Jesus' teaching about violence is very clear-cut: "I say to you, love your enemies, and pray for those who persecute you. Offer no resistance to one who is evil. When someone strikes you on your right cheek, turn the other one to him as well." Jesus' teaching is one of non-violence. Violence begets violence.
But beginning in the 4th century, Christian teachers began to shift their thinking. Though personally Christians needed to live peaceful and non-violent lives, some people like St. Augustine and St. Thomas Aquinas developed "the just war" teaching. It says that a war is justified if it has a just cause (defense, not aggression), is undertaken with a right intention, has been declared by a legitimate authority, is an absolute last resort, offers a reasonable hope of success, and is proportionate to the wrong suffered. In other words, the damage inflicted must be of the same order as the wrong which was done.
Obviously, this teaching is often ambiguous. National and religious leaders can disagree about whether or not a war is "just" and politics can color the picture even more. Since the middle of the 20th century however, Catholic teaching has returned to a more Scriptural approach, which condemns war in the modern-post nuclear society as "immoral" and "unjust." No war can be proportional in its response anymore, since any war is always flirting with nuclear destruction.