Jus ad bellum

Jus ad bellum (Latin for "right to war") is a set of criteria that are to be consulted before engaging in war, in order to determine whether entering into war is permissible; that is, whether it is a just war.

Definition
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).

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.

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.

Right authority
The principle of right authority suggests that a war is just only if waged by a legitimate authority. Such authority is rooted in the notion of state sovereignty.

Right intention
According to the principle of right intention, the aim of war must not be to pursue narrowly defined national interests, but rather to re-establish a just peace. This state of peace should be preferable to the conditions that would have prevailed had the war not occurred.

Reasonable hope
Just wars must have a reasonable chance of success. According to the principle of reasonable hope, there must be good grounds for believing that the desired outcome can be achieved. This principle involves weighing the costs and benefits of waging war and emphasizes that human life and economic resources should not be wasted on war efforts that are certain to fail.

Proportionality
The principle of proportionality stipulates that the violence used in the war must be proportional to the attack suffered. For example, if one nation invades and seizes the land of another nation, this second nation has just cause for a counterattack in order to retrieve its land. However, if this second nation invades the first, reclaims its territory, and then also annexes the first nation, such military action is disproportional.

Last resort
The principle of last resort stipulates that all non-violent options must first be exhausted before the use of force can be justified.