There is no single Christian view of war, even though some would lead folks to think that all Christians are "right-wing war mongers".
There is no one verse that can be used to define the Bible's opinion on the matter.
The Bible does say, "Thou shalt not kill", but that applies to individuals and relates to murder. The Bible also depicts the histories of several wars and never condemns them (at least from the standpoint of being wars themselves).
My view is that you look at how various scripture themes tie together to determine if the principle of force is valid. There are Christians who have always been pacifists; there are Christians who have always stated that scripture permits force. I personally think those Christians who are pacifists are naive. We live in a fallen world, and to maintain justice sometimes force has to be used. In one sense, war is a logical extension of law. Pacificists would have to contend (in order to be logical) that police should never kill people in the line of duty. Society would break down in a heartbeat if the principle that force can never be used were followed.
This is implying that governments, in order to maintain a peaceful society, sometimes have to use force to do that, and that could be against citizens who commit crimes or against nations who do wrong. It is not saying that governments are direct institutions of God--but that in the scheme of things God uses human institutions to keep peace. Remember, too, this was in the letter to the Romans. The Roman government put Paul in jail for preaching and he still believed they were a legitamate government!
Another approach that has been developed to determine the validity of war is the "Just War Theory" that was developed by Augustine and expanded on by Thomas Acquinas. A good summary of this is at:
The problem is, people will always disagree on the conditions of when war is just. There are Christians who actually have trouble with the fact that an incredibly brave German pastor, Dietrich Bonhoeffer, participated in a plot to assassinate Hitler. He was captured and executed for it.
Another view is the priniciple of moral gradation. This is the idea that sometimes something that is not great must be done to stop something that is worse. There is a phrase (admittedly not scriptural, but I think sensible) that says, "All that is necessary for the triumph of evil is that good men do nothing." So even though some might contend that is was wrong for Bonhoeffer to want to kill Hitler it was appropriate because to stand by and do nothing would permit much more evil.