At a glance
The New Testament does not address the matter of war, apart from using warfare in metaphors that describe the Christian life or illustrate the proclamation or the gospel, and in Jesus warning that those who use the sword are sooner or later destroyed by it, according to Fr. Richard P. McBriens Catholicism.
According to The Modern Catholic Encyclopedia, early Christians were forbidden from taking part in war because of the danger of idolatry in military service (soldiers might be asked to offer sacrifices to the emperor), and secondly because killing, even of an aggressor, was judged a direct violation of Jesus command that Christians love their enemies and overcome evil with good.
After 312 A.D., when the Emperor Constantine made Christianity the state religion, things began to change. Christians no longer were a persecuted minority and church and state became allies. During the period of close relationship between the church and society, Christianitys commitment to pacifism declined and the first outlines of just war teaching appeared, according to this encyclopedia.
Augustine, particularly, began developing a teaching justifying war, arguing that violence could be a requirement of Christian love if it were necessary to protect ones neighbor, especially the innocent and the weak, from unjust harm.
Aquinas later elaborated on that position and outlined specific criteria. According to McBrien, three basic categories must be met to justify war:
Beyond those three categories are a complex set of criteria and principles that include that war is a last resort, that there is reasonable hope of success, that there is proportionality between the evil produced and the evil hoped to be avoided or the good hoped to be achieved.
The just war theory also requires immunity of noncombatants from direct attack.
National Catholic Reporter, October 5, 2001