Can someone who follows the Prince of Peace wage war? Can we kill our enemies, yet still say we love them, especially when they would say they don't want to die? That's a straightforward application of the Golden Rule (Matthew 7:12) found in the Bible. There's the insight that A.A. Milne had in favor of pacifism, or at least avoiding participation in war. His reasoning was: Why should we be willing to kill total strangers we meet on the battlefield when other strangers, our political rulers, say we should go and kill them? Could Christians go out and kill other Christians, their brothers in Christ, even members of the same church, merely because (often) unbelievers with power say they should do so?
Jesus, when he went on trial before Pilate, said He was a king but that:
"'My kingdom is not of this world. If My kingdom were of this world, My servants would fight, so that I should not be delivered to the Jews; but now My kingdom is not from here.'" (John 18:36).
Here Jesus denied that His followers should fight for Him against others. Furthermore, if Jesus' kingdom is not derived from this world, which is the main meaning of the Greek, then Christians should not tie themselves to this world's affairs so closely, such as become part of the military. Our citizenship is in heaven, not here (Philippians 2:20). We are to be like the patriarchs who were in the Promised Land, but didn't inherit the promises during their human lifetimes (Hebrews 11:13-16, 39-40). They placed their priority on the next life, not the present one.
Arguments for joining
In regard to a Christian being allowed to join the military please consider Numbers 1: "Take a census of all the congregation of the children of Israel, by their families, by their fathers’ houses, according to the number of names, every male individually, from twenty years old and above - all who are able to go to war in Israel. You and Aaron shall number them by their armies." (Numbers 1:2-3, NKJV)
Throughout the Old Testament, it is clear that the men over the age of 20 of the children of Israel were trained to fight. This, of course, is when there was a nation of Israel and when they were governing themselves, supposedly under the laws set down by God.
In Israel there was only a small "standing" army most of the time. A call could go out, however, and thousands of fighting men could be gathered to do battle quite rapidly (for the time).
It is true that there is no specific commandment stating one should join the military and fight for his country. However, consider the following the Apostle Paul wrote to Timothy: "But if anyone does not provide for his own, and especially for those of his household, he has denied the faith and is worse than an unbeliever." (1Timothy 5:8)
The implication of this Bible verse is clear. Providing for his own would include not only the necessities of life - such as food, clothing, shelter and training - but also providing for their safety. This would mean protecting one's family from harm, and - by interpolation - protecting one's community, state and nation through either the military or the police.