Carl von Clausewitz's On War has been called, rightly, "not simply the greatest, but the only truly great book on war." It is an extraordinary attempt to construct an all-embracing theory of how war works. Its coherence and ambition are unmatched in the literature on war. Compare On War, for example, with a book that is often named in the same breath: Sun Tzu's Art of War. Whereas Sun Tzu essentially presents an eclectic collection of pithy phrases that contain the dos and don'ts of war, Clausewitz provides us with a comprehensive analytical construct that illuminates the complex, deeper workings of a phenomenon that is central to human endeavor. Nonetheless, On War is also full of instances of sharp observation, biting irony, and memorable phrases that have given a long series of readers, since its posthumous publication in three volumes between 1832 and 1834, a treasure trove of quotes and insights of which "war is a continuation of politics by other means" is merely the most famous.

The life and thought of Carl von Clausewitz were molded by the French Revolution and the Napoleonic Wars. Born in 1780, Clausewitz spent his whole career, from the age of twelve until his death in 1831, in the Prussian army. He fought in all the major Prussian campaigns against France, from the desultory, early fighting against Revolutionary France in 1793-4, through the cataclysmic 1806 campaign, to the hard-fought victory over Napoleon that was won between 1812 and 1815. His most fateful experience in the wars was the 1806 Battle of Jena-Auerstedt. Unlike in Prussia's glorious past, when the country had fought many battles in long drawn-out wars without eversuccumbing, this time, in one day, the Prussian army was destroyed and the state thrown to the mercy of the "God of War," Napoleon. This nightmare impressed upon Clausewitz, as no other event, the importance of understanding the true nature of war, and thus provided a major inspiration for the writing of On War. Older interpreters of Clausewitz have claimed that he was disappointed with his life and career and died a broken, bitter man. A supposed failure to synthesize his desire to be a man of action and a scholar led to extreme frustration and, ultimately, an early death. More modern scholarship has argued that he was actually quite successful. Despite doubts about his noble lineage, Clausewitz was marked out as a promising officer early on and entered the newly formed Kriegsschule (War College) in 1801. In 1804, he became adjutant to Prince August, one of the younger sons of the king of Prussia. As part of the prince's entourage, Clausewitz gained access to the highest social circles and met the love of his life, Countess Marie von Br&uulm;hl, a member of the top German nobility. Caught up in the emerging Romantic movement, Clausewitz penned letters to Marie evincing a yearning for battlefield glory-if only to convince Marie's doubting mother of his suitability as a son-in-law (she was English, and actually non-noble herself).

Heroic achievement on the battlefield consistently evaded Clausewitz in his fighting career, though he did not shrink from seeking action. In 1812, he even left Prussian service for the Russian army, serving in the famous campaign that spelled the beginning of the end for Napoleon (his eyewitness account was read by Tolstoy and earned him a bit part in War and Peace). This act of disloyalty soured relations with the Prussian king, however, and may explain why after the Napoleonic Wars Clausewitz's career stalled somewhat. Nonetheless, he was a major-general at thirty-eight years old, when he became director of the Kriegsschule, a post he held for twelve years and which gave him ample time to write On War. When revolution once more swept Europe in 1830, Clausewitz was recalled to active duty as chief-of-staff of the army that was sent east to prevent the Polish revolution from spilling over westward. He died from cholera in Breslau on November 16, 1831.

