This Article challenges the conventional moral justification for the international humanitarian law (IHL) principle of proportionality and the collateral damage that this principle permits in war. It argues that the Doctrine of Double Effect—the moral theory on which the principle of proportionality is based—cannot justify attacks that will cause foreseeable harm to “innocent” civilians. A person must do something morally relevant to forfeit his or her right to life. Accordingly, collateral damage to civilians can be morally justified only with respect to civilians who are “non-innocent.” The IHL principle of proportionality is thus morally flawed because it permits, as a legal matter, foreseeable harm to innocent and non-innocent civilians alike in certain circumstances.

At the same time, the principle of proportionality should not be entirely discarded. Despite its ethical shortcomings, it serves a broader humanitarian objective by limiting the scope of warfare. In particular, the principle helps to safeguard the expansive but fragile definition of “civilian” under existing IHL, as well as the related principle of distinction, which prohibits attacks directed against civilians. While the principle of proportionality provides only an intermediate degree of protection for civilians in armed conflicts, this protection is still an improvement over the total war paradigm that existed throughout much of the twentieth century. Responsible states, however, should understand that attacks considered lawful under the IHL principle of proportionality are not necessarily morally permissible, and these states should thus take additional measures to avoid causing incidental harm to innocent civilians.