Beginning with the moral reasoning of Augustine of Hippo and its systematic articulation by Thomas Aquinas, it traces how the just war tradition sought to restrain violence through criteria of legitimate authority, just cause, right intention, proportionality, and discrimination. The lecture contrasts this juridical and ethical framework with the sacralised language of holy war, particularly in the era of the Crusades and the confessional conflicts of early modern Europe. It considers the shift towards a secularised law of nations associated with Hugo Grotius and reflected in the post-1648 European order, and assesses how elements of just war reasoning persist in contemporary international law.

Engaging recent debates surrounding the war in Ukraine, the lecture asks whether Christian moral theology still offers a coherent grammar for judging armed force in a pluralist and often fragmented global order. It argues that while holy war language tends to absolutise conflict, the just war tradition remains a disciplined and morally serious attempt to reconcile political responsibility with the primacy of peace.

Professor Darius von Güttner-Sporzyński FRHistS FRSA is a historian of medieval and early modern Europe and Campus Dean of the Canberra Campus at Australian Catholic University. His research focuses on dynastic politics, queenship, political theology, and the history of East Central Europe. He has published widely on the Jagiellon monarchy, political culture, and the intersection of religion and power, and serves as founding editor of the series East Central Europe, 476–1795 AD/CE. His work combines close analysis of sources with broader reflection on questions of authority, sovereignty, and moral responsibility in European history. At ACU he leads initiatives that integrate research, teaching, and public engagement, seeking to foster rigorous debate on questions at the intersection of faith, reason, and public life.