I shall argue that in some wars both sides are (as a collective) justified, that is, they can both satisfy valid jus ad bellum requirements. Moreover, in some wars – but not in all – the individual soldiers on the unjustified side (i.e. on the side without jus ad bellum) may nevertheless kill soldiers (and also civilians as a side-effect) on the justified side, even if the enemy soldiers always abide by jus in bello constraints. The reason for this is that with regard to some forms of inflicting harm on others one may give special weight to one’s own interests and the interests of those to whom one has special responsibilities when assessing the proportionality of those acts. That is, the proportionality calculation may be agent-relative. This is in particular so in the case of foreseeably preventing innocent and non-threatening people from being saved (for instance, by shooting down a tactical bomber who would have saved them by destroying an ammunitions factory) but less so in the case of the intentional or foreseeable direct harming of innocent and non-threatening people (dropping bombs on people standing near an ammunitions factory). In the light of these considerations, I will then answer the question as to when soldiers may justifiably participate in war (and when not).
When may soldiers participate in war?
