I would have to agree that war is a great evil. It has been used throughout the ages as a tool of proud leaders to impose their will on others. There have certainly been unjust wars. I could see myself opposing an unjust war, and perhaps refuse to serve in one. The issues are confused, of course, by human evil.
War brings out the worst in some people.
We see a clearly articulated teaching of the civil authorities acting as administrators of justice. For rulers hold no terror for those who do right, but for those who do wrong. Then do what is right and he will commend you. For he is God's servant to do you good. But if you do wrong, be afraid, for he does not bear the sword for nothing .