WAR
Note:  all quotes are taken from Robertson-Donaldson translation of ante-nicene writings. (Ten volume set)
Also, I would like to thank David Bercot for collecting what the ante-nicene christians said on this topic!

And when the Spirit of prophecy speaks as predicting things that are to come to pass, He speaks in this way: "For out of Zion shall go forth the law, and the word of the Lord from Jerusalem. And He shall judge among the nations, and shall rebuke many people; and they shall beat their swords into ploughshares, and their spears into pruning-hooks: nation shall not lift up sword against nation, neither shall they learn war any more." And that it did so come to pass, we can convince you. For from Jerusalem there went out into the world, men, twelve in number, and these illiterate, of no ability in speaking: but by the power of God they proclaimed to every race of men that they were sent by Christ to teach to all the word of God; and we who formerly used to murder one another do not only now refrain from making war upon our enemies, but also, that we may not lie nor deceive our examiners, willingly die confessing Christ. (Justin Martyr, 1.176)
...and we who were filled with war, and mutual slaughter, and every wickedness, have each through the whole earth changed our warlike weapons,-our swords into ploughshares, and our spears into implements of tillage,-and we cultivate piety, righteousness, philanthropy, faith, and hope, which we have from the Father Himself through Him who was crucified; (Justin Martyr, 1.254)
I do not wish to be a king.  I am not anxious to be rich.  I decline military command. (Tatian, 2.69)
We have learned not to return blow for blow, not to go to law with those who plunder and rob us.  Instead, even to those who strike us on one side of the face, we offer the other side also. (Athenagoras, 2.129)
The new covenant that brings back peace and the law that gives life have gone forth over the whole earth, as the prophets said: "For out of Zion will go forth the law, and the word of the Lord from Jerusalem; and he will rebuke many people; and they will break down their swords into plowshares, and their spears into pruninghooks, and they will no longer learn to fight." ...  These people (Christians) formed their swords and war-lances into plowshares, ... that is, into instruments used for peaceful purposes.  So now, they are unaccustomed to fighting.  When they are struck, they offer also the other cheek. (Irenaeus,  1.512)
It is not war, but in peace, that we are trained. (Clement of Alexandria, 2.234)
The Scythians, the Celts, the Iberians, and the Thracians are all warlike races.  They are also greatly addicted to intoxication and think that drunkeness is an honorable, happy pursuit to engage in.  But we, the people of peace, feast for lawful enjoyment, not to wantonness.  We drink sober cups of friendship. (Clement of Alexandria, 2.246)
The one instrument of peace is what we employ: the Word alone, by whom we honor God.  We no longer use the ancient psaltery, trumpet, timbrel, and flute.  For those who are expert in war and are scorners or the fear of God were accustomed to make use of them. (Clement of Alexandria, 2.249)
Let our seals be either a dove, a fish, or a ship scudding before the wind ... If there is anyone fishing, he will remember the apostle, and the children drawn out of the water.  We are not to draw an outline of ... a sword or a bow, since we follow peace.  Nor should we draw an outline of ... drinking cups, since we are temperate. (Clement of Alexandria, 2.286)
He bids us to "love our enemies, bless them who curse us, and pray for those who despitefully use us."  And He says: "If anyone strikes you on the one cheek, turn to him the other also; and if anyone takes away your coat, do not hinder him from taking your cloak also." (Clement of Alexandria, 2.293)
An enemy must be aided, so that he may not continue as an enemy.  For by help, good feeling is compacted and enmity dissolved. (Clement of Alexandria, 2.370)
We do not train our women like Amazons to manliness in war, for we wish even the men to be peaceable. (Clement of Alexandria, 2.420)
If, then, we are commanded to love our enemies (as I remarked above), whom have we to hate?  If injured, we are forbidden to retaliate, lest we become just as bad ourselves.  Who can suffer inury at our hands? (Tertullian, 3.45)
How often you inflict gross cruelties on Christians.  You do this, partly because it is your own inclination, and partly in obedience to the laws... Yet, banded together as we are, ever so ready to sacrifice our lives, what single case of revenge for injury are you able to point to?  However, if it were held  to be right for us to repay evil by evil, a single night with a torch or two could achieve an ample vengeance.  But away with the idea of a divine sect avenging itself by human fires! (Tertullian, 3.45)
