In the springtime, when kings go out to battle, it is worth pleading with Christians to reconsider our prevailing ethos of war. I might as soon question the ethics of oxygen than the American military juggernaut. We have so deeply cast our lot with our armed forces that it is difficult to locate where the church ends and Americanism begins. But the Scriptures clamor to be heard on this very point and to them we must go.
There are basically two categories of warfare in the Old Testament. First, God used his covenant people Israel as an instrument of judgment on surrounding nations. It was generally total warfare, a nasty business of razing cities to the ground, sowing salt, executing survivors, kidnapping virgins, collecting foreskins, and dashing babies to bits. The second category of war was that of secular nations used by God to judge and then were judged by God for judging. It was live by the sword and die by the sword - no sooner did God judge Israel with Assyria than he judged Assyria for her wickedness in the matter (Is 10).
Contra wishful thinking, America is not the new Israel. In fact, Israel is not even the new Israel. If you are looking for support for a secular government to be used by God to judge another nation and in turn be blessed for her efforts you won't find it in the Old Testament. Try Greek mythology. If you are looking for a "just war theory" - taking the word war from the OT and baptizing it in some of the humanitarian kindness from the New - you won't find that either. Try Augustine or Geneva.
There's no space to cover Romans 13 here. Suffice to say that it would take some fantastic hermeneutical gymnastics to balance America's worldwide jurisdiction over sovereign states on the point of the sword mentioned there.
Generally a defense for warfare is drawn not primarily from the Bible but our predicament, concerning Hitler, Al Qaeda, Darfur. At best this is thinly veiled scorn for the naivety of our Lord who failed to foresee this. At worst its blatant disregard for everything he said about laying down rights and taking up the cross.
Jesus said "blessed are the peacemakers" and Paul, "for the weapons of our warfare are not of the flesh"; and yet somehow we've spiritualized the peacemakers and materialized warmongers. Its long overdue for the burden of biblical proof to reside with the war makers.