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The philosophy of war is the area of philosophy devoted to examining issues such as the causes of war, the relationship between war and human nature, and the ethics of war. Certain aspects of the philosophy of war overlap with the philosophy of history, political philosophy, international relations and the philosophy of law.
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.
Leo Tolstoy's novel War and Peace contains frequent philosophical digressions on the philosophy of war (and broader metaphysical speculations derived from Christianity and from Tolstoy's observations of the Napoleonic Wars). It was influential on later thought about war. Tolstoy's Christian-centered philosophy of war (especially his essays "A Letter to a Hindu" and "The Kingdom of God is Within You") was a direct influence on Gandhi's Hinduism-centered non-violent resistance philosophy.
While Sun Tzu's The Art of War, focuses 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.
The Indian Hindu epic, the Mahabharata, offers the first written discussions of a "just war" (dharma-yuddha or "righteous war"). In it, one of five ruling brothers (Pandavas) asks if the suffering caused by war can ever be justified. A long discussion then ensues between the siblings, establishing criteria like proportionality (chariots cannot attack cavalry, only other chariots; no attacking people in distress), just means (no poisoned or barbed arrows), just cause (no attacking out of rage), and fair treatment of captives and the wounded. 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.
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.
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 as "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)
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: