Indiscriminate attacks and the proportionality rule: what is incidental civilian harm?

Abstract Proportionality in the law of armed conflict imposes an absolute ceiling on the level of incidental civilian harm that an attack may be expected to cause. Despite its importance, surprisingly little attention has been paid so far to the notion of ‘incidentality’, in other words, to what it means for civilian harm to be incidental in nature. According to what we might call the Lesser Effect Thesis, civilian harm qualifies as incidental only if it is a mere side consequence of the attack, rather than its main or prevalent effect. Proponents of this approach suggest that where an attack is expected to affect a larger number of civilians or civilian objects compared to the number of military objectives, the civilian harm would become prevalent and therefore cannot be considered incidental in nature. The proportionality rule would not apply in these circumstances, and conducting such an attack would be outright indiscriminate. The purpose of this article is to explain why this interpretation amounts to a radical re-reading of proportionality that does not reflect the law, contradicts State practice, and is operationally unsound. Applying the rules of treaty interpretation, the article shows that civilian harm is incidental in nature when it constitutes the fortuitous and undesired consequence of an attack directed against a military objective and that the term ‘incidental’ as used in the law of armed conflict does not imply a relationship of proportion.