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.”
Just war theory is a product of this Christian modification of the Roman theory.