The classical inquiry
Military virtue is the cluster of dispositions a soldier and a commander need to do their work well — physical courage, discipline, endurance, decisiveness, the kind of practical judgement that holds under pressure. The Greek tradition treats them as continuous with the civic virtues but distinguished by the conditions in which they are exercised. Xenophon, more than any other classical writer, gives us the practical handbook: the Anabasis and the Cyropaedia are full of incidents read for what they show about command, supply, the management of subordinates, and the formation of an army that will fight well.
Rome and the long inheritance
The Roman tradition adds the legion — a collective military institution disciplined and trained at a scale Greek armies rarely achieved. Polybius admired it; Scipio, Marius and Caesar each reformed it in turn; the long European tradition of professional military thought (from Frontinus through Vegetius and on) is the inheritance. The Christian moral tradition asks what kinds of war and conduct are compatible with the soldier's other duties.
Why it matters for Virtue & Power
Military virtue is one of the platform's recurring categories because the classical and historical literature would not let us treat it as separable from the rest. A polity's ability to defend itself, the formation of its officers, and the discipline of its citizens are not three subjects but one, read through different windows. See the entries on the Roman Republic figures (Scipio, Marius, Caesar) and the theme of discipline.