The Early Church Fathers on various topics: This was a 3700 hour project which included going through 22896 pages of the 38 volume set called Ante Nicene, Nicene, Post Nicene Fathers. I compiled 255 pages of quotes showing that the Early Church was always and completely Catholic. All of these quotes can be verified and found from the source which is free online.
Marcus Aurelius also, in his expedition to Germany, by the prayers his Christian soldiers offered to God, got rain in that well-known thirst
Athanasius Letter 48 (296-373 ad)
For in other matters also which go to make up life, we shall find differences according to circumstances. For example, it is not right to kill, yet in war it is lawful and praiseworthy to destroy the enemy; accordingly not only are they who have distinguished themselves in the field held worthy of great honours, but monuments are put up proclaiming their achievements. So that the same act is at one time and under some circumstances unlawful, while under others,
and at the right time, it is lawful and permissible.
Basil Letter 106 (329-379 ad)
I have learnt to know one who proves that even in a soldier's life it is possible to preserve the perfection of love to God, and that we must mark a Christian not by the style of his dress, but by the disposition of his soul.
Basil Letter 155 (329-379 ad)
To forget you in my prayers is impossible, unless first I forget the work to which God has called me, for assuredly, faithful as by God's grace you are, you remember all the prayers of the Church; how we pray also for our brethren when on a journey and offer prayer in the holy church for those who are in the army, and for those who speak for the sake of the Lord's name, and for those who show the fruits of the Spirit.
John Chrysostom Homily 5 on First Thessalonians (347-407 ad)
When thy son is grown up, before he enters upon warfare, or any other course of life, consider of his marriage.
Augustine Reply to Faustus the Manichean Book 22 pa 74 (354-430 ad)
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."
Sozomen Ecclesial History Book 6 Ch 3 (375-477 ad)
but when the soldiers discovered the cause of his refusal, they loudly proclaimed that they were themselves Christians.
Leo the Great Letters 167 question 14 (395-461 ad)
For that which a man has vowed to GOD, he ought also to pay. Hence he who abandons his profession of a single life and betakes himself to military service or to marriage, must make atonement and clear himself publicly, because although such service may be innocent and the married state honourable, it is transgression to have forsaken the higher choice.
Apostolic Constitutions book 7 par 2 (400ad)