed attack occurs or is imminent. It is further subject to a requirement of immediacy. 1. Textually, Article 51 of the United Nations Charter refers to a situation in which ‘an armed attack occurs’. Clearly, this covers incidents in which the effects of the armed attack have already materialized, that is, when the cyber armed attack has caused, or is in the process of causing, damage or injury. It also encompasses situations in which a cyber operation is the first step in the launch of an armed attack. The paradigmatic case involves cyber operations directed against another State’s air defences to ‘prepare the battlefield’ for an air campaign. 2. The majority of the International Group of Experts took the position that even though Article 51 does not expressly provide for defensive action in anticipation of an armed attack, a State need not wait idly as the enemy prepares to attack. Instead, a State may defend itself once the armed attack is ‘imminent’. Such action is labelled ‘anticipatory selfdefence’.40 This position is based on the standard of imminence articulated in the nineteenth century by US Secretary of State Webster following the Caroline incident. In correspondence with his British counterpart, Lord Ashburton, regarding a British incursion into American territory to attack Canadian rebels during the Mackenzie Rebellion, Webster opined that 40
For support regarding the notion, see Derek W. Bowett, Self-Defence in International Law 188–9 (1958). Bowett finds support for this in the travaux of the Charter’s drafting committee. Ibid. at 182 (quoting Report of the Rapporteur of Committee I to Commission I, 6 U.N.C.I.O. 459 (13 June 1945)).