Jus ad bellum (/juːs/ YOOS or /dʒʌs/), literally "right to war" in Latin, refers to "the conditions under which States may resort to war or to the use of armed force in general".^[1] Jus ad bellum is one pillar of just war theory. Just war theory states that war should only be condoned under 'just' conditions.^[2] Jus ad bellum simply limits the causes for which war can be considered justifiable.^[2] The other parts of just war theory include jus in bello (just actions in war) and jus post bellum (justice after war).^[2]

The history of jus ad bellum dates back to early religious and philosophical ideas. There are references to the idea of morally fought wars dating back to ancient civilizations. Thousands of years later, these ideas still stand in today's society.

Many of the ancient civilizations have shown some degree of understanding of just war. One example can be found in Ancient Egypt. Early Egyptians viewed themselves as the cosmological center of civility.^[3] Therefore they put their faith in the gods alongside the pharaoh to have just reasons to engage in war. These include self defense, defense of their allies and against evil powers.^[3] Beyond the Egyptians, there have also been tracings of these core just war and jus ad bellum elements in Ancient Mesopotamia, Anatolia, the Levant and Hatti.^[3]

Many philosophers in the early centuries have dabbled in defining what is ethically and morally acceptable in war. However, St. Augustine is considered to be the father of just war theory. St. Augustine was a Roman Catholic who constructs the idea from a christian perspective. He is the credited founder of the term jus ad bellum itself.^[4] Later came philosopher St. Thomas Aquinas in the Summa Theologicae. Aquinas lists criteria from the christian perspective that were intended to protect civilians and guarantee that wars were not just fought for the interest of private parties.^[5][6][7]

Francisco de Vitoria and Hugo Grotius grew the idea of just war theory into international law. Vitoria's main argument presented that war should serve the common good. States should not be going to war for revenge or power. ^[8] Hugo Grotius was another key thinker within the evolution of just war. He has been named father of international law. In his work, On the Law of War and Peace, Grotius emphasized the need for proportionality and accountability.^[9]

Today organizations such as League of Nations and United Nations stand to prevent unjust wars. The League of Nations was one of the first international organization established with a primary goal of maintaining world peace. It was established in 1920 after the World War I in order to protect the world from dealing with such tragedies again.^[10]Unfortunately they were not entirely successful in that mission due to the occurrence World War II. Thus the United Nations was born. In the UN Charter, Article 2, paragraph 4 states: "All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any State, or in any other manner inconsistent with the purposes of the United Nations."^[11] Article 51 of the UN Charter later 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."^[12] These put the idea of jus ad bellum into writing by requiring states to agree that they will only use armed force in self-defense, or with the approval of the United Nations Security Council.

Since the 1950s, declarations of jus ad bellum have dropped dramatically.^[13] In "Why States No Longer Declare War" Tanisha Fazal notes the drop in formal declarations of war since the 1950s.^[13] 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.

One of the more recent topics being discussed in scholarship is the application if international law in the realm of cyberspace. As cyber attacks continue to increase across the globe, a conversation needs to be had about the jurisdiction of cyber crimes. When is the use of force justified in these uncharted spaces? ^[14]

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.^[15]

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.^[16] 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.^[5]

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."^[17] 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 this principle, there must be good grounds for concluding that aims of the just war are achievable.^[16] This principle emphasizes that mass violence must not be undertaken if it is unlikely to secure the just cause.^[18] 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.^[19] Essentially, this is meant to gather coalition building and win approval of other state actors.

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.^[20]

The principle of Proportionality is the idea of maintaining a balance . The desire to go to war must balance with the destruction or loss of life that the war will cause. If one is more than the other that is a breach in proportionality. There should never be extreme damage and loss of life for a cause that isn't equally productive. ^[21]

• Casus belli
• Jus post bellum
• Just war theory
• List of Latin phrases
• Mandatory war
• Tyrannicide

• Slater S.J., Thomas (1925). "Book 6: On War" . A manual of moral theology for English-speaking countries. Burns Oates & Washbourne Ltd.

• War & Law
• Crimes of War
• Characteristics Of Intractable Conflicts
• Internet Encyclopedia of Philosophy
• Rutgers: Book/Reading List
• Essay hosted at USAF site
• Joseph R. Cerami, James F. Holcomb (Editors). U.S. Army War College guide to strategy. Strategic Studies Institute, 2001. ISBN 978-1-58487-033-3., pp. 19–30. Chapter 3. Ethical issues in War, An overview, Cook, Martin L.
• Stanford encyclopedia entry for war
• Brander, Kenneth Rabbi. "Is All Fair in Love & War?" Just & Unjust Wars through the prism of Jewish and Secular Thought", part 1 and part 2