The Just War principle of jus ad bellum explicitly requires a reasonable prospect of success; the prevailing view about personal self-defence is that it can be justified even if the prospect of success is low. This chapter defends the existence of this distinction and goes on to explore the normative basis of this difference between defensive war and self-defence and its implications. In particular, the chapter highlights the rationale of the ‘success condition’ within Just War thinking and argues that this (...) condition is grounded in assumptions about the nature of political authority and responsibility that do not apply to personal self-defence. This important difference is commonly ignored or side-lined by contemporary theorists who regard the principles of these two types of conflict as very closely aligned and the moral differences between them as matters of degree only. (shrink)