Justice should be observed even in war and with enemies. This is proved by the example of Moses and Elisha. The ancient writers learned in turn from the Hebrews to call their enemies by a gentler term. Lastly, the foundation of justice rests on faith, and its symmetry is perfect in the Church.
139. How great a thing justice is can be gathered from the fact that there is no place, nor person, nor time, with which it has nothing to do. It must even be preserved in all dealings with enemies. For instance, if the day or the spot for a battle has been agreed upon with them, it would be considered an act against justice to occupy the spot beforehand, or to anticipate the time. For there is some difference whether one is overcome in some battle by a severe engagement, or by superior skill, or by a mere chance. But a deeper vengeance is taken on fiercer foes, and on those that are false as well as on those who have done greater wrongs, as was the case with the Midianites. For they had made many of the Jewish people to sin through their women; for which reason the anger of the Lord was poured out upon the people of our fathers. Thus it came about that Moses when victorious allowed none of them to live. On the other hand, Joshua did not attack the Gibeonites, who had tried the people of our fathers with guile rather than with war, but punished them by laying on them a law of bondage. Elisha again would not allow the king of Israel to slay the Syrians when he wished to do so. He had brought them into the city, when they were besieging him, after he had struck them with instantaneous blindness, so that they could not see where they were going. For he said: “You shall not smite those whom you have not taken captive with your spear and with your sword. Set before them bread and water, that they may eat and drink and return and go to their own home.” Incited by their kind treatment they should show forth to the world the kindness they had received. “Thus” (we read) “there came no more the bands of Syria into the land of Israel.”
140. If, then, justice is binding, even in war, how much more ought we to observe it in time of peace. Such favour the prophet showed to those who came to seize him. We read that the king of Syria had sent his army to lie in wait for him, for he had learned that it was Elisha who had made known to all his plans and consultations. And Gehazi the prophet's servant, seeing the army, began to fear that his life was in danger. But the prophet said to him: “Fear not, for they that be with us are more than they that be with them.” And when the prophet asked that the eyes of his servant might be opened, they were opened. Then Gehazi saw the whole mountain full of horses and chariots round about Elisha. As they came down to him the prophet says: “Smite, O God, the army of Syria with blindness.” And this prayer being granted, he says to the Syrians: “Follow me, and I will bring you to the man whom you seek.” Then saw they Elisha, whom they were endeavouring to lay hold of, and seeing him they could not hold him fast. It is clear from this that faith and justice should be observed even in war; and that it could not but be a disgraceful thing if faith were violated.
141. So also the ancients used to give their foes a less harsh name, and called them strangers. For enemies used to be called strangers after the customs of old. This too we can say they adopted from our writings; for the Hebrews used to call their foes “allophyllos,” that is, when put into Latin, “alienigenas” (of another race). For so we read in the first book of Kings: “It came to pass in those days that they of another race put themselves in array against Israel.”
142. The foundation of justice therefore is faith, for the hearts of the just dwell on faith, and the just man that accuses himself builds justice on faith, for his justice becomes plain when he confesses the truth. So the Lord says through Isaiah: “Behold, I lay a stone for a foundation in Sion.” This means Christ as the foundation of the Church. For Christ is the object of faith to all; but the Church is as it were the outward form of justice, she is the common right of all. For all in common she prays, for all in common she works, in the temptations of all she is tried. So he who denies himself is indeed a just man, is indeed worthy of Christ. For this reason Paul has made Christ to be the foundation, so that we may build upon Him the works of justice, while faith is the foundation. In our works, then, if they are evil, there appears unrighteousness; if they are good, justice.
Source: On the Duties of the Clergy (New Advent)