Biblical Teachings that support fighting in wars
Bible teaches that Christians should defend the weak, and should fight against evil.
Old Testament: full of war and violence
Proclaim this among the nations; prepare for war! (Joel 3:9)
New Testament: preaches agape and peace because Jesus was present to show the people the nature of God
However there is an exception. Christians might use this incident to show that Jesus thought it was acceptable to use violence in some circumstances:
He overturnered the tables of the money-chargers and the benches of those selling doves. (Mark 11:15)
Biblical Teachings that opposes war and violence
At the Sermon of the Mount, Jesus said:
Blessed are the peacemakers. (Matthew 5:9)
Love your enemies, and pray for those who persecute you. (Matthew 5:44)
Put your sword back in its place, for all who draw the sword will die by the sword. (Matthew 26:52)
The Just War
Thomas Aquinas' three conditions:
1. War must be started and controlled by the authority of the state or ruler.
2. There must be a just cause.
3. War must be fought to pomote good or avoid evil. Peace and justice must be restored afterwards.
Further conditions were later added by de Vitoria:
4. War must be the last resort, all other solutions must have been tried first.
5. There must be proportionality in the way the war is fought. Only enough force should be used as is needed to achieve victory.
6. War must do more good than harm.