The proportionality principle is on one level the easiest to understand, but on another level it is the most difficult. On the first level, the principle of proportionality simply says that the amount of force used as a tactic of war must be neither too strong nor too weak for the task to be accomplished. If a single enemy marksman is firing at two of one's own soldiers, it is not allowed to bomb an entire company of soldiers to stop the one shooter. On the second level, the problem is to identify the relevant ways in which tactics and tasks are to be related and how tasks themselves are to be individuated. The second level is enormously difficult. In this chapter, I will propose a relatively new way to think about the jus in bello principle of proportionality that sees it as intimately entwined with the principle of humane treatment.

Proportionality does not merely say that tactics must not cause more suffering than they prevent. Proportionality is only an attenuated utilitarian principle, for there is a core concern that sets a fairly rigid moral limit. There must be no morally less costly way to accomplish a military objective that is thought to be necessary.