We willingly yield to the sword.  So what wars would we not be both fit and eager to participate in (even against unequal forces), if in our religion it were not counted better to be slain than to slay? (Tertullian, 3.45)
The Christian does no harm even to his enemy. (Tertullian, 3.51)
God puts His prohibition on every sort of man-killing by that one inclusive commandment: "You shall not kill." (Tertullian, 3.80)
"Nation will not take up sword against nation, and they will no more learn to fight."  Who else, therefore, does this prophecy apply to, other than us?  For we are fully taught by the new law, and therefore observe these practices. ... The teaching of the new law points to clemency.  It changes the primitive ferocity of the primitive execution of war upon the rivals and enemies of the Law into the peaceful actions of plowing and cultivating the land. (Tertullian, 3.154)
Now inquiry is made about the point of whether a believer may enter into military service.  The question is also asked whether those in the military may be admitted into the faith - even the rank and file (or any inferior grade), who are not required to take part in sacrifices or capital punishments ... A man cannot give his allegiance to two masters - God and Caesar. ... How will a Christian man participate in war?  In fact, how will he serve even in peace without a sword?  For the Lord has taken the sword away.  It is also true that soldiers came to John (the Baptist) and received the instructions for their conduct.  It is true also that a centurion believed.  Nevertheless, the Lord afterward, in disarming Peter, disarmed every soldier. (Tertullian, 3.73)
"And they will beat their swords into plowshares, and their spears into pruning hooks."  In other words, they will change the dispostions of injurious minds, hostile tongues, blasphemy, and all kinds of evil into pursuits of moderation and peace.  "Nation will not lift up sword against nations." That is, they will not stir up conflict.  "Neither will they learn war any more" - that is, the provocation of hostilities.  So you should learn form this that Christ was not promised to pursue peace.  Now, you must deny either that these things were foretold (although they are plainly seen) or that they have been fulfilled (although you read of them). (Tertullian, 3.339)
I think we must first inquire whether warfare is proper at all for Christians.  What point is there in discussing the merely incidental, when that on which it rests is to be condemned?  Do we believe it is lawful for a human oath to be added to one that is divine?  It is lawful for a man to come to be pledged to another master after Christ has become his Master?  Is it lawful to renounce father, mother, and all nearest kinsfolk, whom even the Law has commanded us to honor and love next to God himself? ... Is it lawful to make an occupation of the sword, when the Lord proclaims that he who uses the sword will perish by the sword?  Will the son of peace take part in the battle when it does not become him even to sue at law?  Will he who is not the avenger even of his own wrongs, apply the chain, the prison, the torture, and the punishment? (Tertullian, 3.100)
Is the (military) laurel of triumph made of leaves, or of corpses?  Is it adorned with ribbons, or with tombs?  Is it wet with ointments, or with tears of wives and mothers?  It may be made of some (dead) Christians too.  For Christ is also believed among the barbarians.  (Tertullian, 3.101)
Our religion commands us to love even our enemies, and to pray for those who persecute us. (Tertullian, 3.105)
The existence of many kingdoms would have been a hindrance to the spread of the doctrine of Jesus throughout the entire world ... This was because of the need for men everywhere to engage in war and fight on behalf of their native country - which was the case before the times of Augustus ... How, then, was it possible for the Gospel doctrine of peace to prevail throughout the world?  For it does not permit men to take vengeance even upon their enemies.  It was only possible because, at the coming of Jesus, a milder spirit had been everywhere introduced into the conduct of things. (Origen, 4.444)
The statement (of Celsus, a pagan critic) is false "that in the days of Jesus, others who were Jews rebelled against the Jewish state and became His followers."  For neither Celsus, nor those who think like him, are able to point out any act or part of Christians that hints of rebellion.  In fact, if a revolt had led to the formation of the Christian commonwealth, the Christian lawgiver would not have altogether forbidden the putting of men to death.  So it could not have derived it existence in such a way from the Jews.  For they were permitted to take up arms in defense of the members of their families and to slay their enemies.  Yet, Christ nowhere teaches us that it is right for His own disciples to offer violence to anyone, no matter how wicked.  For He did not consider it to be in accord with His laws to allow the killing of any individual whomever.  For His laws were derived from a divine source.  Indeed, if the Christians truly owed their origin ot a rebllion, they would not have adopted laws of so exceedinlgy mild a character.  For their laws do not allow them on any occasion to resist their persecutors, even when it was their fate to be slain as sheep. (Origen, 4.467)
