Proportionality

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Definition of proportionality[edit | edit source]

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’.[1] 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.[2] 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’.[3]

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.[4] The decision must be made in good faith.[5]

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.[6] 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.[7] 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,[8] 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.[9]

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.[10] 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.[11] Finally, when different types of incidental civilian harm are anticipated, the harms must be assessed in combination, and not in isolation of each other.[12]

The ‘concrete and direct’ military advantage that is assessed is that which is ‘substantial and relatively close’.[13] Conversely, ‘advantages which are hardly perceptible and those which would only appear in the long term should be disregarded’.[14] Among others, the expected military advantage to be assessed cannot be merely speculative.[15] Additionally, advantages that are solely political, psychological, economic, financial, social, or moral in nature do not constitute ‘military advantage’ under the principle of proportionality.[16] 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’.[17]

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’.[18] At the same time, the determination of excessiveness also has an objective element since it ‘must be based on that of the “reasonable commander”’.[19]

Publicly available national positions that address this issue include: Common position of the African Union (2024) (2024), National position of Costa Rica (2023) (2023).

National positions[edit | edit source]

African Union (2024)[edit | edit source]

"51. [...] The African Union also emphasizes the importance of the principle of proportionality, which prohibits attacks that are expected to cause incidental civilian harm that would be excessive to what is necessary to achieve a definite military advantage."[20]

Costa Rica (2023)[edit | edit source]

"47. The principle of proportionality prohibits parties to armed conflicts from launching a cyber-attack against a military objective, which may be expected to cause incidental civilian harm that would be excessive in relation to the concrete and direct military advantage anticipated. In Costa Rica’s view, the incidental harm to be taken into consideration includes any incidental loss of functionality of civilian computers, systems or networks. For Costa Rica’s understanding of the notion of loss of functionality, refer to para. 20 of this position."[21]

Appendixes[edit | edit source]

See also[edit | edit source]

Notes and references[edit | edit source]

  1. Additional Protocol I, Article 51(5)(b). On the applicability of the proportionality principle in cyber space see: Ministry of Defense of France, International Law Applied to Operations in Cyberspace (9 September 2019); Federal Government of Germany, ‘On the Application of International Law in Cyberspace’ (March 2021) 9-10; Federal Department of Foreign Affairs, ‘Switzerland's position paper on the application of international law in cyberspace’ (May 2021) 9-10; Tallinn Manual 2.0, rule 113.
  2. ICRC Customary IHL Study, Rule 14. Tallinn Manual 2.0, rule 113.
  3. See ICRC, International humanitarian law and the challenges of contemporary armed conflicts, (2019) 16.
  4. Laurent Gisel, The principle of proportionality in the rules governing the conduct of hostilities under international humanitarian law: international expert meeting, 22-23 June 2016 (ICRC 2018) (Expert Report on Proportionality) 9, 52-53.
  5. Yves Sandoz, Christophe Swinarski and Bruno Zimmerman (eds), Commentary on the Additional Protocols (ICRC 1987) (Commentary on Additional Protocol I), para 1978; Emanuela-Chiara Gillard, Proportionality in the Conduct of Hostilities: The Incidental Harm Side of the Assessment (Chatham House 2018) 28.
  6. ICRC, International humanitarian law and the challenges of contemporary armed conflicts, (2015) 41. See, also, Emanuela-Chiara Gillard, Proportionality in the Conduct of Hostilities: The Incidental Harm Side of the Assessment (Chatham House 2018) (Proportionality in the Conduct of Hostilities) 13-18; Tallinn Manual 2.0, commentary to rule 113, paras 6–7.
  7. Expert Report on Proportionality 43-45; Proportionality in the Conduct of Hostilities 18-20.
  8. U.S. Department of Defense, Law of War Manual, June 2016 (revised December 2016) (DoD Law of War Manual), para 5.12.1.3.
  9. See, the Cyber Law Toolkit entry for ‘Attack (international humanitarian law)’ (‘In the assessment of what constitutes the ‘reasonably expected’ effects of an operation that have to be considered, some States, including Denmark, Finland, New Zealand, Norway, Switzerland, or the United States, have clarified that this includes harm due to the foreseeable direct and indirect (or reverberating) effects of an attack’).
  10. Tallinn Manual 2.0, commentary to rule 92, paras 10–12; ICRC, International Humanitarian Law and Cyber Operations during Armed Conflicts: ICRC position paper (November 2019) 7-8.
  11. Tallinn Manual 2.0, commentary to rule 113, para 5.
  12. Additional Protocol I, Article 51(5)(b).
  13. Commentary on Additional Protocol I, para 2209.
  14. Commentary on Additional Protocol I, para 2209. For a discussion of different State interpretations of this requirement see ICRC Customary IHL Study, Rule 14.
  15. William H. Boothby, The law of targeting (Oxford University Press 2012) 94-95.
  16. International Law Association Study Group, Final Report: The Conduct of Hostilities and International Humanitarian Law Challenges of 21st Century Warfare, 25 June 2017, 31.
  17. Expert Report on Proportionality 13.
  18. Commentary on Additional Protocol I, para 2210.
  19. Final Report to the Prosecutor by the Committee Established to Review the NATO Bombing Campaign Against the Federal Republic of Yugoslavia’ [50]. See also Expert Report on Proportionality 52.
  20. African Union Peace and Security Council, "Common African Position on the Application of International Law to the Use of Information and Communication Technologies in Cyberspace" (29 January 2024) 8.
  21. Ministry of Foreign Affairs of Costa Rica, "Costa Rica's Position on the Application of International Law in Cyberspace" (21 July 2023) 13 (footnotes omitted).

Bibliography and further reading[edit | edit source]