The Christian just war tradition did not spring fully formed from Augustine's pen — it was shaped by a prior generation's engagement with Roman civic values after Constantine. Ambrose of Milan, writing his pastoral treatise On Duties (De Officiis, c. 386 AD) for clergy, drew directly on Cicero's discussion of virtuous courage to argue that defending the fatherland and the weak was a moral obligation. Ambrose introduced the criterion of right intention — the soldier should fight from love of justice, not passion — and he explicitly praised soldiers who died defending others. Augustine built on Ambrose's foundation but gave it a theological depth Ambrose lacked: in the City of God and his letters, Augustine argued that war is a tragic necessity in a fallen world, authorized only by legitimate rulers, aimed only at restoring peace, and to be waged with grief rather than relish. Augustine did not systematize these into a checklist — that came with Aquinas — but he supplied the theological grammar of intention, authority, and peace-as-goal that defined the tradition for a millennium.
Ambrose argues that courage displayed in defense of the fatherland against barbarians, or in defense of the weak against robbers, is a "fuller justice" — combining bravery with love of neighbor. He explicitly applies this to the soldier's vocation, grounding it in the Ciceronian virtue tradition baptized into Christian ethics. The soldier who dies defending others has fulfilled love of neighbor in the highest way. (NPNF2-10)
Advising a Christian general considering monastic withdrawal, Augustine urges Boniface to remain at his post: "Think, then, of this first of all, when you are arming for the battle, that even your bodily strength is a gift of God...Do not think that it is impossible for any one to please God while engaged in active military service." Augustine grounds the Christian soldier's vocation in justice and love of neighbor, not martial glory. (NPNF1-01)
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