Clausewitz did not suffer fools gladly. On War, like his other writings, is full of damning criticism of other military theorists. His first piece of published writing was a withering critique of one of the most famous theorists of his day, Heinrich Dietrich von B&uulm;low (1757-1807). Later, he was equally uncomplimentary about the hugely influential interpreter of Napoleon's generalship, Antoine-Henri de Jomini (1779-1869). Closer colleagues did not get off lightly either. Clausewitz's account of the 1812 campaign contains a long list of the generals fighting on the Russian side and adds brief comments about their military and moral qualities. Descriptions vary from the exceedingly negative "real Russian" or "mischievous Cossack" to "passably honest man" and "German man; his main pillars of support are his proven honesty and the Czar" (the posthumous edition of this account carefully toned down these personal characterizations and left the list out altogether). Although very reserved in public, Clausewitz was widely known to be outspoken in private. The king of Prussia and many in his close circle-who were indeed not spared Clausewitz's negative judgements-can thus perhaps be forgiven their reservations regarding his suitability for high military command or a senior diplomatic posting (he sought the ambassadorship to Britain in the 1820s). Yet Clausewitz's critical attitude also extended to himself. The careful reader of On War will find many passages where Clausewitz posits an anonymous view that actually is (or used to be) his own, which he then proceeds to dispatch with precision and relish. In many ways, On War contains an argument with himself, and the quality of the argument derives in large part from Clausewitz's willingness and ability to take on anyone-including not least himself. Unsurprisingly, Clausewitz published very little during his lifetime and the project of his life, On War, remained unfinished even though he spent many years on it and extensively revised the manuscript many times over. In the end, it was his wife who edited his manuscripts and posthumously published them in ten volumes (of which On War comprises the first three).

Reflective and studious by nature, Clausewitz read a wide variety of works throughout his life. On War, as a result, is inspired by many different ideas and theories, which the author seeks to weld together into a single theory that is very much of his own making. He employs a miraculously varied body of theory and method to analyze war and distill general truths. In a sense, Clausewitz was too early in trying to accomplish his ambition. The fields of inquiry on which he drew were mostly incipient academic disciplines. Philosophy, mechanics, and mathematics might be said, by his time, to have developed their own methods and ways of forming theory, but history, sociology, and psychology were still decades, or more, away from becoming "modern" disciplines with their specific theories and methods. Moreover, Clausewitz received no formal schooling in any of these fields. One could argue, therefore, that Clausewitz lacks a proper single, coherent analytical instrument to tackle his subject. Yet its truly interdisciplinary nature gives On War a freedom and flexibility that, in conjunction with Clausewitz's exceedingly strong critical and analytical powers, manage to arrive at an amazingly effective and persuasive set of arguments that consistently say something meaningful about the whole of the phenomenon of war and its many parts. A modern academic (or, for that matter, military) environment, with its strong disciplinary boundaries and discipline-specific method, is not likely to permit the writing of a work of comparable breadth and imagination.

The structure of On War requires some explanation and illustrates Clausewitz's eclectic sources of inspiration. Clausewitz never presents an introduction in which he explains how he is going to tackle his subject. There are, however, many clues in the work, and the most direct one explaining the structure is found in Book II, chapter 1, where Clausewitz defines strategy and tactics. Strategy, he writes, "is the theory of the use of combats for the object of the War," while tactics "is the theory of the use of military forces in combat." Two things are of note. First, strategy is defined as possessing means (combats) that are to be used to attain an end (the object of the War). The object of the War, Clausewitz writes elsewhere, is defined by politics; hence, his most famous quote "war is a continuation of politics by other means." Second, a hierarchical relationship exists between strategy and tactics. Tactics is a means to strategy (as strategy, in turn, is a means to politics). By using its own specific means-armed forces-tactics achieves the results in combats that serve the ends of strategy and, by implication, the ends of the war. Clausewitz's understanding of politics and war, and within war, of strategy and tactics, as consisting of an interconnected hierarchy of ends-means relationships was derived from his understanding of the theory of art and was highly original in his day. B&uulm;low, for example, defined strategy as "the science of military movements outside the range of view of the enemy," and tactics as movements "within" the enemy's view. B&uulm;low also realized that strategy and tactics are connected, but he completely missed the fundamental idea that military activity must have a purpose to give it meaning. War, to Clausewitz, could make no sense without a clearly delineated political aim and a subordinate strategic aim: "No War is commenced, or, at least, no War should be commenced, if people acted wisely, without first seeking a reply to the question, What is to be attained by and in the same?" (Book VIII, chapter 2). It is all the more remarkable that Clausewitz had already developed his definitions of strategy and tactics by the age of twenty-four.

