I'm tempted in my discussion of war and peace, to start from the top, outline the full Biblical case for nonviolence, and enter into dialog with major objectors to that case. I'm not going to do that--now at least--primarily because I really don't think I've got much to add to what has been said far more eloquently by others. I will rather reiterate a few main points.
First, the case for Christian nonviolence rests primarily with the character, teaching, and demeanor of Jesus himself. Try as one might to say otherwise, we have to confront the reality that Jesus lived a life of peace, taught love for enemies, and explicitly commanded that his followers return good for evil. Never in his entire ministry, did Jesus qualify any of those commands with an exception for when a disciple was working in a state-sanctioned capacity.
The objection is often raised that Jesus' commands (in particular the Sermon on the Mount and its parallels) must be taken in the larger context of God's commanding and/or condoning warfare in the Old Testament (for an insightful struggle with issues of O.T. Violence, I recommend Greg Boyd's occasional blog series on this subject). Indeed this context must be considered, in the same way that ALL of the "You have heard it was said. . .but I say" statements should be seen, as Jesus clarifying, strengthening, and otherwise modifying accepted principles given before. When Jesus made explicit statements addressing an issue, as he did with our behavior toward our enemies, they must be taken as the final word on the issue. Of all the places we CANNOT qualify Jesus' commands on the basis of other Biblical perspectives, those places where he was most explicit seem to me to be the ones we must be most cautious. Simply put, where Jesus speaks, his words trump every other consideration. Otherwise, someone or something else is Lord.
By far the most common objection I hear to this perspective is Romans 13:1-7, which clearly states that governments, who have their authority from God, wield the sword to reward those who do good and to punish those who do evil.. However, we have got to keep in mind that this passage is bookended by commands directly to the believer to live a completely different paradigm marked by love and self-sacrifice. Romans 12:17-21 explicitly commands us to love our enemies, and Romans 13:8-10 reiterates the message about loving our neighbors (don't forget how Jesus defined "neighbors" in Luke 10:29-37). This "bookending" suggests rather strongly that, whatever the state's rights or responsibilities may be vis-a-vis violent force, it is categorically not the acceptable way for followers of Jesus.
Furthermore, the command to submit to earthly authorities has no bearing on our current state in the U.S., where we have a volunteer army, not a compulsory one. Anyone who joins the military today may do so for a variety of reasons, but it is not submitting to the authorities to do so when the authorities have not demanded it. Therefore, even if Romans 13 might mandate submission to a draft (I am not suggesting for a moment that it does), this is a useless argument in favor of military service in America today.
But this brings me to a point that I have not heard discussed in either camps who traditionally advocate nonviolence, nor those who traditionally favor military service. Throughout the Old and New Testaments, we are repeatedly shown that we are each accountable for our own actions. One of the biggest concerns I have with military service is the fact that the soldier must submit himself to the authority of the military chain of command, and accept the commands to do actions (destruction of lives and property) that would by any definition be immoral except for the context (warfare) in which they are done. The problem is, even Augustine's "Just War" doctrines make it clear by the very things they proscribe, that not all wars, nor all actions within a war, are just. In the chain of command, it is rare that the individual soldier is privy to sufficient information to accurately weigh the justice of the command he has been issued. He is required to trust the intrinsic morality of the chain of command, and on that basis to commit actions that would be sinful in any other circumstance.
This, I submit, is abdication of one's moral responsibility. All the famous hypotheticals about big men breaking into your house and threatening your wife and kids, are personal, intimate pictures where the (ill-founded) presumption is that a clear-cut moral picture is visible. But when an air force pilot is directed to bomb a village in Afghanistan, he must rely on the entire command and intelligence structure to have gotten it "right," that the village or house he's targeting actually contains a combatant or terrorist leader who must be eliminated, and that whatever civilian deaths may accompany this attack are both unavoidable and a sufficient price to pay in order to get our bad guy. The result may indeed be the death of a "bad guy," but it can just as well be the annihilation of a wedding party. Either way, our churches have absolved our pilot of responsibility for the morality of his action. There is absolutely no Scriptural case for this.
To put it as bluntly as I can, I propose that no Christian has the freedom--ever--to yield to another person the right to determine that an otherwise-sinful action is, after all, moral. We are accountable for our own actions, and Romans 13 does not give us a pass when the state commands otherwise.