Any book on the nature of war needs to identify its subject. So how does Clausewitz define war? What are the boundaries of that definition? What are its limitations, if any, in the contemporary world?
In Book I of On War Clausewitz tackles the problem of definition in two distinct ways. One is bottom-up, focusing on the very practical business of war, namely fighting and killing; the other is top-down and begins by imagining war in its most abstract form.
War as fighting
Clausewitz goes ‘straight to the heart of the matter’. ‘War is nothing but a duel on a larger scale’ – a physical contest between people, each using force ‘to compel our enemy to do our will’.  [i] ‘There is only one means in war: combat’ (das Gefecht).  In essence ‘war is fighting’ (Kampf).  It is the spilling of blood that makes war ‘a special activity, different and separate from any other pursued by man’. 
The focus on combat is sustained. On War has over 600 references to battle (Schlacht – which also means slaughter in German). No armchair theorist, Clausewitz was actively engaged in combat on at least 20 occasions between 1793 and 1815, and received a bayonet wound to the head in May 1813.[ii]
Obviously, Clausewitz does not equate all fighting with war. Wrestling may be ‘fighting of a kind’  but it is not war. Nor does he include murders, gang-fights, riots, massacres and the like in his definition. Human beings fight and kill one another in many ways and for many reasons without this necessarily constituting ‘war’. War, Clausewitz insists, must be ‘a serious means to a serious end’.  There are two requirements.
First, war entails ‘a clash between major interests.’  For Clausewitz it is the interests of states that constitute the ‘serious end’. Individuals and groups other than states do not normally wage war. Second, ‘serious means’ refers to fighting by soldiers as part of a state’s military organisation. Combat, Clausewitz says, ‘is not a contest between individuals’ but between soldiers who are ‘recruited, clothed, armed and trained’ to be able to ‘fight at the right place and the right time’.  Most of the references to fighting in On War are to clashes between national armies under the command of a state.
But Clausewitz recognised that war could be more complex.[iii] He knew of the American War of Independence when irregular forces played a significant role in defeating the British (though he does not mention the conflict in On War). He knew more of the Vendée uprising in which lightly-armed peasants fought against France’s revolutionary regime from 1793-96. And he was very familiar with the war in Spain where Napoleon’s army had struggled against a combination of partisans, irregular troops and the armies of England, Portugal and Spain itself. Clearly, war could embrace combatants other than uniformed regulars.
What interested Clausewitz most about these wars were the tactics employed, notably the use of mobile forces, often lightly-armed, to harass enemy soldiers, attack weak points or gather intelligence. Such tactics were often favoured by insurgents unable to recruit large, regular armies or mount major attacks. Like others before him, Clausewitz recognised that standing armies could also employ some of these tactics. Today these might be termed ‘special operations’ but were then known as guerrilla or ‘small war’. While posted to the War College in Berlin in 1810-11 he gave a series of lectures on what he termed ‘little war’ (Kleinkrieg).[iv]
But what he did not contemplate was that war could be conducted by insurgents or non-state groups alone, with partisans and irregular forces employing ‘small war’ tactics. He did not anticipate that such groups might drive out an occupying power or defeat regular forces by relying on nationalism and/or ideology simply by sustained use of irregular methods of war. The fate of Spain, Clausewitz believed, was determined primarily by the armies of England and France.
At the same time Clausewitz understood the importance of governments mobilising popular support and participation in war. Napoleon had done this with spectacular success and Clausewitz, deeply impressed, urged Prussia to follow suit after its humiliation by the French army at Jena in 1806. He advocated what he called people’s war (Volkskrieg) even more vigorously after Prussia had been forced to join Napoleon’s invasion of Russia in 1812. People’s war for Clausewitz was above all a means of strengthening a nation’s fighting forces both materially and psychologically rather than a free-standing form of warfare.
