If we believe in the Bible, we believe in the Christian future. This means that the salt is not seasoning to no effect, and that the light is not shining to no effect. The labor we put forth to make the world a better place is not labor wasted (1 Cor. 15:58). Moreover, if we understand how Christ's kingdom is one that will increase everlastingly (Is. 9:7), we understand that the lack of waste is not promised because we will be rewarded in another dimension somewhere for work done here, but rather that the work we do here to till the ground for the planting of Christ's wheat will result in a harvest here. What we do in the name of Jesus matters, and it matters here.
The salt and light is designed to get into everything, transforming it. The temptation to live in "separated communities," while understandable, is still a temptation. How does the Word spread? The Word spreads as those who believe it and talk about it spread (Acts 8:4). Now as we are scattered all over tarnation, talking about Jesus, where is it appropriate to go?
Geographically, it is appropriate to go anywhere because we were told to preach the gospel to every creature (Mark 16:15). We are not limited in that way -- it can be Asia, Australia, or North Dakota. The Lord Jesus is the promised life of the whole world.
But when we get there, we discover that we will have to make a living, provide for our families, and live a quiet and peaceable life (1 Thess. 4:11-12). So what professions may we scatter into? Or, to put it a bit stronger, what professions may we scatter into without incurring the just censure of the church?
Nobody believes that the Christian mission means that we should send our daughters into prostitution in order to be salt and light there, gradually transforming what has unfortunately become kind of a tawdry business. Nobody thinks that if we get enough of our young people selling cocaine, we can make it into a Christian affair. If anybody were foolish enough to actually do that, a faithful church would respond to it with church discipline. Now, is joining the army or navy in this same category, or is it in a different one?
If it is in the same category, then the church must authoritatively pronounce against it, and enforce her strictures. If it is not, then the pacifist case is entirely wrong, and there is no room left for a sympathetic quasi-pacifism.
A quick side note: this is a separate question entirely than whether or not it is appropriate to join that army over there. That is a question raised by just war theory, and not by pacifism. A man can believe it is appropriate to become a realtor, and yet avoid working for a particular realty business because of their dishonest business practices. That is a separate question. A just war theorist is not one who can justify any war. Just war theory assumes that many (if not most) wars are to be be rejected.
This issue is like falling out of the sky onto a steep roof -- you have to roll down one side or the other. If it is lawful for a Christian man to go into military service, then salt and light travels there, and has its influence there, in an analagous way to what it does in architecture, theater, education, and construction. If it is not lawful, then Christians should respond to an announcement that a young man has just joined the Marines with the same horror that we would show when the youngest daughter declares that she has a job as a stripper.
Pacifists, to their credit, at least as far as consistency requires, congregate in anabaptist communities where this is done, and this attitude cultivated. But quasi-pacifists just raise troubling questions, enough to create a deep sense of unease and perhaps false guilt, but not so much that anybody ever has to act on the basis of what they believe.
When I counsel young men who are going into the military, I talk to them about the challenges they will face, and how they must have thought through the issues so that they will not compromise their Christian convictions. Those situations, which will arise, must not catch them by surprise. But this is the same attitude that Christians must have going into every line of work. For the rest, we must do our jobs heartily, as unto the Lord.