ANSWER: What the Bible says about the military and whether a Christian is allowed to join or not is a serious matter that deserves a balanced discussion. The below brief arguments, one against and one for serving, are offered to encourage a deeper study of this difficult subject.
The basic, most relevant Biblical text against serving in the military comes from the Sermon on the Mount. The whole passage will not be quoted here since it is so very familiar to most people (Matthew 5:39, 44 - 45). These verses are Jesus' instructions to "turn the other cheek" and not resist evil. He tells us to love your enemies and even BLESS those who act against us. Can Christians evade the literal application of these verses in regard to the military?
The founder of the Quakers, George Fox, realized that people could not avoid the literal meaning of Jesus' words. It is easy to see, however, that not having a military at all would be impractical and impossible to implement since other nations would seek to attack us (e.g. the Nazis, etc.). The literal application of these verses concerning the military could also be applied to joining a police force. Romans 13 tells us that God uses unbelievers to maintain law and order (Romans 13:1).
God's word does not authorize true Christians to join groups that defend a country or even the police. It is a point of faith, but people who believe in His ways can count on protection from him. Such protection would apply to nations IF they repented and worshipped the Eternal in spirit and in truth. In three or four cases, ancient Israel let God wage war for them instead of going into battle. Can someone who follows Jesus, who is the Prince of Peace, wage war? Can we kill our enemies, yet still say we love them, especially when they would say they do not want to die?
Argument for serving
An argument from God's word can be made to justify a believer joining the military. The book of Numbers, chapter 1, records a census taken by the children of Israel of all the males, twenty years old and above, who are able to wage war. Throughout the Old Testament it is clear that men were trained to fight. This occurred when Israel was governing itself by the laws set down by the Eternal. Although the nation only had a small standing force (most of the time), a call could go out and thousands of fighting men could be gathered to do battle quite rapidly.
It is true that there is no specific commandment stating one should fight for his country. However, the Apostle Paul wrote to Timothy that if a believer does not feed, clothe, and provide other basics for his family, he is not only not acting like a Christian but he is behaving even WORSE than those who reject God (1Timothy 5:8)!
What holy writ statess is clear. A person who provides for his own family not only has to be concerned about aquiring the necessities of life, such as food, clothing, shelter, and so on, but also providing for their safety. This means protecting them from harm, and, by interpolation, protecting one's community, state, and nation through serving in the military or the police.