Because Jesus and His apostles called upon us to love even our enemies, there is a strong streak of pacifism among Christians. For example, consider these passages.
Matthew 5:38-42 (Today’s New International Version)
Eye for Eye
“You have heard that it was said, ‘Eye for eye, and tooth for tooth.’ But I tell you, do not resist an evil person. If anyone slaps you on the right cheek, turn to them the other cheek also. And if anyone wants to sue you and take your shirt, hand over your coat as well. If anyone forces you to go one mile, go with them two miles. Give to the one who asks you, and do not turn away from the one who wants to borrow from you.
Romans 12:17-21 (Today’s New International Version)
Do not repay anyone evil for evil. Be careful to do what is right in the eyes of everyone. If it is possible, as far as it depends on you, live at peace with everyone. Do not take revenge, my dear friends, but leave room for God’s wrath, for it is written: “It is mine to avenge; I will repay,” says the Lord. On the contrary:
“If your enemy is hungry, feed him;
if he is thirsty, give him something to drink.
In doing this, you will heap burning coals on his head.”
Do not be overcome by evil, but overcome evil with good.
Why the confusion? Why are some Christians pacifists? Are they mistaken? I think so. When we must considered them within the context of the entire Bible, I think they make the mistake of trying to understand such passages isolation. Without a doubt, God expects us love our enemies. Nonetheless, he also expects us to hate and oppose evil, and sometimes that requires us to fight and to kill. What God does not permit is vengeance. That belongs to Him alone.
Consider the example of our soldiers. What makes them the world’s finest? When these young men and women finish fighting, they nurse their wounded and bury their dead. Then instead of punishing their enemies, they set aside their arms and help their enemies rebuild.
Others have studied the Bible much more intently than I. Many are wiser, and some are better writers. Thus, I recommend this relatively short post, Why Does God Allow War? by Max Lucado.