Christians were taught not to avenge themselves upon their enemies ... They would not have made war (although capable) even if they had received authority to do so.  For they have obtained this reward from God:  that He has always warred on their behalf.  On certain occasions, he has restrained those who rose up against them and desired to destroy them ... On special occasions, some have endured death for the sake of Christianity, and those individuals can be easily numbered.  However, God has not permitted the whole nation (of Christians) to be exterminated. (Origen, 4.467, 468)
Perhaps the so-called wars among the bees convey instructions as to the manner in which wars should be waged in a just an orderly way among men - if ever there arise a necessity for them. (Origen, 4.533)
To those who inquire of use from where we come, or who is our founder, we reply that we have come agreeably to the counsels of Jesus.  We have cut down our hostile, insolent, and wearisome swords into plowshares.  We have converted into pruning hooks the spears that were formerly used in war.  For we no longer take up "sword against nation," nor do we "learn war any more."  That is because we have become children of peace for the sake of Jesus, who is our Leader. (Origen, 4.558)
Celsus (a pagan critc) adds ... "How could God command the Israelites through Moses to gather wealth, to extend their dominion, to fill the earth, to put their enemies of every age to the sword, and to destroy them utterly? ... For, on the other hand, His Son, the man of Nazareth, promulgated laws quite opposed to these.  He declared that no one can come to the Father who loves power, riches, or glory.  Jesus said that to anyone who has given them one blow, they should offer to receive another.  So is it Moses or Jesus who taught falsely?  When the Father sent Jesus, did He forget the commands that He had given to Moses?  Or did He change his mind, condemn His own laws, and send forth a Messenger with opposite instructions?" ... 
(Origen's reply) We would observe that it must be impossible for the legislation of Moses,taken literally, to harmonize with calling of the Gentiles and with their subjection to the Roman government.  On the other hand, it would be impossible for the Jews to preserve their civil economy unchanged if they were to embrace the gospel.  For Christians could not slay their enemies.  Nor could they condemn those who had broken the law to be burned or stoned, as Moses commands. ... In the case of the ancient Jews, who had a land and a form of government of their own, to take from them the right of making war upon their enemies, of fighting for their country, of putting to death or otherwise punishing adulterers, murderers, or others who were guilty of similar crimes, would have beento subject them to sudden and utter destruction whenever the enemy fell upon them.  For, in that case, their very laws would restrain them and prevent them from resisting the enemy.  Yet, that same providence that of old gave the Law, and haw now given the gospel of Jesus Christ, has destroyed their city and their temple, not wishing the Jewish state to continue any longer ... However, this providence has extended the Christian religion day by day, so that it is now preached everywhere with boldness.  And this is in spite of the numerous obstacles that oppose the spread of Christ's teaching in the world.  However, since it was the purpose of God that the nations should receive the benefits of Christ'sr teaching, all the devices of men against Christians have been brought to nothing.  For the more that kings, rulers, and peoples have persecuted them everywhere, the more Christians have increased in number and grown in strength. (Origen, 4.617, 618, 621)
(Celsus) "You surely do not say that if (in compliance with your wish) the Romans were to neglect their customary duties to gods and men, and were to worship teh Most High, ... that He would come down and fight for them, so that they would not need any other help than His.  For this same God ... promised of old this and much more to those who served Him.  Yet, see in what way He has helped the Jews and you!  Instead of being masters of the whole world, the Jews are left with not so much as a patch of ground or a home.?
