Admittedly, and I think anyone who is honestly approaching this issue has to recognize this, there is scant Biblical grounds for a Christian to take up the sword. There are lots of philosophical and pragmatic arguments. But the New Testament is unequivocal about it's condemnation of the use of violence either in aggression or self-defense. I am going to try to lay out the arguments in favor in this post so that there can be a balanced discussion.
Again, the issue here is not about the state. Was the United States justified in going to war in World War II? I certainly think that you can make that case pretty easily. I am talking about followers of Jesus Christ doing the same.
One place you might look is into the Old Testament. Certainly there is no lack of examples of God’s people going to war against the pagan nations. God commanded His people specifically to war against other nations to drive them out of the land and in doing so often called for a scorched earth style conquest where no one was spared. Is that a applicable for Christians under the New Covenant though?
There is the centurion with the ill servant in Matthew 8: 5-13. As a leader of men he humbly comes in faith to Christ and asks Him to heal his servant. Jesus commends his faith and heals his servant, but doesn’t condemn the centurion for his line of work. Of course Jesus also heals and forgives all sorts of sinners without advocating that people commit sins.
How about the "buy a sword" passage? Jesus says to His disciples in Luke 22:36-38:
He said to them, "But now let the one who has a moneybag take it, and likewise a knapsack. And let the one who has no sword sell his cloak and buy one. For I tell you that this Scripture must be fulfilled in me: 'And he was numbered with the transgressors.' For what is written about me has its fulfillment." And they said, "Look, Lord, here are two swords." And he said to them, "It is enough." (Luke 22:36-38)
So there is a place in one of the Gospels where Jesus tells His disciples to buy a sword. That seems a bit puzzling since a few verses later Jesus chastises Peter for striking someone with a sword. I am not sure why he seemed to be telling Peter to make sure he has a sword and then chastising him for using it.
Romans 13 details the legitimate use of the sword by the government. We are to be in subjection to those rulers and pay what taxes we owe (as I pointed out this would include taxes legally owed that might go to pay for war). So I have seen the argument that serving in the military, where mandated by law (like in a draft situation), is covered under by this chapter. But what about Shadrach, Meshach, and Abednego? They disobeyed the law of man because they refused to disobey the law of God. Also in Acts 4: 19-20 we see the disciples claiming that following the law of God was more important than the law of man, even though in doing so they faced the possibility of imprisonment and execution.
In searching for common sense Christian writers speaking in Albert Mohler, someone I greatly respect as a thinker and theologian, wrote about the idea of "just war" in April of 2004: Is War Ever Justified? A Reality Check. Dr. Mohler puts forth an interesting, well-thought out but strangely Bible-less argument.
War is a demonstration of the utter sinfulness of sin. In the name of the Prince of Peace, Christians must seek to establish and maintain our faltering and transient efforts at peacemaking until our Lord comes to establish the only peace that endures. In this fallen world, we must honestly acknowledge that peacemaking will sometimes lead to war. In the final analysis, war is the worst option imaginable, until it is the only option left.
I think that is lacking. For an essay, which granted makes lots of sense to this sinner, written by one of the preeminent contemporary theologians in America there is a deafening silence where Christ has spoken. Jesus doesn't say "blessed are the peacemakers except when they run out of options". The cross stands as a stark reminder that sometimes, perhaps oftentimes, we are called to be persecuted, to be offended, to be attacked and even killed.
That is a pretty scant list. Am I missing something? Can you think of other verses that would support taking up the sword, whether in warfare or in self-defense?