At the other end of the spectrum from the harsh reality of combat is the idea of pure war. For Clausewitz this is war free of all constraint and limitation. He mostly refers to absoluter Krieg which is best translated as ‘pure war’, following Kant’s practice of identifying the unadulterated essence of a concept or activity.[v] ‘Pure war’ is thus not to be found in the real world though sometimes Clausewitz lapses. In admiration of Napoleon’s military triumphs, he remarks that ‘with our own eyes we have seen warfare achieve this state of absolute perfection’. 
In strict terms, however, the idea of ‘pure war’ means stripping war of all its real-world characteristics – soldiers and armies, generals and statesmen, the social and political context. It means war without its normal dynamics such as strategic interaction and friction. It is thus ‘a wholly isolated act, occurring suddenly and not produced by previous events in the political world’. It is simply collision – ‘a clash of forces freely operating and obedient to no law but their own’. 
Clausewitz stresses that this is a ‘logical fantasy’ and can never occur in the real world.  To understand actual war one must move from concept to reality. Now ‘the whole thing looks quite different’  – and far more complex.
First, we must replace abstract entities with human beings and real organisations with all their emotions, limitations, variety and unpredictability. War is not a collision between inanimate objects but ‘always the collision of two living forces’. 
Second, in real war interaction occurs between combatants over a period of time. At tactical, strategic (campaign) and national levels each side responds to the actions of the other, evaluating its options in the light of possible reactions. Belligerents rely on information and judgement but these will vary greatly in quality and reliability. The goals of warring states, moreover, will be influenced by the course of the war. Real war is a complex of interactions, multi-layered and often unpredictable.
Third, the complexity of actual war is evident in what Clausewitz calls a ‘remarkable trinity’ [wunderliche Dreifaltigkeit] of passion, reason and chance that underlie war (and, one might add, all serious human activity). The passion of war is the ‘primordial violence, hatred and enmity’  that motivate people to fight. The reason of war is the calculation of means to achieve ends and the reckoning of costs and benefits.
Finally, chance and uncertainty beset the whole enterprise. This unholy trinity varies not only from war to war but also within each war.[vi]
The function of war
Clausewitz also seeks to define war by its function in human affairs: ‘what does it do?’ rather than ‘what is it?’. His answer has two elements that are fused in the German word Politik. This refers both to ‘policy’ – the aims and ambitions of individual states – and to ‘politics’ – the workings of human interaction on a large scale.
War as an instrument of policy
This is Clausewitz’s best-known depiction of the function of war though earlier thinkers also speculated along these lines.[vii] War occurs when states seek goals that clash with the goals of other states and choose to pursue them through violent means. The decision to use force must be mutual. As Clausewitz observes wryly, wars actually begin when the defender decides to fight in preference to simply surrendering to the aggressor.  Both take up war as a means to differing ends.
Clausewitz’s key insight is that policy – which originates in a combination of passion and reason – does not cease to exist once war breaks out but runs through the entire course of hostilities. It explains not only the motives for war and the objectives set but also the degree of effort made by belligerents.  In its simplest expression: ‘war is nothing but a continuation of policy with other means’.  It is therefore ‘only a branch of political activity [and] in no sense autonomous’. 
Some wars have ambitious goals, evoke huge effort and cause great destruction; others seek only marginal advantage and show little ‘hostile spirit’.  A war may start as one type but transition to the other. Escalation may occur since war contains an inherent tendency for each side to increase its effort in order to outdo the other, making for a rise to ‘extremes’.  Alternatively, ambitions may dwindle and costs mount up so that war becomes ‘nothing more than armed neutrality’ .
Clausewitz’s position here is not that war is necessarily an instrument of policy but rather that war ought to be treated as an instrument of policy. He acknowledges that this is no easy task. A government can set wise or foolish objectives – these are matters for policy. [606-7] But whatever their goals they should constantly seek to understand what war can and cannot achieve and the costs and risks involved.
