A summary of St. Augustine’s doctrine on Just War:
A. There is a greater merit in preventing war by peaceful negotiation and conciliation than in vindicating rights by bloodshed.
B. Peace attained by conciliation is better than peace attained by victory.
C. There is a natural society of mankind which gives rise to certain rights and duties relevant to the morality of war.
D. The absence of a superior tribunal before which a prince can seek redress can alone justify him in making war, except when resisting actual attack.
E. Saving the direct intervention of God, the following conditions must in addition be fulfilled before war can be just:
(1) It must have a just cause: this can only be a grave injury received (e.g. actual invasion: unlawful annexation of territory: grave harm to citizens or their property: denial of peaceful trade and travel) or a great injustice perpetrated upon others whom it is a duty to help (e.g. the same injuries as above, violation of religious rights).
(2) It must be necessary: i.e. the only available means of restoring justice or preventing the continued violations of justice.
(3) It must be the consequence of a formal warning to the offending state and must be formally declared.
(4) It must be declared and waged only by the sovereign authority in the state (i.e. one who has no political superior) and, if the defense of religious rights are involved, with the consent of the Church.
(5) The good to be attained by war must be reasonably supposed to be greater than the certain evils, material and spiritual, which war entails.
(6) A right intention must actuate both the declaration, conduct and conclusion of war. That intention can only be the restoration or attainment of true peace.
(7) Only so much violence may be used as is necessary: in the case of defense, only so much as is necessary to repel the violence of the aggressor.
F. The moral responsibility for war lies upon the sovereign authority, not upon the individual soldier or citizen: his duty is to obey, except in a war which he is certainly convinced is wrong.
G. Priests may not fight even in just war.
H. The duty of repelling injury inflicted upon another is the common obligation of all rulers and peoples.
– The Catholic Tradition of the Law of Nations, John Eppstein –
Lucas G. Westman