One last response to Witherington’s criticisms of Defending Constantine , and I’d be an ingrate if I didn’t express my appreciation for the many positive things that Witherington said about the book. I’m grateful that he thought the book worth interacting with at all.
His final criticism concerns my attempt to offer a non-pacifist way of reading the Sermon on the Mount. Witherington writes, “the attempt to minimize Jesus’ call to non-violence in Mt. 5 is weak. How exactly is killing a fulfillment of ‘love your enemies’ do them no harm, or turn the other cheek, or overcome evil with good? It isn’t.”
I agree with his main point: The argument I mounted is weak; it is the section of the book that I am least satisfied with, and something I need to work on more. At this point, I can only sketch out the reasoning that led me to those counterintuitive conclusions. I found Daniel Bell’s discussion in Just War as Christian Discipleship: Recentering the Tradition in the Church rather than the State 
The primary issue is, how are we to square Jesus’ teaching in the Sermon with God’s approval of war in the Old Testament. We might say that Jesus is offering a completely new ethic, a non-violent one to replace the violent ethic of the Torah. Jesus Himself preempts that interpretation at the beginning of the sermon with His insistence that He has come to “fulfill” and not “abolish” the Law and Prophets (Matthew 5:17). Whatever “fulfill” means here, it doesn’t mean “cancel out” or “abolish.” Jesus claims that His teaching doesn’t destroy the Law but rather provides the correct interpretation of the Law. What Jesus teaches is the righteousness that the Law always aimed at.
Another popular option has been to claim that the Sermon is instruction for personal but not for political ethics. I’ve been convinced by NT Wright and others that this option doesn’t work either. As Wright has shown, some of the specific instructions have to do with the Christian disciple’s stance toward Roman authorities: “whoever shall force you to go one mile, go with him two.” The immediate context assumes that the disciples are in a position of weakness, but what happens if they are in positions of authority. Are they permitted to ignore the Sermon in favor of some “natural” political ethics? Much of the Christian tradition has danced this two-step - a Jesus ethic individually and in the church, a natural-law ethic in public life. I don’t think that this works. Here is where Bell’s discussion of Augustine helps: If Christians are going to justify participation in war at all, they are going to have to justify it in a way consistent with the Sermon on the Mount and the other teaching of the New Testament. If nothing else, just war arguments that ignore Jesus are at the mercy of pacifist arguments. In fact, “realist” defenses of just war tradition are ineffective against pacifism because they are simply the inverse side of pacifism.
The section of Defending Constantine that Witherington is responding to includes a couple of pages attempting to apply the Sermon to political ethics - an attempt to read the Sermon as a mirror of magistrates. A good bit of the Sermon is straightforwardly applicable: Politicians as well as private persons need to control anger and lust, speak the truth, resist with good rather than evil, avoid hypocritical displays, trust in God and not money - and politicians have to practice these virtues not only in their private lives but in their public acts.
But Matthew 5:43-48 seems impossible to apply to political acts. How can a ruler love his enemies? Should he even try? And, especially, how do we square this with the Old Testament endorsement of war? Here is the best I can do, and it is admittedly weak. First, it is fairly easy to make the argument that violent, even lethal, intervention can be an act of love for the victim. To return to the example I discussed yesterday, Stephen saw Moses’ killing of the Egyptian as an act of deliverance: Moses acted, we can say, out of love for his Israelite brother who was being beaten. We can extend the principle of various kinds of war: A ruler calls out the troops to defend wives and children against an aggressive invader; at times, a ruler might send troops in to rescue a people from a domestic or foreign tyrant. There would be a lot of circumstantial judgments to make in those cases, but at least the argument can make sense.
The argument that is harder to make sense of is that opposing enemies can be an act of love for the enemy that is opposed. It is clear from Scripture that there are loving acts that cause physical harm and pain. Augustine points to the example of fatherly discipline: Fathers cause physical pain to their children out of love for them, and as discipline that prevents the children from doing greater harm to themselves (Proverbs 13:24). The analogy works smoothly if we are talking about various sorts of civil punishments: We can see how the physical pain of, say, whipping might be an act of loving discipline, or the social and psychic pain of imprisonment might also correct the offender. Or, suppose a mugger aims a gun at a victim, and I whip out a gun and shoot him in the leg; if I’m a good shot, I might prevent the mugger from committing a more serious crime than theft. (There are all sorts of other options for resisting a mugger, most of them are practically and morally preferable to taking a shot at him.) Again, we can reason by analogy to wartime situations: Repelling an aggressive invader prevents the aggressor from doing evil that, whether he knows it or not, does deep harm to himself.
When we introduce killing, the reasoning isn’t so satisfying. Is killing a mugger doing him a kindness? Is it an act of love to send him to God as an attempted mugger rather than as a murderer? Is it better to send a troop of soldiers to judgment before they slaughter and rape? Is that better for the soldiers to be killed than to be permitted to do evil? Put it in terms of the golden rule: If I were about to commit a horrific crime, would I want to be stopped? The rod causes pain, but saves from Sheol (Proverbs 23:14): Might killing the body in some circumstances be a means of saving the soul? (Of course, the circumstances in which this might be true are very limited. This is not an argument for abortion - kill the babies before they have a chance to do irreparable harm. The killing that I have in view is the killing that Scripture permits - in war, and as punishment for crime.) In any case, the logic of Augustine’s position pushes toward a positive answer to these questions, and as far as I can see at this point, it’s the only way to make the case. Other ways of defending just war lurch either into a public/private, moral man/immoral society dualism or into natural law, or simply ignore Jesus, none of which is a viable alternative in my judgment.
As Bell makes clear, just war as discipleship is not a permissive ethic of war, or an open door for militarism. On the contrary, it’s a stringent and difficult and even paradoxical political ethic, one that refuses to leave Jesus behind when maki ng judgments about the exercise of power.