Many Christians, within the Catholic Church and without it, have taken the position that all war -- or even all violence of any kind -- is wrong. Dorothy Day, whose cause for canonization has been opened, noted in The Long Loneliness that the Catholic Worker movement (which she founded together with Peter Maurin) was not only pacifist but anarchist. The pacifist viewpoint is gaining ground, whether in pure or more diluted forms, among Christians of the rising generation, partly perhaps because of the leftward swing that the churches in America are experiencing.
I was, and remain, highly sympathetic to the pacifist position. At present my eligibility for the military is a non-issue, due to being gay, but if and when the current policies are repealed I intend to apply for the status of a conscientious objector.
This is not because I am a pacifist. I am not. The Scriptures make it clear, in passages such as Romans 13, that the secular authorities have been established by God -- not, admittedly, with the same directness, the same purpose, or the same protection from the Holy Spirit afforded to the Church. The Church was founded by God Himself and continues to be indwelt, in the whole and in all her members, by that same God. This is not true of any state. Some regimes, Christian ones included, have made valiant attempts at theocracy: the Byzantine Empire, Tudor England, Calvinist Geneva, the Massachusetts Bay Colony. But that is as alien to the reality of Christian belief as every other sin that guises itself as an angel of light.
Yet I digress. The Bible states plainly that secular rulers, in their own capacity, are representatives of God -- not a revelatory sense, but in a practical sense. In order for life to function at all, there must be some practical restraint of human evil, some order, some justice in the practical and not merely the ideal realm.
Set apparently against this are such passages as "Do not resist an evil person ... turn the other cheek." And in the context of daily squabbles between villagers, which would probably have been to the fore in the minds of Jesus' original audience, that is exactly right (surprise). Every trace of resentment, of bitterness, of revengefulness, must be given no quarter by the Christian in his own heart. But Jesus gives no indication that He is here setting forth any political theory, and it would be pretty surprising if He did, given that politics was a subject He rarely addressed directly, except to say that thing so blasphemous to American conservatives even within the Church: "Pay your taxes."
The problem with rigorous pacifism -- that is, the rejection of all forms of violence as intrinsically wrong, or even simply as worse than any of the evils it would prevent -- is that it is, in this respect, actually inconsistent with Catholic teaching. For of course, to insist that the use of force is always wrong does not merely eliminate all possibility of a just war; it eliminates the possibility of a just government (how could there be policemen, or prison guards, or a justice system in general, or even laws?) -- thus bringing us from pacifism to anarchism. Nor could there be any sort of discipline in schools; so we proceed from political to educational anarchy. Nor, realistically, could any punishments be conducted by parents, if such punishments are morally worse than simply letting the child go its merry way. Strict pacifism thus lands us at a rather austere picture of humanity, in which neither government nor school nor family exist in any practical sense of the words -- and so, quite apart from making the Old Testament pretty startling unjust in every possible respect, putting one in mind of Hobbes' description of man's life without society: "Nasty, poor, brutish and short."
Some people would allow for internal violence, but say that it ought always to stop short of war. For convenience, this may be labeled the semi-pacifist position. This is rather attractive, but unfortunately it suffers the fatal flaw of inconsistency. If it is moral for a government, when necessary, to impede or even harm its own citizens to prevent them injuring others, why is it suddenly immoral for them to do the same to foreigners? Are the lives of its own citizens worth less? Is it butting into other countries' business by defending its own citizens?
Many semi-pacifists would doubtless point out that very few wars have ever been fought for strictly defensive reasons. That is true. That is why, in a rather vague sense, Christians should indeed be "anti-war"; most wars are wrong. There is, according to the consensus of Catholic theologians, such a thing as a just war, but its criteria are rarely met, for they are stringent:
1. They must be fought in self-defense. All aggressive wars are intrinsically unjust. Note that this contains a tacit acknowledgment of how horrible war is even int he act of saying it can be defended, for no just defense. can take place unless an unjust attack has already taken place. (Some would argue that this criterion should be read strictly, so that wars which claim to be pre-emptively defensive are disqualified.)
