Dwight D. Eisenhower, U.S. Army General who commanded the Allied invasion of continental Europe, stated “Nothing can stand against the argument of military necessity”. This highlights the significance of the principle of military necessity during armed conflicts, which forms an integral part of the modern international humanitarian law (hereafter IHL). Military necessity (hereafter MN) consists of “measures which are indispensable for securing the ends of the war, and which are lawful according to the modern law and usages of war.” Thus, the principle of the MN permits necessary measures to target a legitimate military objective without undermining other principles of IHL, most importantly, the principle of proportionality.
The rationale behind this contribution is to examine firstly, the prerequisites for legitimization of Computer Network Attacks (hereafter CNAs) as military objectives, and, secondly, the issue of collateral damage raising from the dual-use of computer networks i.e. concurrent civilian and military use.
Only military objectives shall be attacked
Computer network refers to an information infrastructure, wired or wireless, that facilitates exchange of data between computers. The operations to disrupt, deny, degrade, or destroy information resident in computer and computer network through cyber or kinetic means, may be described as a CNAs. Computer networks may constitute a military objective and the attacks on it are permitted insofar as the two conditions of Article 52(2) of Additional Protocol – I, 1977 (hereinafter ‘’AP I’’) are satisfied: firstly, the computer networks by its nature, location, purpose, or use must make an effective contribution to the military action of the adversary and secondly, the attacks on the computer networks must offer a definite military advantage to the attacker. In cases of dual-use, due to the increase in military communications and traffic which travels in tandem across the civilian network, the entire network may effectively contribute to the military action of the adversary. It is impossible to determine with certainty as to which routes any military data packets will take and the entire network may be classified as a military objective. Nevertheless, if a small part of the network provides the same military advantage when compared to the destruction of the whole structure, the CNAs should be directed towards the smaller part of the network without paralyzing the entire structure. In order to be in line with IHL principles, the military advantage must be concrete rather than speculative and refers to the advantage gained from an attack as a whole, not from its isolated or particular parts.
The perplexing case of CNAs
The principle of distinction necessitates distinguishing between civilian objects and military objectives and accordingly direct the operations only towards military objectives. Further, in case of doubt whether a computer network which is normally dedicated to civilian purposes is being used to make an effective contribution to military action, it shall be presumed not to be used in that manner. The attack on the dual use infrastructure which allows large amounts of military communications to be sent across civilian networks, may lead to disproportionate civilian damage. This mandates a comparison between definite military advantage on the one side and the collateral civilian damage on the other side.
The issue of collateral damage
Collateral damage is prohibited if an attack causes a significant incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which is excessive in relation to the concrete and direct military advantage anticipated. When a dual-use computer networks is attacked, it exposes the risk of significant incidental loss due to the knock-on effects of attacks i.e. which cause more harm to civilians than the direct effect of the attack itself. One of the generally accepted rule in IHL is the need to avoid or at the least minimize incidental damage to civilians and civilian objects. Hence, a careful analysis is required as the civilians are unaware of the system being intertwined and the impact on the civilians must not be excluded from the proportionality calculation.
CNAs are often seen as a case of excessive collateral damage due to their overwhelming effect on the civilian cyber infrastructure which is intertwined with the military use. Prior to conducting CNAs, it is mandatory to meticulously analyze whether there exists an effective contribution to the military action of the adversary and also the concrete military advantage of the attacker vis-a-vis the incidental loss for civilians due to the knock-on effects. Thus, military commanders must consult network specialists to ascertain the level of the effect and must take all feasible precautions to minimize or avoid incidental damage to civilians.
 General Dwight Eisenhower, Order of the Day, Dec. 24, 1943, Report of the American commission for the protection and salvage of Artistic monuments in War areas (1946), p. 48-49.
 International Humanitarian Law (IHL) is applicable in times of an armed conflict to prevent unnecessary or superfluous injury by limiting the means and methods of warfare. It further ensures that the civilians and civilian objects are protected at all times.
 Instructions for the Government of Armies of the United States in the Field (Lieber Code) (24 April 1863), Article 14.
 Michael N. Schmitt (ed.), Tallinn Manual on the International Law Applicable to Cyber Warfare (Cambridge University Press, Cambridge 2013) (Tallinn manual), p.258.
 Knut Dormann, ‘Computer network attack and international humanitarian law’, Cambridge review of international affairs (2001).
 Tallinn manual, Rule 38.
 Ibid; Jean-Marie Henckaerts et al (eds), Customary International Humanitarian Law, Vol I: Rules (Cambridge University Press 2005) (ICRC Study) Rule 8.
 Heather Dinniss, ‘The nature of objects: Targeting networks and the challenge of defining cyber military objectives’, 48 Isr.L.Rev. 39 (2015), p.53.
 Yves Sandoz et al, Commentary on the Additional Protocols of 8 June 1977 to the Geneva Conventions of 12 August 1949 (Martinus Nijhoff 1987) ¶ 2226-2228.
 Bothe et al, Commentary on the Two 1977 Protocols Additional to the Geneva Conventions of 1949 (Martinus Nijhoff 1982), p.326.
 Heather Dinniss, Cyber Warfare and the Laws of War (Cambridge University Press 2012), p.191.
 ICRC Study, Rule 7
 Protocol Additional to the Geneva Conventions of 12 August 1949, and Relating to the Protection of Victims of International Armed Conflicts (AP I), Article 52 (3).
 Ibid, Article 51 (5) (b); ICRC Study, Rule 14.
 Christopher Greenwood, ‘The Law of Weaponry at the Start of the New Millenium’ (Naval War College, 1998), p. 202.
 AP I, Article 57 (2) (a) (ii).