War as part of human society
A second function of war is found in Clausewitz’s assertion that war is ‘part of man’s social existence’.  It is inherent in the system of states that emerged from around 1500. Since war cannot be eradicated from human affairs, a state must be prepared to fight in order to defend its interests, its honour and even its survival. Also critical for security are alliances and the balance of (largely military) power among states, topics to which Clausewitz devotes considerable attention. He warns, for example, that allies can never be fully trusted since they will ultimately pursue their own interests. 
In this context Clausewitz sees the function of war as that of settling disputes: war is thus a ‘clash between major interests, which is resolved by bloodshed (sich blutig löst)’. [149, emphasis added] How is this to be done? The simplest method is to disarm the enemy so that he is powerless to prevent you imposing your will. More complex is the use and threat of force such that an opponent will sooner or later choose acquiescence rather than resistance.
If war holds out the promise of resolving conflicts, however, it rarely produces permanent results – as Clausewitz acknowledges. Even a decisive victory may turn out to be a passing triumph while defeat as may prove ‘a transitory evil’ for the defeated.  Prussia’s ‘catastrophe’ at Jena in 1806 is clearly in Clausewitz’s mind here. Any self-respecting state will seek ways to restore its honour and independence. War cannot guarantee solutions, only that things will be different.
Modern War and the Modern State
Clausewitz’s understanding of war was developed in the context of the modern state that emerged in Europe from around 1500. There were many factors at work: greater internal order, more efficient administration that facilitated collection of taxes and conscription of citizens, growing international trade, and technological advances, both civilian and military. The Enlightenment also encouraged greater faith in reason as a guide to human affairs.
European states ceased to feel threatened by barbarians outside the gates while still fearing war among themselves. But these modernising states could hope that war, if it could not be prevented, might be made more civilised. European armies were slowly becoming more disciplined, more educated and more professional in the exercise of violence. There were also efforts to separate fighting from civilian life partly out of humanitarian sentiment, partly to avoid economic disruption, partly to reflect military codes of honour. Expanding diplomatic contacts meant that states knew more about the outside world and might better judge their true interests. The resort to war promised to be more rational and conduct of hostilities more controllable.
These changes tied in with Clausewitz’s view that war reflects the ‘social conditions’ within states and the relations between them.  Hence war conducted by civilised states differs from war fought by ‘uncivilized’ (ungebildet) peoples. Primitive warriors, Clausewitz believed, knew little of limitation or restraint. They put prisoners to death and lay waste to cities for no reason other than vengeance or wanton cruelty.  Lacking political purpose and rational control, their ‘wars’ are driven by sheer hatred.  By contrast, wars between ‘civilized nations’ are ‘far less cruel and destructive than wars between savages’. The simple reason is that ‘[s]avage peoples are ruled by passion civilized peoples by the mind’. 
Yet Clausewitz is far from saying that modern war is bloodless. ‘Even the most civilised of peoples’ he acknowledges, ‘can be fired with passionate hatred for each other’.  He has little time for laws of war: their effect on the conduct of war is ‘imperceptible’ and ‘hardly worth mentioning’.  Humanitarianism in war is sheer folly: it invites an enemy ‘with a sharp sword [to] hack off our arms’.  If there is some constraint on war it is through reason which may be found in the political element. Also important is the concept of military honour which requires amongst other things the fair treatment of prisoners and the sparing of non-combatants. Though Clausewitz says little explicitly on this topic, it underlies much of his thinking about his profession.
The changing face of war
How has Clausewitz’s understanding of war fared in in the contemporary world? Is it relevant to the many internal conflicts that have occurred since 1945? Has it adapted to the atomic age when resort to nuclear weapons could well result in mutual annihilation? Is it ultimately misguided in promoting the idea that war can be an instrument of policy rather than an expression of culture or human nature?
Fighting among groups other than states, of course, existed long before the modern era, has continued to exist, and will no doubt persist into the future. Ferocity of will and improvisation often allow such warriors to triumph with little planning or control.[viii] Leaders of armed groups may be little more than brigands or warlords with large personal ambitions.
