Christian critics of just war theory sometimes point out that the tradition originates not with Christian thinkers but with pagans, Romans like Cicero. True enough, says James Brundage in his contribution to The Holy War (p. 102), but the Christians who took up the Roman theory modified it.
“In Roman thought,” Brundage says, “the term ‘just war’ tended to have as much ceremonial as moral content. A bellum justum et pium was a war that had been properly declared, with full observance of the appropriate public ceremonies and religious rites.” The theory was not “utterly devoid of moral content: aequitas required that a just war have a just cause,” and this typically mean “a violation of Rome’s legal interests in foreign territories, infringement on Roman territory itself by foreign powers, or disrespect for the immunities of Rome’s allies or her representatives on alien soil.”
Augustine employed the same terminology, but he did not simply defend the Roman way of war. To the existing tradition, Augustine added substantive and universal moral standards, the familiar just war criteria: “a declaration of war by a legitimate authority; . . . a reasonable and morally acceptable cause for the war; . . . the war must be necessary, that is, there must be no other way of achieving the legitimate objective; and the war must be fought by acceptable means.”
Just war theory is a product of this Christian modification of the Roman theory.