Contra Faustum ("Against Faustus the Manichaean") - ca. 400
https://www.newadvent.org/fathers/140622.htm
Early Christianity was largely pacificist. Critics of Christianity such as Celsus implied that they refused to fight for the Roman Empire (ca 175).[1] St. Basil the Great (330-379), likely with Mt 5:39 in mind, held that Christians who had shed blood while serving in the military should not present themselves for communion for three years,[2] Yet once the Imperium became Christian after 380 should not Christian soldiers defend a now Christian empire? Augustine (354-430) encouraged Christians to fight by developing a framework of jus ad bellum (“right to war”) by which it would be just for Christians to join the military. Here he argues against Faustus, a Manichaean who attacked Old and New Testament ethics.
71. …[A]s for Faustus' objection to the despoiling of the Egyptians [Exod 3:22; 11:2; 12:35-36], he knows not what he says. In this Moses not only did not sin, but it would have been sin not to do it. It was by the command of God, who, from His knowledge both of the actions and of the hearts of men, can decide on what everyone should be made to suffer, and through whose agency. The [Israelite] people at that time were still carnal and engrossed with earthly affections; while the Egyptians were in open rebellion against God, for they used the gold, God's creature, in the service of idols, to the dishonor of the Creator, and they had grievously oppressed strangers by making them work without pay. Thus, the Egyptians deserved the punishment, and the Israelites were suitably employed in inflicting it. … [T]his remains certain, that this was [commanded] for some good reason, and that Moses could not lawfully have done otherwise than what God told him, leaving to God the reason of the command, while the servant's duty is to obey.
72. But, says Faustus, it cannot be admitted that the true God, who is also good, ever gave such a command. I answer, such a command can be rightly given by no other than the true and good God, who alone knows the suitable command in every case, and who alone is incapable of inflicting unmerited suffering on anyone. …
74. … [T]he account of the wars of Moses [Exod 17:8-13; Num 21] will not excite surprise or abhorrence, for in wars carried on by divine command, [Moses] 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? … 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 [the Baptizer], 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 anyone.” 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” [Luke 3:14].
75. A great deal depends on the causes for which men undertake wars, and on the authority they have for doing so; for the natural order which seeks the peace of mankind, ordains that the monarch should have the power of undertaking war if he thinks it advisable, and that the soldiers should perform their military duties in behalf of the peace and safety of the community. When war is undertaken in obedience to God, who would rebuke, or humble, or crush the pride of man, it must be allowed to be a righteous war; for even the wars which arise from human passion cannot harm the eternal well-being of God, nor even hurt His saints; for in the trial of their patience, and the chastening of their spirit, and in bearing fatherly correction, they are rather benefited than injured. No one can have any power against them but what is given him from above. For there is no power but of God [Rom 13:1], who either orders or permits. Since, therefore, a righteous man, serving it may be under an ungodly king, may do the duty belonging to his position in the State in fighting by the order of his sovereign — for in some cases it is plainly the will of God that he should fight, and in others, where this is not so plain, it may be an unrighteous command on the part of the king, while the soldier is innocent, because his position makes obedience a duty, — how much more must the man be blameless who carries on war on the authority of God, of whom everyone who serves Him knows that He can never require what is wrong?
From De Civitate Dei contra Paganos("On the City of God Against the Pagans"), Book XIX, Chapter 7
This text, dating from around 420, was written in response to pagan claims that the sack of Rome by barbarians in 410 was one of the consequences of the abolition of pagan worship by Christian emperors. Augustine countered that Christianity saved the city from complete destruction and that Rome’s fall was the result of internal moral decay. He further outlined his vision of two societies, the saved “City of God” and the damned "City of Man”), respectively representing faith and unbelief .
https://www.newadvent.org/fathers/120119.htm
[The diversity of human languages causes conflict between peoples but] the imperial city has endeavored to impose on subject nations not only her yoke, but her language, as a bond of peace, so that interpreters, far from being scarce, are numberless. This is true; but how many great wars, how much slaughter and bloodshed, have provided this unity! And though these are past, the end of these miseries has not yet come. For though there have never been wanting, nor are yet wanting, hostile nations beyond the empire, against whom wars have been and are waged, yet, supposing there were no such nations, the very extent of the empire itself has produced wars of a more obnoxious description — social and civil wars— and with these the whole race has been agitated, either by the actual conflict or the fear of a renewed outbreak. If I attempted to give an adequate description of these manifold disasters, these stern and lasting necessities, though I am quite unequal to the task, what limit could I set? But, say they, the wise man will wage just wars. As if he would not all the rather lament the necessity of just wars, if he remembers that he is a man; for if they were not just he would not wage them and would therefore be delivered from all wars. For it is the wrongdoing of the opposing party which compels the wise man to wage just wars; and this wrong-doing, even though it gave rise to no war, would still be matter of grief to man because it is man's wrong-doing. Let everyone, then, who thinks with pain on all these great evils, so horrible, so ruthless, acknowledge that this is misery. And if any one either endures or thinks of them without mental pain, this is a more miserable plight still, for he thinks himself happy because he has lost human feeling.
[1] Origen, Contra Celsum, Book 8 , Chapter 73
[2] Peter Brock, Pacifism in Europe to 1914 (Princeton University Pres, 2013), 23