The ability of non-state actors to take up arms has grown enormously in recent times. Weapons are more accessible, more varied and more destructive. Violent attacks can be carried out with relative ease within states or across international borders. Force can be used against any targets and for any cause. This sort of fighting displays characteristics that are the antithesis of what Clausewitz saw as modern war and can be labelled ‘anti-modern’ (rather than pre-modern or post-modern).
Not all such violence is of sufficient scale and scope to warrant the term ‘war’. Where is the line to be drawn? For Clausewitz, as we have seen, war requires the clash of ‘great interests’. What has happened since 1945 is that the idea of ‘great interests’ has been broadened. Prior to WWII the general view was that ‘war’ meant conflict between two states or at least entities that looked like states – as in the American Civil War. But after 1945 pressure grew to apply the term ‘war’ to a wider range of conflicts, and this became most evident with regard to the laws of war.[ix]
In 1977 Additional Protocols to the Geneva Conventions extended their coverage to hostilities directed against colonial rule, foreign occupation and racist regimes (as in South Africa). The requirement for uniformed armies was changed to organised, armed groups under responsible command; the scope of hostilities was widened to situations where belligerents exercised control over territory such that they could carry out ‘sustained and concerted military operations’; and ‘combatants’ need not wear uniforms but must carry arms openly while preparing for and during a military action. Significantly, the term ‘armed conflict’ replaced ‘war’ with its state-oriented connotation.
Clausewitz was not interested in legalistic definitions of war and would perhaps approve of the adoption of more or less objective measures to determine whether ‘war’ existed. He may well have recognised as war certain armed struggles where there is a clear political objective such as overthrowing an oppressive government or securing independence from an imperial power; where there is a measure of central control over the use of violence; and where those fighting may wear a uniform of sorts and somewhat resemble a modern army. To this extent Clausewitz’s ‘war’ retains its relevance.
Where he would draw the line is where the current law of armed conflict also stops. Fighting cannot be recognised as war when fighters rely on tactics and choose targets that are essentially civilian rather than military; when their attacks are small-scale and not part of a wider campaign; when they lack central control; and when there is no prospect of success. In such cases governments will likely treat them as criminals rather than enemies with whom some resolution of the conflict might be achieved, whether by force, negotiation or a combination of both.
By 1945 the demands of modern war had led to weapons of mass destruction capable of destroying entire cities in an instant. Soon after, missiles were developed that could deliver nuclear weapons to any part of the globe in a matter of hours or even minutes. Modern war appeared to have burst its natural bounds – it was now ‘hyper-modern’. In all probability a nuclear war would see no combat among soldiers, no campaigns, no political direction of a sustained national effort. It would resemble Clausewitz’s imaginary ‘pure war’: ‘an isolated act’, taking the form of ‘a single short blow’ with weapons already in existence, and proving ‘decisive’ with a ‘final’ result. [78-9]
Strategists were immediately divided about the continuing relevance of Clausewitz’s view of war. Some argued that nuclear war could never serve as an instrument of policy since it was likely to escape the control of governments and the cost of a nuclear exchange would be out of proportion to any reasonable objective. Moreover, even an unspoken threat of nuclear attack might panic an enemy into striking first. Others, however, claimed that Clausewitz’s admonitions about war as an instrument of policy were now all the more important: do not take the first step without considering the last, means must be matched to ends, wars have a natural tendency to escalate, and political control must be maintained at all times.
From this debate a consensus emerged that the role of nuclear strategy was not to fight war but to avert war – by convincing any opponent that they would gain nothing and perhaps lose everything from initiating the use of nuclear weapons. The term ‘Cold War’ came to define a situation in which threats – explicit and implicit – were managed among the nuclear powers. The most likely causes of a nuclear war became accident or misunderstanding rather than deliberate decisions.
Some of this thinking may have been comprehensible to Clausewitz. But he would certainly have found strange national strategies aimed above all at deterring war rather than actually preparing to fight one. The idea that strategy might deliberately abandon rationality with threats that ‘leave something to chance’ (in Thomas Schelling’s formulation) would also have been troubling. Debates over nuclear strategy, moreover, would lack historical examples that could provide guidance. Like the idea of pure war, nuclear strategy could appear disconnected from the real world, ‘a kind of war by algebra’. 
