Crosswalk.com recently posted an article from Salim Munayer called "Is There a Christian Attitude Toward War?" that I thought did a wonderful job of tackling this difficult issue. Munayer outlines three broad attitudes that can describe Christian thought. The first is to adopt an "in war as in war" approach, which means that "the reality of war necessitates a change in our moral understanding and behavior." Honestly, I can't believe how many Christians I know who take this tack. Believing that the end justifies the means, Christians promote war as a necessary good for God to accomplish his purposes in the world.
Now it can clearly be argued that God often directed the Hebrews to attack cultures and civilizations around them, even commanding that they be destroyed and punishing the Jews when they didn't destroy them. But it is a big leap to say that we should be promoting any kind of "war as a solution to evil" in our world today, declaring God to be on our side. I would suggest two reasons for this, one of which Munayer make a great case for; namely that Israel was under the theocratic rule of God, and no nation today can claim that status. To quote Munayer: "the particularity of our context makes it difficult and extremely dangerous to abstractly apply the principles of Joshua to the present conflict." My second reason would be that it seems to me to violate the way that Jesus taught us to live in the New Testament. He made it clear that his kingdom was not of this world, i.e. not obtained through normal political or military processes. Yes, he said that he came to bring not peace, but a sword (see Matthew 10.34), but he was talking specifically about the interpersonal relationships that we sometimes value over truly following his way of life. He's here talking about priorities, not war. He later in Matthew tells Peter not to use his sword trying to protect Jesus from arrest, because "all who draw the sword will die by the sword" (Mt. 26.52).
I am especially concerned with certain dispensationalist leaders who promote some version of the "Left Behind" eschatology that re-promotes modern day Israel to covenant-nation status with God and teaches that anyone who disagrees with them--or by default agrees with their "enemies" --sets themselves up against God himself. I believe this to be bad hermeneutics; there is no geo-political nation-state that can today claim status as God's chosen nation (see Romans 9-11). Certainly not all dispensationalist leaders teach this, but several prominent ones have so bought into the end times scenario of the position that they see Israel and the US as God's arm of destruction on evil. This is a dangerous position for anyone to take--acting on God's behalf--especially when it involves the taking of another human life!
A second approach is pacifism: the idea that all war is unjust by definition, and that killing of another human is wrong in any context. I have to admit, this position seems to more closely fit the teachings of Jesus and of the early church. It has looked more palatable to me in that past few years. But I have come to believe that it looks that way because I don't believe that the war in Iraq met the demands what Christians have long called "just war" theory.
Munayer calls this third position a "selectivist" approach. This approach argues that it is dangerous to make any absolute decisions about the morality of war. Some should be fought. Others should not. Just war theory, as stated by Constantine and then expanded by others through the centuries, promotes the idea that there are certain inalienable criteria that must exist before a war is considered just. See http://www.justwartheory.com/#INTRODUCTION for an overview of the idea from philosophical standpoint.
I hope that the key component in any Christian's view of war--and any other issue--is critical biblical and prayerful thinking.