I have written on the distinction between jus ad bellum, conditions that justify going to war in the first place, and jus in bello, the standards required by God in the conduct of war. We have to keep these distinctions straight because, if we don't, we are going to muddle just war theory hopelessly. But to sort through it, you are going to have to bear with me for a moment.
Concerns about just conduct in war are relevant to the thinking and ethical behavior of every combatant. Every soldier has to know what goes and what doesn't go. What are the rules of engagement? If this happens, what may we do? If that happens, how can we respond? And what are we to avoid doing under every circumstance? Jus in bello refers to God's standards for combat, with the recognition that an army's commanders may supplement (not alter or replace) these standards with standards of their own. To rape and murder all the women in a village would obviously be a violation of the former. To pursue an enemy band beyond a certain point because it might result in too great a risk to civilians would be an example of the latter.
Jus ad bellum concerns the grounds for going to war. If Iraq had indeed been in possession of weapons of mass destruction, and if they had been planning to use them on us (two ifs that were overtaken by subsequent revelations), then that would have been a legitimate ground for going to war. But the average soldier is not in a position to determine whether or not this is a legitimate ground for a legitimate war, and in most cases, it is none of his business. A military man is under orders, under authority -- his commander says go and he goes, stay and he stays (Matt. 8:9). Or, more famously, he hurries up and waits.
But this does not mean that the average soldier (or citizen) never has a legitimate stake in ad bellum issues. If the king says that "we are going to war with the rascals in that other country, capture their king in battle, and then sacrifice him to Odin and Thor on the steps of my palace, as a sign our national dedication to the gods of war and thunder in all things," the average Christian footsoldier should be thinking something along the lines of hmmmm. More about this shortly, because fundamental paganism, if it is there, will always surface clearly at some point. The basic gods always demand their glory, and they always get it. And so if the average believer in Christ needs to see it, he will be able to see it.
I have also written that the militaries of this world will be transformed in exactly the same way that every other lawful vocation will be transformed -- through the participation and testimony of consistent Christians. I say lawful because, of course, we are not looking to make prostitution more effective by getting Christians into it. But we do look for that with regard to cooking, film-making, geological exploration, cabinet making, and bomb design.
But here is where the first great temptation comes. Secularists like it when they experience the fruit of greater effectiveness that comes with a larger number of Christians. But they don't like what comes after that, when the presence of Christians gets to the level where it starts to affect the missions undertaken.
In his book Imperial Grunts, Robert Kaplan refers to the rise of evangelicalism in the American military, and the transformation of military effectiveness that it helped to bring about. He quotes a liberal analyst and military man who said that "moral fundamentalism was the hidden hand that changed the military for the better. But you try to get somebody to admit it! We never could have pulled off Macedonia or Bosnia with the old Vietnam Army" (pp. 118-119).
Throughout his book, Kaplan speaks of evangelicals and their simple faith in God with great admiration. He quotes a lay chaplain's message before one mission. "To trust in the Lord is the best way to hunt down the devil" (p. 334). He admires this because, so far, it is helping accomplish what he believes the essential missions to be and not interfering with them. He likes the tribal god of the Christians, because it makes them more effective in expanding the empire. And Christians do have that effect . . . temporarily.
But at some point, somebody will always bring out the other gods. This same Robert Kaplan, in his realpolitik book Warrior Politics, argues, as his subitle baldly states it, "Why Leadership Demands a Pagan Ethos." Now if the justifications for going to war (ad bellum) depend upon this kind of appeal, then every Christian must say, "Nothing doing." Justifications for going to war should depend far more on Moses, Jeremiah, Isaiah, and the apostle Paul than they do on Sun-Tzu, Thucydides, Machiavelli, and Hobbes. Unlike the weapons of mass destruction debate, this standard rests not on the assessment of the facts, which are not available to all, but on an evaluation of public faith, which is.
The problem of keeping all this straight, incidentally, is compounded by weak sister Christians, who do their level best to fulfill Nietzsche's caricature of the Christian faith -- as something based on ressentiment, an envious and hostile attitude toward any form of strength. But there is a warrior ethos that is in full submission to Christ. That does exist, and at the leadership level -- not just at a level that keeps your footsoldiers away from painted ladies and cocaine.
If men like Kaplan, who know very well that the world is a dangerous place, see that we need a warrior caste (and we really do), and we in the meantime have allowed the notion to take root that such a need cannot be supplied from the ranks of Christians . . . well, then, we are going to receive what we have in effect asked for. Put another way, weak sister Christianity (by which I mean pacifists and their sentimentalist fellow travelers) will be the best way to ensure that the ranks of the military leadership fill up with the wrong kind of hard men. This is why Christian pacifism is really a functional heresy, and, as heresies go, it has been one of the bloodiest.