Jus ad bellum

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.[3] In his Summa Theologica St. Thomas Aquinas notes that to be a just war, war has not only to be declared publicly, but also must be declared by the proper authority.[4]

Proper authority is what differentiates war from murder: "It is the rules of warfare that give the practice meaning, that distinguish war from murder and soldiers from criminals".[5] A soldier is treated as a prisoner of war and not a criminal because they are operating under the proper authority of the state and cannot be held individually responsible for actions committed under the orders of their military leadership.

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. Wars cannot be fought simply to annex property or install a regime change. Current doctrines of "anticipatory self-defense" or preemptive strikes, sometimes associated with the Bush Doctrine, have challenged concepts of right intention/just cause. Right cause includes humanitarian intervention, particularly when actions "shock the conscience". The responsibility to protect covers more in depth the nature of humanitarian intervention.

According to this principle, there must be good grounds for concluding that aims of the just war are achievable.[3] This principle emphasizes that mass violence must not be undertaken if it is unlikely to secure the just cause.[6] This criterion is to avoid invasion for invasion's sake and links to the proportionality criteria. One cannot invade if there is no chance of actually winning. However, wars are fought with imperfect knowledge, so one must simply be able to make a logical case that one can win; there is no way to know this in advance. These criteria move the conversation from moral and theoretical grounds to practical grounds.[7] Essentially, this is meant to gather coalition building and win approval of other state actors.

The principle of proportionality stipulates that the violence used in the war must be proportional to the military objectives. This objective is meant to preclude "scorched earth" policies. The level of military victory must be proportional to the level of destruction that ensues. For example, if there is one enemy combatant in a shopping plaza full of 400 civilians, it would not be considered proportional to blow up the plaza. However, if there is a high value military target in an area with far fewer civilians (a car or a private home) an attack and the collateral damage may be considered justifiable under the rule of proportionality.

The principle of last resort stipulates that all non-violent options must first be exhausted before the use of force can be justified. Diplomatic options, sanctions, and other non-military methods must be attempted or validly ruled out before the engagement of hostilities. Further, in regard to the amount of harm—proportionally—the principle of last resort would support using small intervention forces first and then escalating rather than starting a war with massive force such as carpet bombing or nuclear warfare.[8]

Since the 1950s, declarations of jus ad bellum have dropped dramatically.[11] In "Why States No Longer Declare War" Tanisha Fazal notes the drop in formal declarations of war since the 1950s.[11] This may be as a result of the nuance of war in modern times, given the rise of non-state actors such as terrorist groups.