That would depend on the individual Christian, and what their attitude toward national defense would be.
The Fifth Commandment addresses the issue of murder, not national defense or justice. The Scriptures set a precedent for the inherent value of human life. However, in cases of national defense or public justice, there is the great paradox of having to take one life in order to preserve another. Killing in self defense and in defense of others who are in danger is completely valid. St. Augustin once said that it is the sad duty of just men that sometimes they must fight wars for the sake of justice.
Furthermore, I would suggest we look at the attitude of God toward warfare in general and toward soldiers in particular. Did you know that God in the Old Testament is called YHWH Sabaoth - literally translated it means, "Lord of Armies" or "General YHWH." There is even a passage of Scripture that says that
YHWH is a great Warrior, mighty in battle. In a very real sense, the God of Scripture is a war God. And this is also true in the New Testament, for the
Lamb of God (Jesus) is said to make war against the enemies of God.
Furthermore, consider also that there were at least four great men of faith in the early church who were soldiers. There is a story of
a Roman soldier who came to Jesus to have his servant healed, and Jesus not only healed the servant, but commended the soldier for his great faith.
It was
a Roman soldier at the cross of Jesus who was the first to recognize the crucified Lord as the Son of God.
And the first Gentile Christian was a Roman soldier.
The fourth was
a Philippian soldier who guarded St. Paul.
Now as for whether an individual Christian should serve in the Armed Forces, that depends. Some Christians are able to realize that taking life in defense of the lives of others is within the scope of their calling. Others may be less comfortable with it. When I was in, I didn't really give it much thought; I was a security augmentee, which meant I could have been sent to combat, but never was. But I wasn't exactly a devout Christian in those days, either. Now, I'm not so sure I would want to kill, except in a case where I know I must in order to protect someone else (for example, if a gunman were trying to kill someone, and only I could stop them).
Another factor is whether or not a Christian can serve in a support capacity. One can serve in the military and not necessarily be put in front line combat. But that is an issue that must be thoroughly discussed with one's recruiter beforehand, not when combat is taking place.
The Code of Chivalry in the Middle Ages was essentially an attempt to answer this question: and ultimately, it required that the soldier (or knight, if you will) must be conscientious of justice in all that he does. If he fights, he must fight for justice. If he refrains, he must do so because it is just. Never should he fight (or refrain from doing so) solely for his own benefit.
In short, I do not consider serving in the Armed Forces - or for that matter as a policeman or other law enforcement capacity - a violation of the Fifth Commandment. However, it does put the Christian in a place where he must exercise wisdom and diligently seek to serve justice.