Abstract Finding the western just war criterion of reasonable chance of success to be a contribution to ethical decision making about armed conflict requires dealing with a number of critiques. Specifying ?probability? rather than the alternatives ?hope? or ?chance?, and raising standards of evidence involved, makes the term less vague. Expanding the concept of ?success? to include morally defensible aims that can be achieved without military victory enriches the understanding of the moral relationship between ends and means in armed conflict. (...) Asking decision makers to accept moral responsibility for the costs of possible failure is a unique contribution to the just war criteria. The enriched concept of reasonable probability of success thus offers morally significant insights to prewar jus ad bellum decisions, and can benefit ethical decision-making about whether to continue once fighting has begun. (shrink)