I believe that the majority view (historically anyway) of the church is that Scripture teaches that it is proper to conduct "just" wars. Since the American military (and government) does not distinguish between just and unjust wars and requires all servicemen and women to fight regardless, is it advisable for Christians to volunteer for military service?
In regards to the Christian understanding of just war, we can consider the explication of the sixth commandment ("Thou shalt not kill.") in the Westminster Larger Catechism:
So, yes, we do acknowledge the propriety of "just war."
Q. 135. What are the duties required in the sixth commandment?
A. The duties required in the sixth commandment are, all careful studies, and lawful endeavors, to preserve the life of ourselves and others ... by just defense thereof against violence,...
Q. 136. What are the sins forbidden in the sixth commandment?
A. The sins forbidden in the sixth commandment are, all taking away the life of ourselves, or of others, except in case of public justice, lawful war, or necessary defense;
You asked, "Since the American military (and government) does not distinguish between just and unjust wars and requires all servicemen and women to fight regardless, is it advisable for Christians to volunteer for military service?"
I would have to differ with the premise of your question. In that civil governments bear "the power of the sword, for the defence and encouragement of them that are good and for the punishment of evil doers" (Westminster Confession of Faith, chapter XXIII.I), all governments, including the American government, must necessarily make decisions about the justice or injustice of war. Whenever a government, any government, chooses to go to war or to take military action, they are, in effect, making the determination that it is just to do so.
Of course, governments are capable of acting unjustly and have often done so. But in their own eyes, at least, they do what they do on the basis of what they perceive is best, or prudent, or expedient, or beneficial. The decision is made on the basis of perceived "justice." Even atrocities have been perpetrated on the basis that it seemed "just" to those who have carried them out; and in the end governments may only be trying to 'justify themselves'. But the point is this, the question of whether to go to war or not, that is, to wield the power of the sword, is always a matter of justice.
The problem, of course, is that justice, true justice, is not defined by men but by God. The Scriptures clearly teach that the decisions of civil governments ultimately will be weighed in the balance of God's justice. Simply because a government decides to use force does not insure that true justice is being done.
In one sense, the American government and its military is no different from any other government in history. Whether the government is a monarchy, or a constitutional republic, those who make the decisions to wield the sword are making decisions that impinge on the question of justice. The soldiers of the armies of such governments are thus being called on to carry out those decisions, and such governments expect them to do so.
It is here that we point to the biblical understanding of conscience. An extremely important principle is stated in the Westminster Confession of Faith (XX.II) as follows:
2. God alone is Lord of the conscience, and hath left it free from the doctrines and commandments of men, which are, in anything, contrary to his Word; or beside it, if matters of faith, or worship. So that, to believe such doctrines, or to obey such commands, out of conscience, is to betray true liberty of conscience: and the requiring of an implicit faith, and an absolute and blind obedience, is to destroy liberty of conscience, and reason also.
Your question is an important one, not just for the military man or woman, but for us all. As Orthodox Presbyterians we believe being a soldier is a lawful calling as we believe being a doctor or nurse or a teacher or a salesman or an athlete is a lawful calling. The OPC is not a pacifist denomination. We believe it is lawful to be a soldier and to fight wars.
Does that mean, then, that every war is just? Of course not! So what is a soldier to do when confronted with unlawful commands or asked to fight unjust wars? We could ask a similar question of others. What should a doctor or nurse do when the hospital requires him or her to participate in abortions for the sake of convenience? What should a teacher do when required to teach evolution rather than creation as fact? What should a salesman do when asked to shortchange the customer? What should the athlete do when required to compete on the Sabbath?
What would happen to the soldier who refuses to obey unlawful commands? That is a weighty question, but remember many Christians in many different vocations have suffered for preserving conscience and determining to obey God rather than men. Pray that we all would have the courage to do so. We remember the sober words of Jesus, "My friends, do not be afraid of those who kill the body, and after that have no more that they can do. But I will warn you whom to fear; fear the One who after He has killed has authority to cast into hell; yes, I tell you, fear Him" (Luke 12:4-5). How many of us have been asked to sacrifice our lives for conscience' sake? Yet in Christ all of us have been asked "to present our bodies a living and holy sacrifice, acceptable to God, which is our spiritual service of worship" (Romans 12:1).
In the end, we do well not to bind beyond or contrary to the Word of God the consciences of those who would be soldiers. Even so, we believe the responsibility of bearing the sword is weighty. Therefore, we ought to encourage those who do choose to be soldiers with our prayers and counsel and together endeavor with them to bring every thought captive to the obedience of Christ. We must each count the cost of following Christ.
May God bless you richly.
R. Daniel Knox, Pastor
Grace Orthodox Presbyterian Church