(Origen's reply) What would happen if, instead of only a relatively few persons believing (as at the present), the entire empire of Rome believed?  They would pray to the Word, who of old said to the Hebrews, when they were pursued by the Egyptians: "The Lord will fight for you, and you will hold your peace."  And if all the Romans united in prayer with one accord, they would be able to put to flight far more enemies than those who were defeated by the prayer of Moses ... However, He had made the fulfillment of His promises dependent on certain conditions - namely, that they would observe and live according to His Law ... But if all the Romans embraced the Christian faith (according to the supposition of Celsus), they would overcome their enemies when they prayed.  Or rather, they would not war at all. For they promised to save five entire cities for the sake of fifty righteous persons.  Men of God are assuredly the salt of the earth.  They preserve the order of the world.  And society is held together as long as the salt is uncorrupted ... When God gives to the Tempter permission to persecute us, then we suffer persecution.  And when God wishes us to be free from suffering - even in the middle of a world that hates us - we enjoy a wonderful peace, trusting in the protection of Him who said, "Be of good cheer, for I have overcome the world." (Origen, 4.666)
In the next place, Celsus urges us "to help the king with all our might, and to labour with him in the maintenance of justice, to fight for him; and if he requires it, to fight under him, or lead an army along with him." To this our answer is, that we do, when occasion requires, give help to kings, and that, so to say, a divine help, "putting on the whole armour of God." And this we do in obedience to the injunction of the apostle, "I exhort, therefore, that first of all, supplications, prayers, intercessions, and giving of thanks, be made for all men; for kings, and for all that are in authority; " and the more any one excels in piety, the more effective help does he render to kings, even more than is given by soldiers, who go forth to fight and slay as many of the enemy as they can. And to those enemies of our faith who require us to bear arms for the commonwealth, and to slay men, we can reply: "Do not those who are priests at certain shrines, and those who attend on certain gods, as you account them, keep their hands free from blood, that they may with hands unstained and free from human blood offer the appointed sacrifices to your gods; and even when war is upon you, you never enlist the priests in the army. If that, then, is a laudable custom, how much more so, that while others are engaged in battle, these too should engage as the priests and ministers of God, keeping their hands pure, and wrestling in prayers to God on behalf of those who are fighting in a righteous cause, and for the king who reigns righteously, that whatever is opposed to those who act righteously may be destroyed!"
And as we by our prayers vanquish all demons who stir up war, and lead to the violation of oaths, and disturb the peace, we in this way are much more helpful to the kings than those who go into the field to fight for them. And we do take our part in public affairs, when along with righteous prayers we join self-denying exercises and meditations, which teach us to despise pleasures, and not to be led away by them. And none fight better for the king than we do. We do not indeed fight under him, although he require it; but we fight on his behalf, forming a special army-an army of piety-by offering our prayers to God.  And if Celsus would have us to lead armies in defence of our country, let him know that we do this too, and that not for the purpose of being seen by men, or of vainglory. For "in secret," and in our own hearts, there are prayers which ascend as from priests in behalf of our fellow-citizens. And Christians are benefactors of their country more than others. For they train up citizens, and inculcate piety to the Supreme Being; and they promote those whose lives in the smallest cities have been good and worthy, to a divine and heavenly city, to whom it may be said, "Thou hast been faithful in the smallest city, come into a great one," where "God standeth in the assembly of the gods, and judgeth the gods in the midst; "and He reckons thee among them, if thou no more "die as a man, or fall as one of the princes." (Origen, 4.667, 668)
Wars are scattered all over the earth with the bloody horror of camps.  The whole world is wet with mutual blood.  And murder - which is admitted to be a crime in the case of an individual - is called a virtue when it is committed wholesale.  Impunity is claimed for the wicked deeds, not because they are guiltless - but because the cruelty is perpetrated on a grand scale! (Cyprian, 5.277)
Christians do not attack their assailants in return, for it is not lawful for the innocent to kill even the guilty. (Cyprian, 5.351)
The hand must not be spotted with the sword and blood - not after the Eucharist is carried in it. (Cyprian, 5.488)
When the worship of God was taken away, men lost the knowledge of good and evil ... They then began to fight with one another, to plot, and to achieve glory for themselves form the shedding of human blood. (Lactantius, 7.141)
If only God were worshipped, there would not be dissensions and wars.  For men would know that they are the sons of one God. (Lactantius, 7.143)