Instrument of policy?
Clausewitz is also criticised by those who claim that he fails to take into account fundamental drivers of war. It is true that he approaches war from the demand side, as something that states require for their purposes. And he says little about the supply side of war, about why groups, including states, may see war as valuable in itself rather than simply as a means to an end. While Clausewitz recognises that hatred can exist between peoples, critics argue that war originates from deeper factors that undermine the notion of war as simply a rational instrument of policy.
One line of attack is that Clausewitz’s idea of war ignores culture and therefore ‘does not fully encompass the causes of war’.[x] John Keegan, for example, asserts bluntly that ‘war is not a continuation of policy by other means’ because it ‘reaches into the most secret places of the human heart, places where self dissolves rational purpose’.[xi] Communities embody this underlying truth and fight, not for ‘political’ reasons but instinctively for the sake of the tribe or society, for religion or ideology, or simply as a way of life. A straitjacket of means and ends may be imposed on war, but this does not capture its true nature. On this interpretation societies value war for itself – a view Clausewitz could never countenance in relation to modern war.
A related criticism is that Clausewitz neglects the individual psychology of war. Fighting, Martin van Creveld suggests, ‘can be a source of joy, perhaps even the greatest joy of all’. The simultaneous risk of death and prospect of glory make it ‘one of the most exciting, most stimulating’ of human activities.[xii] War tests the manhood of young men and separates the brave from the unworthy. Duty, obedience and self-sacrifice become sacred values and are reinforced by ceremony, uniforms, flags and medals. There is always a supply of people ready, even keen, to fight whether in a modern, disciplined army or a rag-tag anti-modern outfit.
There is no ‘right’ definition of war – only definitions that are more or less useful for a given purpose. Clausewitz is interested in war in his own time because it reflected enormous changes taking place in politics and society. His principal concern is that war should serve as an instrument of policy for states with effective governments and regular armed forces – and be used to protect their independence and their honour. It is also an activity that can be to some extent ‘civilised’ by reason and by its separation from civilian life.
[i] Citations in English are from On War, translated and edited by Michael Howard and Peter Paret, Princeton UP, 1976. Page numbers are in square brackets. Citations in German are from the 19th edition of Vom Kriege, ed. Werner Hahlweg, Dümmler, Bonn, 1980.
[ii] See Donald Stoker, Clausewitz: His Life and Work, OUP 2014, Appendix ‘Clausewitz’s Battles’, pp. 289-90
[iii] See Beatrice Heuser, ‘Small Wars in the Age of Clausewitz: The Watershed Between Partisan War and People’s War’. Journal of Strategic Studies, vol. 33 no. 1 (February 2010), pp. 150-4
[iv] Peter Paret, Clausewitz and the State, Clarendon Press, Oxford, 1976, p. 188
[v] Clausewitz was familiar with Kant’s ideas. See Paret, Clausewitz and the State, p. 162. Clausewitz himself occasionally refers to the ‘pure concept of war’ (reiner Begriff des Krieges). 
[vi] For a discussion of the relationship between Clausewitz’s three trinities of passion, reason and chance; government, army and people; and combat, strategy and policy, see Hugh Smith, ‘Clausewitz’s Divisions: Analysis by Twos and Threes’, Infinity Journal, vol. 5 no. 3 (Fall 2016)
[vii] Christopher Duffy, The Military Experience in the Age of Reason, Routledge, London, 1987, p. 154
[viii] Azar Gat, War in Human Civilization, Oxford UP, 2008, pp. 446-7
[ix] Geoffrey Best, War and Law Since 1945, Clarendon Press, Oxford, pp. 142-3
[x] Gat, War in Human Civilization, pp. 669-70
[xi] A History of Warfare, Hutchinson, London, 1993, p. 3
[xii] The Culture of War, Ballantine, NY, 2008, pp. xi , 411