Jesus taught his disciples to love their enemies (Matt 5:38) and to kill no one (Luke 18:19).
Jesus made his triumphal entry into Jerusalem on a donkey rather than a war-horse (Luke 19:28-40)
Jesus rebuked Peter for taking up a sword when He is arrested (Luke 22:49-51)
36Jesus said, “My kingdom is not of this world. If it were, my servants would fight to prevent my arrest by the Jews. But now my kingdom is from another place.” John 18:36
39 But I tell you not to resist an evil person. Matt 5:39
Respond non-violently if someone takes you to court Matt 5:40
Respond non-violently if a Roman Soldier asks you to carry his stuff Matt 5:41
44 But I say to you, love your enemies, bless those who curse you, do good to those who hate you, and pray for those who spitefully use you and persecute you Matt 5:44
Since Christians should not kill (as is taught in the Bible and by every Christian teacher for the first three centuries of Christianity) even to keep Jesus from being arrested and crucified, a nation that follows Christ’s principles should not go to war against its enemies. Non violent resistance against evil is a better strategy, and we see many cases where non-violent movements have proven highly successful in the 20th century – Gandhi’s non-violent protests conquered the powerful British Empire and Martin Luther King Jr. non-violent protests changed American history.
There are also other biblical cases that can be made – for the last 16 centuries most Christians have believed that sometimes war is necessary as a last resort – the just-war theory.