As long as war has existed certain rules seem to have been applied. Ever since ancient times, some form of etiquette was informally followed due to the values and beliefs of a culture. For instance, many cultures throughout time spared women and children, or other innocents, from slaughter when at war. No guidelines were formally recognized (at least of that which is known), so warfare relied solely on the morals of a certain person or group of people. During the middle ages models for the ethics of war began to arise, although the very first contributor to the ethics of polities and governing was believed to be Aristotle. His general belief was that a nation and its people would not be happy if its sole purpose was to gain dominance over other nations. He also thought that the person in power should not have such a goal. Instead, Aristotle felt nations should have a goal of establishing peace and balance in a society with the ultimate achievement being a just society. Aristotle also believed it to be in the nature of humans to acquire land as a means of expansion and growth. Natural as it was to have the desire to acquire land, Aristotle thought it rather self-defeating to take more than necessary due to the increased possibility of attack from an opposing force.

Saint Augustine is thought to be the founder of the original Just War Theory, working from influence of Aristotle. Saint Augustine linked the beliefs and values of the Christian religion to much of Aristotle’s theology. He was searching for a way to make war legitimate under a belief system that condemned violence towards others. He believed that by waging war through the church in the name of peace and in the defense of justice, the idea of war could be made justifiable to Christians.
Saint Thomas Aquinas added to Saint Augustines' theory by forumulating three basic rules of conduct to make war acceptable, which included:

Legetimize authority was granted when there was a wrong to be undone. Peace could be preserved through this by guiding an opposing force back on the path of justice by means of war, or by the defense of the church itself from a threat. The one who held the power of authorization was the established ruler of a society.
According to St. Aquinas, "...a nation or state has to be punished, for refusing to make amends for the wrongs inflicted by its subjects, or to restore what it has seized unjustly."
Right intentions were those which were for the advancement of justice and peace in a society. There were no other reasons in the eyes of Aquinas which could make war acceptable under the Christian faith.

A man by the name of Raymond of Penafort developed five necessary requirements to be met in order for a war to be just:

1. War may only be waged by the followers of the Church and not by the actual clergy, for it was morally wrong and therefore unacceptable for clergy to engage in acts of violence.

2. The implementation of war be with the intention of the preservation of one's country.

3. War must have be a last resort, used only after all other means of intervention or negotiation had been exhausted.

4. The intention of war should have been to attain peace through the establishment of a righful balance of power.

5. The only entities with the authority to declare war were the Church (the members of the faith) and the Prince of a nation.

French natural law theorist Emer De Vattel made large contributions to the modern Just War Theory, as did John Locke, an English philosopher who made contributions in the field of science and politics. Up to the end of the Middle ages, theJust War theory was based on the Catholic faith's belief system, morals and ambitions.
Vattel and Locke applied the basis of Just War theory more universally. As the more ideological Just War theorists before had established, there were two branches by which Just War could be defined, "jus ad bellum" - the justice of war, and "jus in bello"- the fair conduct of war. (The actual terms "jus in bello" and "jus ad bellum" weren’t applied to the rules of the Just War theory until the 1940’s during the W.W.II trials, but are now commonly used to define the set of terms categorized into two rather distinct groups.)

So, at the beginning of the 16th century, the theory of Just War was sectioned into two separate doctrines, the "Holy War Doctrine", and the "Just War Doctrine". The Holy War Doctrine relied on the classic ideological stance on justice and etiquette, with the Just War Doctrine based upon natural law.

Natural law condemned the waging of war for all purposes, except for that of self-defense. Of course, while sharing the same core values and beliefs, many followers of natural law had differing opinions pertaining to the definition of self-defense as a just means to wage war.

One of the opposing views to the Just War theory is the realist perspective. The origin of the realist theory dates back to Thucydides with his "Melian Dialog". The general belief is that conflict is not only unavoidable, but it is part of human nature. Whenever there is more than one unit of world power, conflict will be inevitable due to the mutual goal of growth. Growth involves not only the consumption of natural resources, but in many cases the acquisition of territory. Even if a State (or unit of world power) is not actively growing as an economy, the consumption of resources for the purpose of maintaining a certain level of stability is still necessity.

Another main argument being made on the justice of war is the Pacifist theory. The followers of Pacifism split into two distinct groups "...1) a more consequentialist form of pacifism (or CP), which maintains that the benefits accruing from war can never outweigh the costs of fighting it; and 2) a more deontological form of pacifism (or DP), which contends that the very activity of war is intrinsically wrong, since it violates foremost duties of justice, such as not killing human beings."

Many who believe in nonviolence agree with the belief that conflict between States or nations can be settled successfully by other means, such as treaties, negotiations, diplomacy, or possibly by the intervention of a third party (which has currently been displayed by Egypt in the Israel-Palestine conflict.) It is believed that a "win-win" situation should be able to be achieved by intellectual means, although this is often not brought about with success because of the lack of flexibility or understanding on either side.

Pacifism is often an accompaniment to religion, and while some people actively pursue the stance that violence should be resisted by nonviolence, some people generally are against violence but do not choose to act upon their beliefs (typically the CP group of Pacifists). So, Pacifism has no absolute definition or set of guidelines, although the concept of violence as an unacceptable form of power or expression is a universal concept of the theory.

Criticism which has arisen about the validity of the Pacifist theory mainly goes to argue that it is not possible to group all forms of violence (as the DP group of believers typically does) for judgement on the same level. There are in fact many levels of violence which are not necessarily morally "wrong" or "right". For example, is it right to declare the violence which naturally occurs between lions over a carcass as morally wrong? They are obviously using violence as a means of resolving a conflict, but does it automatically make it wrong (this can be applied on the human level with the point being that some forms of violence are natural and on the basic instinctual level of daily survival)? Or is it right to place a woman who acts out of self-defense against an attacker on the same level as her attacker when passing a judgement on the morality of the situation?