Clausewitz's definitions are reflected in the three middle books of On War. Book III covers strategy in general, Book IV focuses on the means in strategy: the combat; and Book V moves down into the tactical and discusses the means in tactics: the armed forces. Books VI and VII deal with the two basic modes of operation in war: the defense and the attack. The place of the first two books is straightforward: Book I defines the nature of war and discusses the main elements that are either a help or hindrance to war, revealing its nature, while Book II presents the basic theoretical considerations defining Clausewitz's approach to the topic. The final book, Book VIII, in keeping with the pragmatic objective of the work as a whole, is intended to tie all the strands together and suggests what one must weigh in one's mind when designing a "Plan of War." But if, as Clausewitz insists, the ends determine the means, why are there no books on the aims in strategy and tactics? The aim in tactics does crop up throughout On War and is defined as "victory." What constitutes victory, Clausewitz insists, depends on the strategic purpose it serves. If war is an instrument of politics, then strategic purpose obviously depends on political objectives, and the former should vary in line with variations in the latter. As political objectives are the prerogative of governments, and Clausewitz, as a soldier is concerned with his area of professional expertise, one could claim that there is no need for an extended separate discussion of strategic aims. Shouldn't Clausewitz simply focus on analyzing the peculiarities of the means at the disposal of governments and generals? After all, isn't his book "On War" and not "On Politics and War"? Although Clausewitz repeatedly emphasizes that it is important for both governments and generals to understand war so that they will not make demands of it that it cannot meet, he does actually have a very clear, specific idea of the nature of strategic purpose, or the aim in war. In fact, this fundamental idea is at odds with his other fundamental idea, that war is an instrument of politics, since it makes war independent of politics.

Clausewitz derives the idea of the strategic aim that any general should pursue in war from his understanding of the nature of war. Like any good author, he defines war on the first page of the first chapter of On War: "War ... is an act of violence intended to compel our opponent to fulfill our will." Again one notes the emphasis on means and ends here. There is also an identification of what Clausewitz considers to be the essence of war: violence. The use of violence, Clausewitz then argues, sets off an escalatory spiral that makes war lose sight of its aim. In order to prevail, one side needs to use more force than the other; and, as both sides know this, both are forced to use as much force as possible as quickly as possible. The ultimate result-and Clausewitz is at pains here to emphasize that this is an ideal, not a possible reality-is "absolute war": a war that is absolved, or set free, from reality, and marked by a single, instantaneous discharge of violence. In that discharge, the original aim of the war gets lost. Clausewitz immediately introduces a number of modifications of the absolute that the real world imposes, the most important being that war cannot be compressed into a single, instantaneous act. In effect, this means that there is time and opportunity to exercise judgement and so, as he puts it, "the political object now reappears." However-and this is the key issue with which Clausewitz struggles throughout On War and which he never quite manages to resolve-can the political object exercise a sufficient restraint to break the escalatory spiral?

For much of his life (and much of On War), Clausewitz answers in the negative. Irrespective of the political aim, a general must always aim to make his opponent defenseless by using the utmost energy and force. If he fails to do so, he will hand his opponent a chance to make him defenseless. Once defenseless, the loser cannot resist the victor's will. Whatever the political aim-whether it be limited, like a territorial concession, or maximalist, like a regime's overthrow-it will be achieved. "Real war" should therefore be marked by ceaseless, offensive activity in which armies seek to achieve each other's destruction in great, decisive battles. The failure of the Prussian army in 1806 to understand that escalation is inherent in war led to its destruction at the hands of the man who did understand. In short, the idea of absolute war allows Clausewitz to establish a clear guiding principle that suggests that the utmost use of force makes real strategic sense in all wars. The idea even appears to allow for the retention of the political instrumentality that in absolute war would be lost.

This key idea as to how war works underlies most of the chapters and books in On War. Chapter 2 in Book I, on "Ends and Means in War," explains how "the destruction of the enemy's armed force, amongst all the objects which can be pursued in War, appears always as the one which overrules all others." The rest of the chapters in Book I explain how "genius," "bodily exertion," and "information" can help in the pursuit of this objective, while its achievement is hindered by the elements of "danger" and "friction." To give another example, most of the chapters in the book on strategy deal with moral and operational factors that need to be mastered in order to pursue this objective efficiently and effectively.

