War is among the most terrible of human realities. Yet Catholicism has never condemned all participation in war as morally impermissible. From the early Middle Ages to our own time, the church has consistently asserted that some evils or threats are so grave that they merit a vigorous armed response. “It is the other side’s wrongdoing that compels the wise man to wage just wars,” wrote Augustine, one of the earliest exponents of what is now termed “just-war theory.”
Reichberg, Gregory M. (2004) Preemptive War: What would Aquinas say?, Commonweal. 3 July.