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 (that is, 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. Traditional just war theory (...) and self-proclaimed “revisionist” just war theory think otherwise since the former focuses on the law enforcement or public authority justification for inflicting harm and the latter on the self-defense justification. These are both intrinsically asymmetrical justifications: there is no justified self-defense (properly understood) against justified self-defense, nor is there justified law-enforcement against justified law-enforcement. However, there can, as I will show, be justified self-defense against force that is justified by a necessity justification, and there can be force justified by a necessity justification being used against force that is also justified by a necessity justification. The necessity justification is not intrinsically asymmetrical, and it is an indispensable justification in the context of war. Moreover, 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). (shrink)