The intellectually compelling idea that the aim in war must always be to make the enemy defenseless through the destruction of means of resistance in decisive battle has been a powerful and influential one. It led the military architect of the Wars of German Unification, Field Marshal Helmuth von Moltke (1800-1891), to claim that politics is suspended during wartime and that the escalatory military logic of war must be allowed to run its course:

Policy makes use of war to gain its objects, it acts with decisive influence at the opening and at the end of the war in such a way as either to increase its claims during the progress of war or to be satisfied with lesser gains. With this uncertainty strategy cannot but always direct its efforts towards the highest goal attainable with the means at disposal. It thereby serves policy best, and only works for the object of policy, but completely independent of policy in its actions.

It compelled all European militaries to execute offensive plans in August 1914, despite their awareness of the devastating effectiveness of modern firepower. "Decisive success in battle can be gained only by a vigorous offensive," Major-General (later Field Marshal) Douglas Haig (1861-1928) wrote in the 1909 Field Service Regulations-and, despite the experience of trench warfare in World War I, the postwar edition repeated and defended this statement as entirely correct. All soldiers, before and after the war, accepted Clausewitz's logic that defense does not win wars; if anything, it could invite defeat by leaving the initiative to the enemy. Today, the same idea inspires the American military doctrine of "overwhelming force." Irrespective of political aims, the U.S. armed forces' main strategic preoccupation is with making their enemies defenseless. The war against Iraq over Kuwait, the war against Serbia over Kosovo, the war against the Taliban in Afghanistan and the final war against Saddam Hussein in 2003, were all fought with the intent to destroy the enemy's capacity to resist. Although the first two conflicts clearly had limited political aims, adherence to this one strategic principle never wavered. It reflects an understanding of the relationship between politics and war that is essentially similar to that expressed by Moltke. The U.S. military took-and were granted-a remarkable degree of strategic independence in their prosecution of these wars. Only in the case of Kosovo was there an attempt to "politicize" the war by circumscribing which targets in Serbia were to be attacked-interference that was fiercely resisted by the U.S. Air Force and the joint chiefs.

Some interpreters of Clausewitz would claim that the above-sketched legacy of his thought constitutes a perversion. That is not quite accurate, as we have seen, but it is true that Clausewitz began to move away from this one idea of the essence of strategy toward an understanding of war and strategy that was more subtle, sophisticated, and comprehensive, and in which the key concern was the search for a closer correlation between political and strategic aims. This was a slow and, ultimately, not completely successful process. That Clausewitz undertook it is a mark of his true intellectual greatness. Many lesser minds-like the generals mentioned-would have been satisfied with the theory that Clausewitz initially laid out. There are several milestones indicating a growing uneasiness which we find scattered in On War. The first was Clausewitz's wonderment at what he called "the suspension of the act of war." In real war, as he knew from his own experience, military activity often came to a standstill. But if escalation was inherent in war, as his guiding theory of absolute war suggested, then this should not happen. At first he sought to explain this phenomenon as yet another modification imposed by reality on the absolute that did not affect the underlying principle of escalation. But, in reading Clausewitz carefully, one notices his nagging doubt that the standstill in war might be an indication of something deeper and fundamental. Clausewitz wrote an essay about this puzzle already in 1817, which he sent to his patron and friend, Count Neithardt von Gneisenau. In On War, the issue is addressed in three places, in three subtly different versions, reflecting various stages in his thinking. Chapter sixteen is wholly devoted to it in terms of strategy, and in Book VIII, chapter 2, there is a brief reference in the context of "absolute and real war." The third and final version appears in the first chapter of the first book, where it is linked to Clausewitz's second problem: the existence of the defense. As he was working on Book VI on the defense, the same uneasiness as with the suspension in the act of war creeps up on him: why should the defense be such a common form of military activity? If escalation is inherent in war, should not offense be the rule and defense the exception? It is fascinating for the reader to follow how Clausewitz argues with himself throughout this book (note in particular also the inspiration drawn from the natural sciences of mechanics and physics). At first, he is quite happy to see defense as a useful phase in operations if a party wants to wait for a better moment to counterattack (a lesson powerfully drawn from the 1812 campaign, when the Russians let Napoleon march all the way to Moscow and, once he had worn himself out, they moved to the offensive and chased him out of Russia). But again, we see that Clausewitz's innate critical sense gets the better of him. Might not the defense (which he usually describes, deliberately pejoratively, as the form of war with the "negative" object, whereas the offense has the "positive" object), after all, be of a more substantial value to the art of war?

