Article 51 of the Charter provides the following:

Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security’.

An armed attack is the prerequisite for the exercise of the right of self-defence, according to article 51 of the UN Charter. However, no certain definition exists for the concept of ‘armed attack’ under the jus ad bellum.

Even the International Court of Justice in the 1986 Nicaragua judgment asserted that

‘a definition of the ‘armed attack’ which, if found to exist, authorizes the exercise of the ‘inherent right’ of self-defence, is not provided in the Charter and is not part of treaty law.’

However, the ICJ adds further on:

‘There appears to be general agreement on the nature of the acts which can be treated as constituting armed attacks. In particular, it may be considered to be agreed that an armed attack must be understood as including not merely action by regular armed forces across an international border, but also “the sending by or on behalf of a State of armed bands, groups, irregulars or mercenaries, which carry out acts of armed force against another State of such gravity as to amount to” (inter alia) an actual armed attack conducted by regular forces, “or its substantial involvement therein”. This description, contained in Article 3, paragraph (g), of the Definition of Aggression annexed to General Assembly resolution 3314 (XXIX), may be taken to reflect customary international law.’

It can be easily observed that the quoted abovementioned definition is now verbatim reproduced as one of the acts of the crime of aggression.

As Yoram Dinstein aptly comments,

‘the phrase “armed attack” is not equivalent to “aggression” (a much broader and looser term, used, e.g. in article 39 pertaining to the powers of the Security Council). An armed attack is actually a particular type of aggression. This is bourne out by the French text, which speaks of “une agression armée”.’

Consequently, given that it has been established supra that cyber warfare operations can fall under the definition of the international crime of aggression, cyber attacks can subsequently constitute an armed attack as well, and thus giving rise to the lawful right of self-defence.