2. Every peaceful means of resolving the conflict must have been exhausted first. This may be because they have been tried and found wanting, or because they have been shown to be impractical or ineffective. (For example, the aggressor may offer to settle a conflict in return for a monetary payment, but if the country cannot afford such a payment, that particular peaceful means would be demonstrably impractical.)
3. The damage the aggressor will inflict must be lasting, grave, and certain. "Hitler is going to kill all the Jews" would meet these criteria; "Hitler is going to make us wear totally stupid clothes" would fall somewhat short.
4. The damage the aggressor would inflict must be worse than the damage the war will do. So, for instance, if the only practical means of keeping your country from being destroyed by an invasion is to destroy it yourself, the war would not be worth it, and would therefore be unjust even in self-defense (defending yourself by ceasing to have a self is not logically plausible, QED).
5. Hand in hand with that, there must be a serious prospect of success. If, let us say, Sealand (a pretend-nation in the North Atlantic, consisting -- quite literally -- in about six or seven people) were invaded by Great Britain, it would be unjust for Sealand to take up arms, because a serious prospect of beating Great Britain does not exist. France, sure.
6. Non-combatants must not be touched. This includes not only children and the elderly (and, in most cultures until recently, women), but even non-combatant military assistants, according to most formulations of Just War Theory. So, you can bomb an AA battery while it is manned, but not a munitions factory during working hours -- because munitions workers are not actually in combat.
There are a lot of elaborations of these principles -- the principle of proportionate force, for instance, which dictates that enough force and no more must be used to stop an attack (which raises serious doubts about whether atomic weapons can ever be justly used); or the principle of fighting for peace, which means that the conflict must be ended as soon as is reasonably possible -- neither side may, for instance, insist on an unconditional surrender (which the United States did with Japan in the Second World War; I understand that attempts to surrender conditionally on the part of the Japanese were turned down, simply on the grounds that it was not an unconditional surrender).
Obviously this knocks nearly, if not absolutely, every war off the 'eligible' list for just wars. And so the rigorous application of Just War Theory would mean an awful lot of conscientious objectors among Christians an awful lot of the time, which, to be frank, I contemplate with some pleasure. After all, can any government -- can our government -- be relied upon to lead us only into just wars?
That being said, any war which really met these criteria would, I think, be not only just, but even morally obligatory. Chesterton said it with greater clarity and succinctness than I could manage: "There is no inconsistency in loving men and fighting them, if we fight them fairly and for a good cause."
The wars that have been most attacked for injustice and wickedness in recent times are the Crusades. It is said that fighting a war for a religion is evil. But surely this is quite backwards. Fighting for power, for money, for pleasure: these are wicked wars. But fighting to defend what we believe is holy -- how could we respect ourselves if we refused to do that? Admittedly the wars called the Crusades were in some respects on shaky moral ground, though for rather different reasons (and far less so) than most people think, mostly in that there was really some imposition of Catholicism involved, not simply the defense of it. Admittedly the actual conduct of every war, the Crusades included, exhibits much injustice; a criticism that can be leveled at all human behavior. But a religion can almost be defined as that which we are willing to die to defend. That is why the martyr and the crusader are, in the eyes of historic Christianity, regarded so similarly; as in Dante's heaven, where the two species mingle in the glories of Mars, their own shed blood dyeing the planet red. Ultimately, the only war really worth fighting is the war to defend what we regard as holy.
Dim drums throbbing, in the hills half-heard,
Where only on a nameless throne a crownless prince has stirred;
Where, risen from a doubtful seat and half-attainted stall,
The last knight of Europe takes weapons from the wall;
The last and lingering troubadour to whom the bird has sung
That once went singing southward when all the world was young.
In that enormous silence, tiny and unafraid,
Comes up along a winding road the noise of the Crusade.
-- GKC, Lepanto 15-22