The argument continues in Book VIII on the war plan. In this book, which is clearly unfinished, Clausewitz moves to a reevaluation of the idea that war is a continuation of politics by other means. He had always understood war as a political instrument, but, as we have seen, his theory of war suggested that war must operate quite independently from politics. Now he begins to claim that the influence of politics could extend into war, possibly to the extent that it pervades all military activity. Chapter 3B makes this point in a famous piece of breathtaking historical sociology that ranges from antiquity to the French Revolution. The focus of the book is an attempt to establish how political objectives translate into strategic objectives. If the political objective is unconditional surrender, then the strategic objective is quite straightforwardly the overthrow of the enemy through the destruction of his means of resistance. But what if one pursues a lesser political aim? Should not a limited political aim require something less than making the enemy defenseless? Yet, however much it made intuitive sense, working out the relationship between politics and war conceptually proved difficult and a precise definition of limited aims in war, on a conceptual par with the maximalist aim of making the enemy defenseless, eluded Clausewitz.

The issue comes to a head in chapter 1, where Clausewitz presents his theory on war as an integrated whole. Arguably, this chapter represents the summation of his thought, and he is believed to have completed it not long before he locked away his manuscripts on his final return to active duty in 1830. It is a difficult chapter, because Clausewitz was still struggling to reconcile his two conflicting ideas outlined above. He begins by presenting the essentialist conception of war: that war is about the use of force and that, despite a number of modifications reality imposes, there is no logical limit to its use. However, he then changes tack and argues that, despite the independent inner escalatory logic, war is influenced by politics. At the end of the chapter, Clausewitz introduces his synthesizing concept: the "wonderful trinity." The trinity is "composed of the original violence of its elements, hatred and animosity, which may be looked upon as blind instinct; of the play of probabilities and chance, which make it a free activity of the soul; and of the subordinate nature of a political instrument, by which it belongs purely to reason." In other words, the tendency toward unrestrained violence is moderated by the game-like qualities of war and political reason.

Debate has raged about the effectiveness of the synthesis, and its continuation indicates the importance of Clausewitz's thinking. The relationship between politics and war remains a critical issue today. The translation of political aims into strategic aims, particularly when the former are limited in nature, is a matter of interest and legitimate concern to all citizens of the Western democracies that continue to use war as an instrument of foreign policy. How much force is enough? What types of force, against which targets, are most effective in achieving our political aims? Clausewitz continues to merit close study. Even Al Qaeda has taken an interest: Clausewitz's work was found in a safe house in Afghanistan in 2002. Clausewitz may not provide clear-cut, recipe-like answers, but he does have the great distinction of identifying the key issues and suggesting clever and insightful ways of thinking about them. As he wrote in one of his historical studies: "We do not consider what we have thought to be a contribution to theory, but the way in which we have thought it."

A note on the translation

What you have before you is one of the three major English translations of On War. It was undertaken by Colonel J. J. Graham and based on the third edition of the German original. Originally published in 1873, the edition was republished by Colonel F. N. Maude in three volumes in 1908. Although the oldest English translation (the second was made by O. J. Matthijs Jolles during World War II and the third by Peter Paret and Michael Howard in the 1970s), it possesses a number of important strengths. In a general sense, its age makes it nearest in time to the original and thus it most closely approximates the intellectual climate of Clausewitz's world. The translation is also faithful to the original in the sense of being literal and consistent in its rendering of Clausewitz's terminology. As a result, the structure and coherence of Clausewitz's thought come through more clearly than tends to be the case with the more modern translations.

Jan Willem Honigis a senior lecturer in the department of War Studies at Kings College London. He teaches and writes on the relationship between politics and war, the development of Western strategic thought, strategies of military intervention, and European security and foreign policy.