Perhaps the greatest and most influential work in the philosophy of war is On War by Carl von Clausewitz. It combines observations on strategy with questions about human nature and the purpose of war. Clausewitz especially examines the teleology of war: whether war is a means to an end outside itself or whether it can be an end in itself. He concludes that the latter cannot be so, and that war is "politics by different means"; i.e. that war must not exist only for its own sake. It must serve some purpose for the state.
While Sun Tzu'sThe Art of War, focusses mostly on weaponry and strategy instead of philosophy, his observations are often broadened into a philosophy applied in situations extending well beyond war itself (see the main Wikipedia article on The Art of War for a discussion of the application of Sun Tzu's philosophy to areas other than war). Parts of Niccolò Machiavelli's masterpiece The Prince (as well as Discourses) and parts of his own work titled The Art of War discuss some philosophical points relating to war, though neither book could be said to be a work in the philosophy of war.
Just war theory
Just war theory is a sect of war philosophy thought to have been founded by St. Augustine. The philosophy of just war theorizes what aspects of war are justifiable according to morally acceptable principles. Just war theory is based upon four core criteria to be followed by those determined to go to war. The four principles are as follows: just authority; just cause; right intention; last resort.
The criterion of just authority refers to the determined legality of going to war, has the concept of war and the pursuit of it been legally processed and justified?
Just cause is a justifiable reason that war is the appropriate and necessary response. If war can be avoided, that must be determined first, according to the philosophy of just war theory.
To go to war, one must determine if the intentions of doing so are right according to morality. Right intention criterion requires the determination of whether or not a war response is a measurable way to the conflict being acted upon.
War is a last resort response, meaning that if there is a conflict between disagreeing parties, all solutions must be attempted before resorting to war.
Traditions of thought
Since the philosophy of war is often treated as a subset of another branch of philosophy (for example, political philosophy or the philosophy of law) it would be difficult to define any clear-cut schools of thought in the same sense that, e.g., Existentialism or Objectivism can be described as distinct movements. The Stanford Encyclopedia of Philosophy refers to Carl von Clausewitz is "the only (so-called) philosopher of war", implying that he is the only (major) philosophical writer who develops a philosophical system focusing exclusively on war. However, discernible traditions of thought on war have developed over time, so that some writers have been able to distinguish broad categories (if somewhat loosely).
Anatol Rapoport's introduction to his edition of the J. J. Graham translation of Clausewitz's On War identifies three main teleological traditions in the philosophy of war: the cataclysmic, the eschatological, and the political. (On War, Rapoport's introduction, 13). These are not the only possible teleological philosophies of war, but only three of the most common. As Rapoport says,
To put it metaphorically, in political philosophy war is compared to a game of strategy (like chess); in eschatological philosophy, to a mission or the dénouement of a drama; in cataclysmic philosophy, to a fire or an epidemic.
These do not, of course, exhaust the views of war prevailing at different times and at different places. For example, war has at times been viewed as a pastime or an adventure, as the only proper occupation for a nobleman, as an affair of honor (for example, the days of chivalry), as a ceremony (e.g. among the Aztecs), as an outlet of aggressive instincts or a manifestation of a "death wish", as nature's way of ensuring the survival of the fittest, as an absurdity (e.g. among Eskimos), as a tenacious custom, destined to die out like slavery, and as a crime. (On War, Rapoport's introduction, 17)
The Cataclysmic school of thought, which was espoused by Leo Tolstoy in his epic novel War and Peace, sees war as a bane on humanity – whether avoidable or inevitable – which serves little purpose outside of causing destruction and suffering, and which may cause drastic change to society, but not in any teleological sense. Tolstoy's view may be placed under the subcategory of global cataclysmic philosophy of war. Another subcategory of the cataclysmic school of thought is the ethnocentric cataclysmic, in which this view is focused specifically on the plight of a specific ethnicity or nation, for example the view in Judaism of war as a punishment from God on the Israelites in certain books of the Tenakh (Old Testament). As the Tenakh (in certain books) sees war as an ineluctable act of God, so Tolstoy especially emphasizes war as something that befalls man and is in no way under the influence of man's "free will", but is instead the result of irresistible global forces. (On War, Rapoport's introduction 16)
The Eschatological school of thought sees all wars (or all major wars) as leading to some goal, and asserts that some final conflict will someday resolve the path followed by all wars and result in a massive upheaval of society and a subsequent new society free from war (in varying theories the resulting society may be either a utopia or a dystopia). There are two subsets of this view: the Messianic and the Global theory. The Marxist concept of a communist world ruled by the proletariat after a final worldwide revolution is an example of the global theory, and the Christian concept of an Armageddon war which will usher in the second coming of Christ and the final defeat of Satan is an example of a theory that could fall under Global or Messianic. (On War, Rapoport's introduction, 15) The messianic eschatological philosophy is derived from the Jewish-Christian concept of a Messiah, and sees wars as culminating in unification of humanity under a single faith or a single ruler. Crusades, Jihads, the Nazi concept of a Master Race and the 19th century American concept of Manifest Destiny may also fall under this heading. (On War, Rapoport's introduction, 15) (See main articles for more information: Christian eschatology, Jewish eschatology)
The Political school of thought, of which Clausewitz was a proponent, sees war as a tool of the state. On page 13 Rapoport says,
Clausewitz views war as a rational instrument of national policy. The three words "rational", "instrument" and "national" are the key concepts of his paradigm. In this view, the decision to wage war "ought" to be rational, in the sense that it ought to be based on estimated costs and gains of war. Next, war "ought" to be instrumental, in the sense that it ought to be waged in order to achieve some goal, never for its own sake; and also in the sense that strategy and tactics ought to be directed towards just one end, namely towards victory. Finally, war "ought" to be national, in the sense that its objective should be to advance the interests of a national state and that the entire effort of the nation ought to be mobilized in the service of the military objective.
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.
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.
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.