Philosophy of war
Ethical categories: Another possible system for categorizing different schools of thought on war can be found in the Stanford Encyclopedia of Philosophy (see external links, below), based on ethics. The SEP describes three major divisions in the ethics of war: the realist, the pacifist, and the just war Theory. In a nutshell:

Realists will typically hold that systems of morals and ethics which guide individuals within societies cannot realistically be applied to societies as a whole to govern the way they, as societies, interact with other societies. Hence, a state's purposes in war is simply to preserve its national interest. This kind of thinking is similar to Machiavelli's philosophy, and Thucydides and Hobbes may also fall under this category.
Pacifism however, maintains that a moral evaluation of war is possible, and that war is always found to be immoral. Generally, there are two kinds of modern secular pacifism to consider: (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. Eugene Victor Debs and others were famous advocates of pacifistic diplomatic methods instead of war. In the late twentieth century social philosophers such as Robert L. Holmes also strove within the halls of academe to apply both theoretical forms of secular pacifism within the context of modern forms of warfare including nuclear war and terrorism.
Just war theory, along with pacifism, holds that morals do apply to war. However, unlike pacifism, according to just war theory it is possible for a war to be morally justified. The concept of a morally justified war underlies much of the concept international law, such as the Geneva Conventions. Aristotle, Cicero, Augustine, Aquinas, and Hugo Grotius are among the philosophers who have espoused some form of a just war philosophy. One common just war theory evaluation of war is that war is only justified if 1.) waged in a state or nation's self-defense, or 2.) waged in order to end gross violations of human rights. Political philosopher John Rawls advocated these criteria as justification for war.

See also: 

References: Notes

Bibliography

Clausewitz, Carl von, On War. Middlesex, England: Penguin Books, 1968. J. J. Graham translation, 1908. Anatol Rapoport, editor. Introduction and notes (c) Anatol Rapoport, 1968
Further reading

Chanakya, Arthashastra [1], especially Book X "Relating to War". Discusses war along with philosophical/religious observations about sacrifice derived from Vedic scripture.
Heindel, Max, The Rosicrucian Philosophy in Questions and Answers - Volume II (The Philosophy of War, World War I reference, ed. 1918), ISBN 0-911274-90-1. Describing a philosophy of war from the point of view of the Rosicrucian Fellowship.
Holmes, Robert L. On War and Morality (Princeton University Press, 1989) -A robust philosophical defense of Pacifism which analyzes the ethical limitations inherent in Just War theories.
Rawls, John, The Law of Peoples.  A discussion of international law in the context of political liberalism which argues against the Clausewitzian conception of war between wholly autonomous states, seeking to replace it with a conception of a "fair and just" international society of peoples adhering to principles of international law.

External links: "War" entry  by Seth Lazar in the Stanford Encyclopedia of Philosophy
Alexander Moseley. "The Philosophy of War". Internet Encyclopedia of Philosophy.