St Agustine (354-430 AD) in the City of God (De Civitate Dei) and other works proposes a theory of how to initiate and conduct a just war. He thinks that:
- Because of the pride, vanity, and fallen nature of man, wars between political states (see [2.6.7]), sects, religions are present in the world.
- Wars have a coercive role in God’s plan for humanity.
- Killing in wars is permissible even for Christian soldiers – as agents of the law, if the killing avoids a greater evil.
- Christians involved in wars should decide and conduct wars in a just way.
In the UML Activity Diagram below, I propose a representation of the “business” process of decision-making and conducting a just war. The actions marked with yellow and cyan are St Augustine’s criteria.

ACTIVITY/Action | Action/Description |
DECISION ABOUT JUST WAR | • Check if there is a just cause for war: “such as to defend the state from external invasion; to defend the safety or honor of the state, with the realization that their simultaneous defense might be impossible; to avenge injuries; to punish a nation for failure to take corrective action for wrongs (legal or moral) committed by its citizens; to come to the defense of allies; to gain the return of something that was wrongfully taken; or to obey a divine command to go to war (which, in practice, issues from the political head of state acting as God’s lieutenant on earth); and in any case, the just cause must be at least more just than the cause of one’s enemies”. • Check if there is a rightly intended will for war: “which has the restoration of peace as its prime objective, takes no delight in the wickedness of potential adversaries, views waging war as a stern necessity, tolerates no action calculated to provoke a war, and does not seek to conquer others merely for conquest’s sake or for territorial expansion”. • Check if the war is declared by a competnet authority • Check if the war is the last resort in order to achieve the objective of the just cause. |
Declare war | Declare war by a competent authority |
CONDUCT JUST WAR | • Plan military action • Check if the military action is proportional to “the wrong to be avenged, with violence being constrained within the limits of military necessity” • “Check if the military action discriminates between objects of violence (that is, combatants) and noncombatants, such as women, children, the elderly, the clergy” • “Check if the military action observes good faith in its interactions with the enemy, by scrupulously observing treaties and not prosecuting the war in a treacherous manner.” • Conduct military action • Check if war’s objective is reached |
Sources
- All quotations from: Mattox, J. Mark, “Augustine: Political and Social Philosophy”, Internet Encyclopedia of Philosophy
- Malene Hasberg Kjaer, “Just war theory in the works of Augustine”, 2016
First published: 30/1/2020