Why would (the just man) carry on war and mix himself with the passions of others when his mind is engaged in perpetual peace with men?  Would he be delighted with foreign merchandise or with human blood - he who does not know how to seek gain?  For the Christian is satisfied with his standard of living.  He considered it unlawful not only to commit slaugher himself, but also to be present with those who do it. (Lactantius, 7.153)
If desire is restrained, no one will use violence by land or sea.  No one will lead an army to  carry off and lay waste the property of others. ... For what are the interests of our country but the detriments of another state or nation?  To extend the boundaries that are violently taken from others, to increase the power of the state, to improve the revenues - all of these things are not virtues.  Rather, they are the overthrowing of virtues. (Lactantius, 7.169)
How can a man be just who hates, who despoils, who puts to death?  Yet, those who strive to be serviceable to their country do all these things ... When they speak of the "duties" relating to warfare, their speech pertains neither to justice nor to true virtue. (Lactantius, 7.169)
Therefore, it is not befitting that those who strive to keep to the path of justice should be companions and sharers in this public homicide.  For when God forbids us to kill, He prohibits more than the open violence that is even not allowed by the public laws.  He also warns us against doing those things that are considered lawful among men.  For that reason, it will not be lawful for a just man to engage in warfare, since his warfare is justice itself.  Nor is it lawful for him to accuse anyone of a capital charge.  For it makes no difference whether you put a man to death by word, or by the sword instead.  That is because it is the act of putting to death itself that is prohibited.  Therefore, with regard to this commandment of God, there should be no exception at all.  Rather, it is always unlawful to put a man to death, whom God willed to be a sacred creature. (Lactantius, 7.187)
It is not right that a worshipper of God should be injured by another worshipper of God. (Lactantius, 7.271)
You allege that those wars of which you speak were sparked because of hatred of our religion.  However, it would not be difficult to prove that (after the name of Christ was heard in the world), wars were not increased.  In fact, they actually diminished in great measure by the restraining of furious passions.  A numerous band of men as we are, we have learned from His teaching and His laws that evil should not be repaid with evil.  Rather, it is better to suffer wrong than to inflict it.  We would rather shed our own blood than stain our hands and our conscience with that of another.  As a result, an ungrateful world is now enjoying - and for a long period has enjoyed - a benefit from Christ.  For by His means, the rage of savage ferocity has been softened and has begun to withhold hostile hands from the blood of a fellow creature.  In fact, if all men without exception ... would lend an ear for a while to His salutary and peaceful rules, ... the whole world would be living in the most peaceful tranquility.  The world would have turned the use of steel into more peaceful uses and would unite together in blessed harmony, maintaining involate the sanctity of treaties. (Arnobius, 6.415)
Those soldiers were filled with wonder and admiration at the grandeur of the man's piety and generosity and were struck with amazement.  They felt the force of this example of pity.  As a result, very many of them were added to the faith of our Lord Jesus Christ and threw off the belt of military service. (Disputation of Archelaus and Manes, 6.179)
NOTE:  As a person can easily see from the above quotations the early christians were against War in all it's forms.  However, when reading the early christian writings one will find a couple of references to christians being in the army.  This seeming contradiction is easily explained when the following two quotes from Tertullian and Hippolytus are read:
" soldier of the civil authority must be taught not to kill men and to refuse to take an oath.  If he is unwilling to complly, he must be rejected for baptism.  A military commander or civic magistrate who wears the purple must resign or be rejected.  If an applicant or a believer seeks to become a soldier, he must be rejected, for he has despised God." (Hippolytus, Apostolic Tradition 16)
"Of course if faith comes later (after someone was already enrolled in the military) and finds someone already occupied with military service, their case is different.  For example, there is the instance of those whom John (the Baptist) received for baptism, and of those most faithful centurions.  I mean the centurion whom Christ approved, and the centurion whom Peter instructed (ie. Cornelius).  Yet, at the same time, when a man has become a believer and faith has been sealed, there must be either an immediate abandonment of the military office, which has been the course of many - or else all sorts of quibbling will have to be resorted to in order to avoid offending God.  And such quibbling is not allowed even outside of military service." (Tertullian, 3.100)
I would also like to add that during this time period (100-325ad) in most cases it was not possible to just leave the military, like it is today.  Often, it meant certain death.  Generals could not have their newly converted soldiers leaving the army since many people who no longer wanted to serve in the military would simply fake a conversion and then they would be excused from service.  Whatever the scenario, it was not possible to leave the army unless your term (ie. time of service) was completed.