ANSWER: What the Bible says about the military and whether a Christian is allowed to join or not are very serious matters that deserve a lengthy discussion. Unfortunately, space does not allow for an in-depth study of this topic in relation to a believer. The below brief arguments made by two of the site's Email Evangelists, one against and one for serving in the military, are offered to encourage a deeper study of the subject.
Arguments against serving
The basic, most relevant Bible text related to the subject of the military comes from the Sermon on the Mount by Jesus. 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 Bible verses? The founder of the Quakers, George Fox, realized that people could not. It is easy to see it as impractical, and impossible to implement, since other nations would seek to attack us - such as the Nazis and communists in the past. 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.
Let every soul be subject to the governing authorities. For there is no authority except from God, and the authorities that exist are appointed by God. (Romans 13:1)
These verses, however, do not authorize Christians to join groups that defend a country or even the police. It is a point of faith, but people who believe in God's ways can count on protection from him. Such protection would also applies to nations. 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? That is a straightforward application of the Golden Rule (Matthew 7:12) found in the Bible.
There is the insight that A. A. Milne (creator of Winnie-the-Pooh) 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 . . . " (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 on the earth (Philippians 2:20). We should place our priority on the next life, not the present one.
Arguments for serving
In regard to a Christian being allowed to join the military please consider Numbers 1 in the Bible.
Take a census of all the congregation of the children of Israel . . . every male individually, from twenty years old and above - all who are able to go to war in Israel. (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 occurred when the nation of Israel was governing itself by 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 fight for his country. However, the Apostle Paul wrote to Timothy that if a believer does not feed, clothe, etc. (provide the basics) for his family, he is not only not acting like a Christian but he is behaving even WORSE than those who believe in God and Jesus (1Timothy 5:8)!
The implication of what this Bible verse says 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.