From A.A. Hodge's Commentary on the Westminster Confession:
5. Christian magistrates may lawfully, under the New Testament, wage war upon just and necessary occasions. The right and duty of self-defense is established by the inalienable instincts of nature, by reason, conscience, the Word of God, and the universal consent of mankind. If it is right for an individual to take life in self-defense, it must be equally right for a community to do so on the same principle.
It is very difficult to decide in particular cases when it is right for a Christian nation to go to war, and it is not our place to consider such questions. But the following general principles are very plain and very certain: -- War is an incalculable evil, because of the lives it destroys, the misery it occasions, and the moral degradation it infallibly works on all sides -- upon the vanquished and the victor, the party originally in the right and the party in the wrong. In every war one party at least must be in the wrong, involved in the tremendous guilt of unjustifiable war, and in the vast majority of cases both parties are in the wrong. No plea of honour, glory or aggrandizement, policy or profit, can excuse, much less justify, war; nothing short of necessity to the end of the preservation of national existence. In order to make a war right in God's sight, it is not only necessary that our enemy should aim to do us a wrong, but also (1.) That the wrong he attempts should directly or remotely threaten the national life; and (2.) That war be the only means to avert it. Even in this case every other means of securing justice and maintaining national safety should be exhausted before recourse is had to this last resort. A war may be purely defensive in spirit and intent while it is aggressive in the manner in which it is conducted. The question of right depends upon the former, not the latter -- upon the purpose for which, and not upon the mere order in which, or theater upon which, the attack is made.