74 Now, if this explanation suffices to satisfy human obstinacy and perverse misinterpretation of right actions of the vast difference between the indulgence of passion and presumption on the part of men, and obedience to the command of God, who knows what to permit or to order, and also the time and the persons, and the due action or suffering in each case, the account of the wars of Moses will not excite surprise or abhorrence, for in wars carried on by divine command, he showed not ferocity but obedience; and God in giving the command, acted not in cruelty, but in righteous retribution, giving to all what they deserved, and warning those who needed warning.
What is the evil in war? Is it the death of some who will soon die in any case, that others may live in peaceful subjection? This is mere cowardly dislike, not any religious feeling. The real evils in war are love of violence, revengeful cruelty, fierce and implacable enmity, wild resistance, and the lust of power, and such like; and it is generally to punish these things, when force is required to inflict the punishment, that, in obedience to God or some lawful authority, good men undertake wars, when they find themselves in such a position as regards the conduct of human affairs, that right conduct requires them to act, or to make others act in this way.
Otherwise John, when the soldiers who came to be baptized asked, What shall we do? Would have replied, Throw away your arms; give up the service; never strike, or wound, or disable any one. But knowing that such actions in battle were not murderous but authorized by law, and that the soldiers did not thus avenge themselves, but defend the public safety, he replied, "Do violence to no man, accuse no man falsely, and be content with your wages." But as the Manichæans are in the habit of speaking evil of John, let them hear the Lord Jesus Christ Himself ordering this money to be given to Cæsar, which John tells the soldiers to be content with.
"Give," He says, "to Cæsar the things that are Cæsar's." For tribute-money is given on purpose to pay the soldiers for war. Again, in the case of the centurion who said, "I am a man under authority, and have soldiers under me: and I say to one, Go, and he goes; and to another, Come, and he comes; and to my servant, Do this, and he does it," Christ gave due praise to his faith; He did not tell him to leave the service. But there is no need here to enter on the long discussion of just and unjust ways.
Source: Reply to Faustus the Manichaean (New Advent)