Proportionality

Definition of proportionality
 {| class="wikitable mw-collapsible" style="background-color:#ffffcc;" ! scope="col" style="background-color:#ffffaa;"| Proportionality
 * The principle of proportionality prohibits attacks ‘which may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated’. The principle of proportionality is codified in Articles 51(5)(b) and 57(2)(a)(iii)(b) of the 1977 Additional Protocol I and reflects customary international law.  The nature of the principle makes it relevant only to attacks directed at military objectives or persons who are lawful targets, where incidental civilian loss of life, injury, damage to civilian objects, or a combination thereof, is expected. These three types of harms are commonly referred to as ‘incidental civilian harm’.
 * The principle of proportionality prohibits attacks ‘which may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated’. The principle of proportionality is codified in Articles 51(5)(b) and 57(2)(a)(iii)(b) of the 1977 Additional Protocol I and reflects customary international law.  The nature of the principle makes it relevant only to attacks directed at military objectives or persons who are lawful targets, where incidental civilian loss of life, injury, damage to civilian objects, or a combination thereof, is expected. These three types of harms are commonly referred to as ‘incidental civilian harm’.

The principle of proportionality is ex ante in nature, as it demands a balancing of the expected civilian harm and the anticipated military advantage. A proportionality assessment must therefore be made in advance of an attack and cannot be judged based on hindsight. The assessment must be made on the basis of a ‘reasonable military commander’s’ assessment of the information which is reasonably available from all sources at the relevant time. The decision must be made in good faith.

The ICRC has expressed the view that all direct and indirect incidental civilian harm that is foreseeably caused by the attack must be taken into consideration in the proportionality assessment. Direct harm relates to consequences that are directly and immediately caused by a cyber attack. All other harms are considered indirect harms; sometimes referred to as the ‘reverberating’ effects of an attack. For example, if it is reasonably expected that a cyber attack against a power grid will cause deaths in a hospital emergency ward due to a lack of power, those deaths must be part of the proportionality assessment. While one military manual claims the assessment of incidental civilian harm is generally understood to be limited to immediate or direct harm, most of them do not limit the assessment in this way and a number of manuals and other relevant official State documents expressly require the consideration of indirect effects.

When considering what constitutes ‘damage’ to civilian objects, some have argued that the damage does not have to be physical, but may include loss or deprivation of functionality. However, Tallinn Manual experts agreed that damage must go beyond inconvenience, irritation, stress, or fear since these consequences do not amount to incidental loss of civilian life, injury to civilians, or damage to civilian objects. Finally, when different types of incidental civilian harm are anticipated, the harms must be assessed in combination, and not in isolation of each other.

The ‘concrete and direct’ military advantage that is assessed is that which is ‘substantial and relatively close’. Conversely, ‘advantages which are hardly perceptible and those which would only appear in the long term should be disregarded’. Among others, the expected military advantage to be assessed cannot be merely speculative. Additionally, advantages that are solely political, psychological, economic, financial, social, or moral in nature do not constitute ‘military advantage’ under the principle of proportionality. When ratifying Additional Protocol I, a number of States explained that they consider the military advantage from an attack to refer to the ‘advantage anticipated from the attack as a whole and not only from isolated or particular parts of an attack’.

When assessing whether the incidental civilian harm will be excessive to the attack’s anticipated concrete and direct military advantage, determining ‘excessiveness’ entails a subjective assessment that allows for a ‘fairly broad margin of judgement’. At the same time, the determination of excessiveness also has an objective element since it ‘must be based on that of the “reasonable commander”’.