In later writings, Augustine links “just war” to political responsibility: the power to declare and counsel war belongs to those who hold supreme authority, not private individuals. This authority-based framing became a key building block for medieval scholastic accounts of legitimate war-making.
In City of God, Augustine argues that even wars aim at peace, and he treats peace as a kind of “well-ordered” harmony. He also warns that even so-called just wars bring misery, helping set a moral problem that later Christian writers tried to discipline with rules.
At the Council of Clermont, Pope Urban II called for armed expedition to aid Byzantium and to recover holy sites, helping launch the First Crusade. This moment mattered because it linked warfare to religious aims and church leadership, creating new pressure to explain when and how war could be morally justified.
Around this period, Gratian compiled the Decretum (also called Concordia discordantium canonum), organizing earlier church rules and cases into a teachable system. By applying the scholastic method to law, the Decretum gave later theologians and canonists a structured legal-theological framework for discussing violence, authority, and wrongdoing.
The Fourth Lateran Council issued major church reforms and also addressed crusading, including organized preaching and spiritual privileges tied to participation. This helped place armed campaigns into a clearer institutional framework, reinforcing the idea that war-making could be regulated by church and law rather than treated only as personal heroism or private violence.
Pope Gregory IX issued an official collection of papal decretals, the Liber Extra (1234), compiled by Raymond of Peñafort. This collection clarified and standardized many legal rules for the Latin Church, strengthening the legal setting in which questions about coercion, punishment, and war were debated and taught.
Thomas Aquinas began writing the Summa theologiae in the mid-1260s as a structured guide for teaching theology. Its method—posing questions, objections, and replies—provided a powerful scholastic format for analyzing moral limits on force, including when war could be justified.
In Summa theologiae (II–II, Q.40), Aquinas lists three requirements for a just war: legitimate authority, just cause, and right intention. This mattered because it turned earlier themes (especially Augustinian ideas) into a concise test that later medieval and early modern thinkers could debate, teach, and apply.
Aquinas argues that coercive force is normally reserved to public authority responsible for the common good, not private persons seeking personal redress. This reinforced a key boundary in medieval ethics: war is not merely an individual choice but an act tied to political office and public responsibility.
Aquinas abruptly stopped dictating the Summa theologiae late in 1273, leaving it unfinished. The break shows that his mature scholastic synthesis of moral theology—including war—was cut short, even as it quickly became central to later teaching and commentary traditions.
Aquinas died on March 7, 1274, while traveling to the Second Council of Lyon. His death marks a natural endpoint for the 1200–1300 focus: the major scholastic formulation of just war criteria associated with Aquinas was already in place, ready to be developed by later scholastics and canonists.
After Aquinas, later moral theologians and legal scholars treated authority, just cause, and right intention as a baseline for evaluating war, even when they disagreed about details. In this way, medieval scholasticism helped turn “just war” from scattered moral reflections into a repeatable framework that could limit and criticize rulers’ decisions to fight.
Medieval scholastic formulations on war, especially Thomas Aquinas (1200–1300)