The nature of modern armed conflicts, combined with traditional interpretations of proportionality, poses serious challenges to the jus ad bellum goal of limiting and controlling wars. In between the jus ad bellum focus on decisions to use force, and the international humanitarian law (IHL) regulation of specific attacks, there is a far-reaching space in which the regulatory role of international law is bereft of much needed clarity. Perhaps the most striking example is in relation to overall casualties of war. If the jus ad bellum is understood as applying to the opening moments of the conflict, then it cannot provide a solution to growing numbers of casualties later in the conflict. Moreover, if it does not apply to non-international armed conflicts, then it is of little use in relation to alleviating the suffering of war for a vast proportion of conflicts in the past half a century and more. IHL is equally unsuited for dealing with overall casualties, as it may be the case that each individual attack is proportionate, but the cumulative number of civilians being killed is slowly rising to intolerable figures. A similar problem arises with regard to assessing other forms of accumulated destruction. This article sets out a new approach to proportionality in armed conflict and the regulation of war. It advocates for a principle of “strategic proportionality,” stemming from general principles of international law and reflected in state practice, and which requires an ongoing assessment throughout the conflict balancing the overall harm against the strategic objectives. The article traces the historical development and aims of the principle of proportionality in war, sets out the scope and aims of strategic proportionality, and provides an analysis of how such a principle can be operationalized in practice.