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, the first one against a believer in the military and the second supporting such a service, are offered to encourage a deeper study of this difficult subject.
The most relevant Bible text, used by some, against serving in the military comes from the Sermon on the Mount. The whole passage will not be quoted here since it is familiar to most people (Matthew 5:39, 44 - 45). Jesus instructs us to "turn the other cheek" and not resist evil. He tells us to love our 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 Philistines attacked Israel many times).
The literal application of these Bible 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.
Ancient Israel let God wage war for them, on three or four occasions, instead of going into battle. Can someone who follows the Prince of Peace wage war? Can we kill our enemies, yet still say we love them?
Argument for serving
An argument from the Bible can also be made to justify a Christian joining the military. 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 believers are allowed to be in the military and fight for their home. The Apostle Paul, however, wrote to Timothy that if a believer does not feed, clothe, and provide other basics for his family, he is behaving even worse than a pagan who rejects God (1Timothy 5:8)!
What the Bible states 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.