Virtually all historical treatments of just war recognize the importance of the account given by Thomas Aquinas in Summa theologiae II-II, q. 40, ‘De bello’, where he outlines three conditions – legitimate authority, just cause, and right intention – for a justifiable use of armed force. It is, however, less well known that within the same section of the work (q. 50, a. 4) Aquinas extended his reflection on just war into a theory of military prudence. By placing generalship under the category of ‘prudence’, rather than ‘art’ or ‘science’, he held that military command involves more than a morally neutral skill with victory as its sole aim. Building on the premise that service to the common good constitutes the overarching purpose of the military profession, Aquinas showed how the virtue of prudence provides an inner compass for decision-making amid the uncertainty and confusion of the battlefield.
Reichberg, Gregory M. (2010) Thomas Aquinas on Military Prudence, *Journal of Military Ethics * 9 (3): 262–275.