What is Just War Theory?
From both a historical and moral perspective, there is a strong presumption against the use of violence and aggression. Just war theory deals with the justification for overriding this strong presumption and waging war. Historically, the just war tradition represents the effort of Western cultures to regulate and restrain violence by establishing widely recognized rules of combat. The theoretical aspect of just war theory, on the other hand, is concerned with ethically justifying war. Moral justification for war has its roots in Christian theology and the writings of St. Thomas Aquinas. Together, traditional rules of combat and moral ideals have helped to form the rules of warfare found in international law. The set of guidelines commonly known as the "war convention" is made up of these moral norms and legal precepts.
Theorists distinguish between the rules of jus ad bellum and jus in bello. The rules of jus ad bellum pertain to the circumstances under which states can acceptably wage war, while the rules of jus in bello serve as guidelines for fighting fairly once war has begun.
Formulated in international law and recognized by most cultures, the rules of jus ad bellum serve as principles to determine when war and the use of violence are justifiable. Only when the criteria of jus ad bellum are met can the use of violent force be permitted.
Having just cause is often thought to be the most important condition of just war. Many hold that the only just cause for war is self-defense against aggression. In 1974, the United Nations General Assembly defined aggression as "the use of armed force by a State against the sovereignty, territorial integrity or political independence of another State." States' rights to territorial integrity and political sovereignty are derived from the rights of individuals to build a common life and rest on the consent of their members. Insofar as a state protects the lives and interests of individuals, it cannot be challenged in the name of life and liberty by any other state. International law holds that a state engaging in war, other than for purposes of self-defense, commits the crime of aggressive war.
However, many have noted that this conception of just cause is far too narrow. First, it is commonly thought that states can defend themselves against violence that is imminent, but not actual. When the threat is clear and the danger close, military acts of "anticipation" are often considered morally justified. For example, many believe that states are justified in conducting pre-emptive strikes in cases where there is a sufficient threat, and failure to exercise military force "would seriously risk their territorial integrity or political independence." There are threats with which no nation can be expected to live.
In addition, many have noted that the "aggressor-defender" dichotomy is an oversimplification. Intervention across national boundaries can sometimes be justified, and the legal existence of a regime does not guarantee its moral legitimacy. They believe that force may sometimes be used to correct grave public evils or to address massive human rights violations. When a government turns savagely upon its own people, it violates their human rights and imposes conditions to which they could not possibly consent. Such a government lacks moral legitimacy, and its political sovereignty and rights to govern are called into doubt. Because governments that engage in massacre are criminal governments, wars of interventions resemble law enforcement or police work.
By Michelle Maiese
 James Turner Johnson, Just War Tradition and the Restraint of War: A Moral and Historical Inquiry. (New Jersey: Princeton University Press, 1981), 4. <http://books.google.com/books?id=6aCSQgAACAAJ>.
 Alex Moseley," Just War Theory," in The Internet Encyclopedia of Philosophy (2009, accessed February 3, 2003). <http://www.utm.edu/research/iep/j/justwar.htm>.
 Michael Walzer, Just and Unjust Wars: A Moral Argument with Historical Illustrations, 2nd Edition. (New York: Basic Books, 1997), 44. 4th Edition (2006) available here.
 Leslie C. Green, The Contemporary Law of Armed Conflict. (Juris Publishing, 2008). <http://www.amazon.com/The-Contemporary-Armed-Conflict-International/dp/1578232422>.
 Ibid., 59.
 Walzer, op. cit 54.
 Walzer, op. cit 85.
 Johnson, op. cit 328.
 David Luban, "Just War and Human Rights" Philosophy and Public Affairs, Vol. 9, No. 2. (Winter, 1980, accessed on February 3, 2003), 160-181. <http://www.jstor.org/stable/2265110>.
 Walzer, op. cit 107.