Also there are plenty of Biblical passages on forgiveness and peacemaking, and many writings of the saints on the same. Are there any passages or writings that speak positively of war?
Here are a few quotes (many more can be found at http://incommunion.org/articles/introduction/quotations
“Blessed are the peacemakers, for they shall be called sons of God.” Who are these? Those who imitate the Divine love of others, who show forth in their own life the characteristic of the Divine energy. The Lord and Giver of good things completely annihilates anything that is without affinity and foreign to goodness. This work He ordains also for you, namely to cast out hatred and abolish war, to exterminate envy and banish strife, to take away hypocrisy and extinguish from within resentment of injuries smoldering in the heart. Instead, you ought to introduce whatever is contrary to the things that have been removed."
-St. Gregory of Nyssa
"How can a man be just who injures, hates, despoils and puts to death? Yet they who strive to be serviceable to their country do all these things: for they are ignorant of what this being serviceable is, who think nothing useful, nothing advantageous, but that which can be held by the hand; and this alone cannot be held, because it may be snatched away."
- Lactantius, the Divine Institutes, Book 6, Chapter 6 [Lactantius was the tutor of the son of St Constantine the Great. He lived approximately from 260 to 339 AD.]
"We who were filled with war, and mutual slaughter, and every wickedness, have each throughout the whole earth changed our weapons of war — 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; and sitting each under his vine, i.e., each man possessing his own married wife. For you are aware that the prophetic word says, ‘And his wife shall be like a fruitful vine.’ Now it is evident that no one can terrify or subdue us who have believed in Jesus over all the world."
- St Justin Martyr, Dialogue, Chapter 110
"If you enroll as one of God’s people, heaven is your country and God your lawgiver. And what are His laws? You shall not kill, You shall love your neighbor as yourself. To him that strikes you on the one cheek, turn to him the other also."
- Clement of Alexandria, Protrepticus, 10
"It is well known that Jesus was born in the reign of Augustus, who, one might say, brought the mass of mankind under a single sovereignty. The existence of many kingdoms would have hindered the spread of Jesus’ teachings over the whole world GÃƒÆ’Ã¢â‚¬Â¡Ãƒâ€šÃ‚Âªbecause everywhere men would have been forced to serve in the army and go to war on behalf of their countryGÃƒÆ’Ã¢â‚¬Â¡Ãƒâ€šÃ‚ÂªHow could this peaceful teaching, which prohibits a man from avenging himself even against his enemies, have gained sway if the whole world situation at the time of Jesus had not been made more peaceful."
- Origen, Against Celsus, 2:30
"Christ, in disarming Peter, unbelted every soldier."
- Tertullian, de Idololatria 19
There are many
more on the site. In light of those, I'm not so sure that even just war is defensable.