As explained in the Introduction, the present study analyses the armed attack requirement from a threefold perspective: ratione materiae – what acts count as armed attacks?; ratione temporis – when does an armed attack take place?; and ratione personae – from whom must the attack emanate? The present chapter will begin by focusing on the ratione materiae aspect, making abstraction of the temporal and personal aspects.

As recent controversies have generally concerned the legality of self-defence against non-State actors (the ratione personae element) and of anticipatory self-defence (the ratione temporis element), one might be inclined to conclude that the ratione materiae element is the least problematic of the three. Still, it raises important matters which are crucial for the evaluation of any self-defence claim. Against this backdrop, a first section will examine to what extent the UNGA Definition of Aggression is determinative of the scope of lawful self-defence (Section 3.1). Second, we will examine whether there exists some sort of de minimis threshold, i.e., a minimum level of scale and effects required for an attack to qualify as an ‘armed attack’ in the sense of Article 51 UN Charter. To this end, we will consider the distinction between ‘armed attacks’ and ‘mere frontier incidents’, and examine the relevance of the intent of the attacker, as well as the ‘cumulative’ impact of repeated attacks (Section 3.2).