Augustine got the Just War ball rolling in the late 4th Century by outlining three kinds of wars he believed God’s people can support.  And his theories were not changed for the most part but instead grew in popularity among Catholics.

Then along came Thomas Aquinas 900 years later, in the 13th Century (1225-1274), to spearhead the next big evolution in Just War theory.  The High Middle Ages in which Aquinas lived were more academic and systematic than the times of Augustine.  So Aquinas felt the need to make Augustine’s teachings more relevant to his culture and its modern situations and did so by systematizing them, setting definite criteria that must be met in order to justify a war.

He crafted three conditions, based on logic/pragmatism and not scripture, for deeming a war legitimate:

1. A just cause

2. A right intention

3. A declaration from a “legitimate authority”

Aquinas also made it clear that he and Augustine saw no glory in war and did not view war or violence as a “positive moral good”(1).  “He made a presumption in favor of peace and held that one who wants to go to war had to be able to explain why the greater good demanded rupture of the peace.”(2) A good explanation, a just war, were the exception and not the rule, not the majority of man’s wars at the time – in Aquinas’ thinking.

Just War theory began gaining massive theological authority among Catholics once Aquinas agreed with Augustine’s basic premise on the justification of war and then added his own criteria on top of it.  So much so that the opinions of these two men approached the status of dogma within the Church.

SOURCES:
1. George Weigel, Tranquilitas Ordinis (new York: Oxford University Press, 1987), p37
2. Air Power History, Vol.39, No.3, Fall 1992, p38. Copyright and published 1992 by the Air Force Historical Foundation