The Christian “just war” principles say that even in war you should not deliberately kill civilians. It also says that war should be waged defensively: you should not attack first. A just war should be a last resort, undertaken only when other measures have been exhausted and when there is a reasonable chance of success. Moreover, retaliation should be proportionate to the original offense. If someone raids your tribe and kills ten people, you are not justified in raiding theirs and killing ten thousand people. The “just war” doctrine is a product of Christianity. It has its roots in Augustine, was developed further by Aquinas, and was then given its first modern expression by such thinkers as Francisco de Vitoria, Hugo Grotius, and John Locke.