To be a Christian is to be in favour of peace. We follow a Lord who is called the Prince of Peace. We look forward to the day when swords will be exchanged for ploughshares, and every implement of war will be destroyed The logic of the gospel demands this stance. Christ himself is our peace, having reconciled us to God and therefore to one another. God brings peace, not by making war on his enemies, but by giving himself in the person of his Son to die for his enemies. Blessed are the peacemakers - they shall be called sons of God.
There is a strong tradition of Christian pacifism which follows this line in the gospel narrative and concludes that principled non-violence is a necessary part of Christian discipleship. I have a lot of respect for this tradition. I think it stands on an important truth: that violence is not the ultimate solution to anything. Moreover, it highlights a vital and thrilling part of the Christian hope: that there will, one day, be peace.
Personally, I am not a pacifist, but belong to what has historically been the majority Christian position: the just war tradition, which maintains that under certain circumstances, and with certain objectives, military action can be right and justified. I am not a pacifist for Biblical, theological, and ethical reasons.
Biblically, I think you can't argue that war is and always has been wrong in every circumstance without cutting out big chunks of the Old Testament. And I don't think you can argue that war is wrong in every circumstance in the Christian era without denigrating the Old Testament. I also want to take seriously the fact that the state has a legitimate right to the sword.
Theologically, I think the pacifist position represents an overly realised eschatology. That is to say, I think it doesn't take seriously enough the fact that the world is still dominated by the present evil age. Moreover, I think there is a naivety here about human sin. The pacifist tends to assume that we should be able to find a peaceful solution to every problem. Of course we should try, but in a world of sinful people there will be conflict.
Ethically, I fear that pacifism often involves leaving the innocent to suffer whilst we stand aloof. I think if often means preserving our own righteousness whilst hoping other people will do our dirty work. I am not prepared in any way to concede that pacifism holds the ethical high ground.
So I am a macro-pacifist. I believe in peace. I do not believe that any problems can be ultimately solved through violence. And yet I do believe that in certain circumstances war is necessary in the context of seeking peace.