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In page Jus ad bellum:

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By contrast, agreements defining limits on acceptable conduct while already engaged in war are considered "rules of war" and are referred to as the jus in bello. Thus, the Geneva Conventions are a set of "jus in bello". Doctrines concerning the protection of civilians in wartime, or the need for "proportionality" when force is used, are addressed to issues of conduct within a war, but the same doctrines can also shed light on the question of when it is lawful (or unlawful) to go to war in the first place.