<P> Jus ad bellum is sometimes considered a part of the laws of war, but the term "laws of war" can also be considered to refer to jus in bello, which concerns whether a war is conducted justly (regardless of whether the initiation of hostilities was just). Jus ad bellum refers to "legitimate reasons a State may engage in war ." These rules focus on certain criteria for what makes a war just . </P> <P> Article 51 of the UN Charter clarifies: "Nothing in the present Charter shall impair the inherent right of individual or collective self - defense if an armed attack occurs against a Member of the United Nations ." </P> <P> An international agreement limiting the justifiable reasons for a country to declare war against another is concerned with jus ad bellum . In addition to bilateral non-aggression pacts, the twentieth century saw multilateral treaties defining entirely new restrictions against going to war . The three most notable examples are the Kellogg - Briand Pact outlawing war as an instrument of national policy, the London Charter (known also as the Nuremberg Charter) defining "crimes against peace" as one of three major categories of international crime to be prosecuted after World War II, and the United Nations Charter, which binds nations to seek resolution of disputes by peaceful means and requires authorization by the United Nations before a nation may initiate any use of force against another, beyond the inherent right of self - defense against an armed attack . </P> <P> 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 . </P>

A key concept for rules of engagement is the inherent right