What does the Bible have to say about serving in the military?

They came forward, seized Jesus and arrested him. And suddenly, one of the followers of Jesus grasped his sword and drew it; he struck the high priest’s servant and cut off his ear. Jesus then said, “Put your sword back, for all who draw the sword will die by the sword. Or do you think that I cannot appeal to my Father, who would promptly send more than twelve legions of angels in my defense?” (Matthew 26:50-53)

Jesus says that all who use the sword (=coercive force, like that of the military) will die by the sword. He commands his disciples to put their swords away. If he wanted be served by an army, he could command an angelic army; but actually, he rejects this possibility.

His followers, seeing what was about to happen, said, “Lord, shall we use our swords?” And one of them struck the high priest’s servant and cut off his right ear. But at this Jesus said, “That is enough.” And touching the man’s ear he healed him. (Luke 22:51)

Again, in response to his disciples question (which is basically the same as your question), "shall we use coercive force to try and do good?", Jesus' response is to command his disciples to stop using coercive force, and to give them an example of what it really means to do good, by healing his enemy, the very enemy whom he will allow to kill him.

“Blessed are the peacemakers: they shall be recognised as children of God. [...]

“You have heard how it was said: Eye for eye and tooth for tooth. But I say this to you: offer no resistance to the wicked. On the contrary, if anyone hits you on the right cheek, offer him the other as well; […]

You have heard how it was said, You will love your neighbour and hate your enemy. But I say this to you, love your enemies and pray for those who persecute you, so that you may be children of your Father in heaven, for he causes his sun to rise on the bad as well as the good, and sends down rain to fall on the upright.” (Matthew 5:9, 38-39, 43-45)

Jesus blesses his disciples as peacemakers, and makes clear that this peace will be made, not by the use of coercive force to kill one's enemies, but by the use of love and prayer and non-resistance to the enemy. He also shows why this peacemaking quality is the sign that his disciples are the children of God: because those who make peace in this way are like the Father, who gives good things to everyone, even his enemies.

The peace sown by peacemakers brings a harvest of justice. Where do these wars and battles between yourselves start? Is it not precisely in the desires fighting inside your own selves? You want something and you lack it; so you kill. You have an ambition that you cannot satisfy; so you fight to get your way by force. […] There is only one lawgiver and he is the only judge and has the power to save or to destroy. Who are you to give a verdict on your neighbour? (James 3:18-4:2; 4:12)

James here is talking about quarrels between Christians. The apostle thinks it is so plainly the case that warfare is evil and unChristian that he uses it as a metaphor to show how quarrelling is also evil and unChristian. The problem with both is that they are motivated by a selfish defense of one's own (or of one's own national) interests, and that they both entail presuming that we can judge our enemies and destroy them. In fact, we should be motivated by love for others, not a desire to promote our interests against theirs (or our country's interests against their country's), and should leave judgement to God.

Do not model your behaviour on the contemporary world, but let the renewing of your minds transform you, so that you may discern for yourselves what is the will of God—what is good and acceptable and mature. […] Never pay back evil with evil, but bear in mind the ideals that all regard with respect. As much is possible, and to the utmost of your ability, be at peace with everyone. Never try to get revenge: leave that, my dear friends, to the Retribution. As scripture says: Vengeance is mine—I will pay them back, the Lord promises. And more: If your enemy is hungry, give him something to eat; if thirsty, something to drink. By this, you will be heaping red-hot coals on his head. Do not be mastered by evil, but master evil with good. (Romans 12:2, 17-21)

Paul exhorts us not to conform to the world, but to conform to the example of Christ. This entails doing good to those who do evil, and explicitly cannot entail doing harm to those who do evil (even Hitler and ISIS). "Be at peace with everyone," even those who do evil to you. You can't control whether they are at peace with you, but you can and must control whether you are at peace with them. Again, an operative principle is that we have no right to judge others worthy of destruction, but must leave this to God.

/r/Christianity Thread