War and peace have been the concern of the church throughout history. Today I like to share some of views of war during church history.

IV   Views of war in church history.
The early Christians followed the example of Jesus to refrain from violence and military service. When the Jews revolted against the Romans, the Christians did not choose side, but deserted Jerusalem just like Jesus told them to do. (Matt.14:15-18)
In the first two centuries the Christians rejected military service without compromise. All writers from that time were pacifists. Here follow some examples:
Justin Martyr wrote ca. 138 AD: Referring to Isa.2:4: "You must be convinced that this has happened. We who used to kill each other, do no longer make war against our enemies. We refuse to lie or deceit our inquisitores; we prepare to die in our faith in Christ". …. "We that were filled with war and mutual killing and all wickedness, each one of us all over the earth, have changed our weapons: Our swords into ploughshares and our spears into farming tools, and we are filled with the fear of the Lord, righteousness, love for humanity, faith, hope, from our Father through the crucified one."
Tatian wrote ca 160 Ad: "I do not wish to become emperor; I do not want wealth; I renounce military positions; I detest sexual sins".
Clement of Alexandria: "We are not trained in war, but peace." "We are a race committed to peace." "We have used only one instrument, the word of peace, through which we honour God." "We do not train our women to become manly as Amasones in war, we wish that even the men should be peaceful."
Origenes, 185-254 AD, wrote: When you aks from where we come or who is our commander, we answer that we come in the covenant of Jesus to follow his counsel to transform our warlike and arrogant swords into ploughshares, and we transform the spears we used to fight with into sickles. For we no longer lift swords against any nation, and we learn not to war any more; because we have become the sons of peace for the sake of Jesus, he who is our commander.
Tertulian, 160-220 AD, wrote: Is it right to occupy the sword, when the Lord proclaim that he who use the sword will fall by the sword? And should the sons of peace, for whom it is even unseemingly to bring anybody to court, engage in war? And should the one that not even avenge the injustice against himself, lead into bondage and prison, torture and executions? … Of cause it is different if the faith comes to one involved in military service. When the faith is accepted and sealed, the military service must be left immediately, and many have done that.
Cyprian, bishop of Cartage 248-258Augustine AD: The whole world is wet with blood from us all; and murder, as when it apply to one single person, is regarded a crime, seems to be a virtue when it is committed collectively. One demands freedom from punishment from these evil deeds, not on the basis that they are legal, but because the cruelty is committed in gross scale.
When Christendom became state religion, the rule was for church leaders to defend military service. The viewpoints on military service and war were wavering in rhythm with the view of the state.
Augustine was the first to formulate the idea of a "just was". He lived in a situation when the citizens experienced that the Roman impire defended them against the barbarians. Augustine said it was right to defend the state and law and order. That was God’s way of ruling the world. Therefore the army were necessary and the soldier acted on behalf of the government. He stated that the only just reason for war is the violation of justice an aggressor is guilty of.
Augustine did not think in the way that is common today: A state has the right to avert attack against his land. He probably justified war with that others first violated justice. He claimed that this violation demanded a punishment to uphold justice. Such violations could be military attack on other countries, but it could also concern lawlessness and despotism in another country that had to be stopped. The aim of war was according to Augustine to restore law, order and peace.
Augustine did not start on bare ground when he developed his viewpoint on war. He was building on concepts and norms already developed in Greek philosophy and Roman laws. The "just war" concept is a pre Christian idea. (And a non Biblical idea!)
Thomas of Aquino developed a more sophisticated doctrine on the "just war". He said that all wars are inconsistent with several moral commandments, not only "you shall not kill". Therefore he demanded a basic moral reason to accept war. He followed Aristoteles in his view that an act is right when it leads to something worthwhile for the individual or even better if it leads to common good. This is different from the duty ethic of Agustin: It is our duty to punish injustice.
Thomas gave mainly three conditions to call a war just:
1. War can only be made by the legitimate ruler. It is his duty to be concerned for the common good of his people. This is also the purpose of war. The punishment should not be a pure revenge, but lead to repentance and peace.
2. There must be a just reason for war. Thomas understood this that someone had incurred a guilt of injustice in one way or another. If a ruler was guilty of gross injustice in his own land, a military intervention from another land would be morally accepted.
3. There must be a right purpose to justify a war: To further good and hinder evil.
The Roman Catholic Church followed the thoughts of Augustine and Thomas, but stretched it sometimes very far. In addition they adopted a  Holy war concept that was used to justify the Crusades, that was used to promote the interest of the church without asking if it was just.
Martin Luther confirmed that a just war can only be started by the king. He restricted the "just war" concept to defensive warfare. He said that the crusades or holy wars never could be just wars.
"Just war" – what is it? According to Thomas:
1. Only the legitimate government can make decision of war.
2. The reason must be a violation of justice that can be punished by military power to bring restoration.
3. The aim of the war must be to further common good and hinder evil.
In 1500-1600 these points were added:
4. One should not start war before every effort has been done to solve the conflicts by peaceful means.
5. The must be a reasonable proportion between the destructions of the war and the good achievements of the war.
Later two more were added:
6. Innocent – civilians shall be spared from attack.
7. If the war is not direct self defence, there must be a reasonable prospect of victory.
The Lutheran tradition is even stricter:
Only  a war of defence is a just war.
Most of the so called heretics have rejected the "just war" concept and worked for peace by abandoning the military system which is the main means of war. Through the whole church history there have been streams and spiritual movements that rejected war and military as something a Christian should not be involved in. We can just mention the Valdes, most of the Anabaptists, the Quakers etc.