Christian just war theory, developed first by Augustine of Hippo and systematized more fully by Thomas Aquinas in the Summa Theologiae, holds that warfare, though always an evil to be regretted, can be morally justified when a specific set of conditions is met, traditionally organized under two headings. Jus ad bellum, the justice of going to war, requires a legitimate authority to declare it rather than a private individual or faction, a just cause such as defense against aggression or the righting of a serious wrong, right intention aimed at restoring peace rather than mere conquest or hatred, war as a genuine last resort after peaceful alternatives are exhausted, and a reasonable prospect of success, since a hopeless war causes suffering without achieving any of its stated justice.
Jus in bello, the justice within war, governs conduct during the fighting itself, chiefly through the principles of discrimination, protecting non-combatants from direct attack, and proportionality, ensuring the force used and the harm caused do not exceed what the just cause requires. The framework was developed as Christian theology's attempt to hold together a scriptural tradition that includes both pacifist strands and accounts of divinely sanctioned warfare, without collapsing into either an absolute pacifism or an unlimited license for Christian rulers to fight.