There are various Christian views on war. The majority view throughout history dates back to Augustine, who taught there is such a thing as just war. Much more could be – and has been! – said on this topic, but I like how the just war concept is summarized in An Introduction to Biblical Ethics. The authors list seven points, noting that the first three points take priority.
1) Just Cause. All unprovoked aggression is condemned. A war for self-defense and protection (including defense of other vulnerable nations) is morally legitimate. Following this first criterion alone would eliminate all war and aggression.
2) Just Intent. The only legitimate intention is to secure a just or fair peace for friend and foe alike, ruling out revenge, conquest, economic gain, or ideological supremacy.
3) Lawful Declaration. Since the use of military force is the prerogative of governments – not of private individuals or parties within the state – a state of war must be officially declared by a lawful government.
4) Last Resort. War may be entered into only when reasonable negotiations and compromise have been tried and have failed. This does not mean gross injustices continue alongside endless negotiations. Last resort is a prudential, secondary consideration, as are the remaining criteria.
5) Limited Objectives. As the goal of just war is peace, war should not be committed to the destruction of another nation’s economy or its political institutions.
6) Limited/Proportionate Means. The weaponry and the force used should be limited to what is needed to repel the aggression and deter future attacks – that is to say, to secure a just peace.
7) Noncombatant Immunity. Since war is an official act of government, only those who are officially agents of government may fight, and individuals not actively contributing to the conflict (including POWs, medical personnel and casualties, as well as civilian nonparticipants) should be immune from attack, unless in cases of supreme emergency, as noted above.
Even if one doesn’t agree with every part of this summary, in my view this is helpful way to think of war from a Christian perspective.
You can find the entire discussion in chapter 24 of Robertson McQuilkin and Paul Copan’s An Introduction to Biblical Ethics.
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