The bold contrast that Koontz draws between those who have converted to the Christian position and those who have not reflects a broader conception of dualist ethics, one that sharply distinguishes the moral obligations of the Church from those of the (unconverted) world. According to the dualist conception, Koontz argues, those "committed to the way of Christ" are expected to live differently from those in "the world." The dualist conception therefore leaves open the possibility of a certain "quasi-legitimate" justification for war, provided it is chosen and waged not by Christians but by the state. This view of the "higher responsibility" of Christians has its origins in another ongoing conflict of interpretations within a number of Protestant traditions. As Koontz observes, the conflict arises out of two closely related scriptural passages, St. Paul's Letter to the Romans 12:9-21 and 13:1-7, and is dramatically evident in the 1527 Schleitheim Confession: "The sword is an ordering of God outside the perfection of Christ. It punishes and kills the wicked and protects the Good."
There can be no doubt that Cartwright is correct, at least where such an ethic exists (and it is by no means a straw man). Dividing the world into two ethical spheres with equally legitimate, divinely sanctioned codes of conduct (even quasi-sanctioned) creates a problem for pacifists when it comes to being a witness for peace in the world. Fortunately for pacifists, and unfortunately for Cartwright's complaint, much of the ethical dualism that is present in pacifist thought is only apparent, the product of semantic imprecision. The fault of pacifists, certainly, but not a great deterrent to their overarching message.
The key is in the language even Cartwright uses when paraphrasing Koontz. Christian pacifists know they "are expected to live differently" from non-Christians and, consequently, they expect non-Christians to live differently than they do. If these expectations are divine expectations which are, or ought to be, understood as identical to divine moral imperatives--if God expects a certain code of conduct from non-Christians in order to rise to the level of ethical living and a separate code of conduct from Christians to meet that same threshold, even if to different ultimate consequences--then Cartwright's problem is real and damning. If, however, the expectations are human expectations, pragmatic realities based on a recognition of the core beliefs which govern any give person or group of persons' behavior, the problem disappears.
To put it another way, the government expects you to obey the law, and I expect basketball players to be tall. Now, when I help a Christian convert flee the country in order to avoid sharing custody with her lesbian ex-wife, the government arrests me and puts me on trial. Rightly so. I violated their expectations which have authoritative force. Meanwhile, Muggsy Bogues was once a big name in the NBA. I didn't try to have him expelled from professional basketball because he didn't conform to my expectations. There is an obvious difference between normative expectations and pragmatic ones, even if Koontz does not take the care to specify which he means and Cartwright doesn't bother to consider the alternative to ethical dualism.
With regard to the use of force by government (e.g. war), it is important for Christian pacifists to be clear about what they mean when they say that they expect, or even that God expects (particularly in this latter case, with its anthropomorphic thrust), governments to employ violence. It is not an affirmation that their use of violence is legitimate, or even quasi-legitimate. It is a recognition that non-Christians in non-Christian institutions will employ non-Christian means to achieve non-Christian ends. To expect them to do otherwise--that is, to expect them to act like Christians--is to either live in a perpetual state of disappointment or, as has been more often the case, to find one's own view of what is Christian being slowly conformed to what is not even as Christians try to Christianize non-Christian instruments of power (e.g. civil government).
It is ultimately a matter of sequence not ethics, and it applies, for Christian anarchists, beyond the narrow scope of war. For example, when I say that I believe US government should legalize same sex marriage, that is not an endorsement of the morality of homosexuality. It is a recognition that it is inconsistent, even hypocritical, for the government to outlaw a behavior solely on the grounds that it violates morality. By the internal logic of the American system of government, in the political vision of the framers of this country, that kind of abridgment of freedom is anathema. I still think gay marriage is wrong, but I realize that expecting a country of non-Christians to behave as if they were Christians achieves nothing except to further open the name of Christ to ridicule.
The same logic then operates for the use of violence. I expect the government to use force not because it is virtuous to use violence beyond the walls of the church but because I understand that civil government necessarily sustains itself through the use of coercive force. The primary problem is not that Washington has a military and likes to use it. The primary "problem" is that Washington isn't Christian. Trying to coerce the state into becoming pacifist has all the logical consistency of going overseas to invade countries so they'll stop being hostile toward us. Which we would never do. Because it's stupid.
The solution to the problem of violence, as with the problem of homosexuality or any other ethical problem, is first to convert the problem people in question. Before I can convince someone that war is wrong, I first have to convince them that God exists, that sin is a problem, that God intervened through the Incarnation to remedy the problem, that the work of Christ inaugurated a new, peculiar existence for those who join themselves to him, and that this new life in Christ comes with a set of covenant expectations. Only then can we share the kind of internal logic necessary to get from the world needs war to thrive to Christ has called you to love your enemy, not resist the evildoer, and bless those who persecute you. There is no dualism there. Just a recognition of the organic nature of the human transformation which occurs when someone comes out of the kingdom of the prince of this world and into the Kingdom of Heaven.