Carl von Clausewitz (17801831) is one of the best-known and most influential theorists of war. He gained enduring fame for On War, which provides deep insight into the theory and conduct of war. The book has influenced military thinkers and practitioners as diverse as Sir Julian Corbett, Vladimir Lenin, Mao Zedong, Harry Summers, and Colin Powell. Since the publication of On War in 1832, soldiers and scholars the world over have proclaimed it a fundamental resource. On War is a basic text at institutions of professional military education around the globe. It is thus fair to say that no serious thinker of military and strategic affairs is ignorant of Clausewitz. While war planners, military historians, and strategists draw heavily from Clausewitz, International Relations (IR) scholars find a place for him in their writings much less often. Aside from reproducing a famous quote or two, many IR scholars pursue their professional enterprise as if Clausewitz never existed. One possible reason for their relative inattention to Clausewitz stems from uncertainty over how to characterize him. If IR scholars are unsure how Clausewitz fits into the grand theories that dominate the fieldRealism, Liberalism, Marxism, and Constructivismor if he fits at all, it seems doubtful that he can guide their thinking.
What Clausewitz Has in Common withAllRealists
To help IR scholars better understand Clausewitz, this essay attempts to determine if we can fairly situate his ideas within the framework of IR Realism. It is appropriate to focus on Realism, because it is the oldest and most prominent theoretical paradigm in international relations. Moreover, even a cursory reading of On War indicates that some of Clausewitzs ideas align closely with those of Realism. For example, he saw war as a legitimate act of statecraft, gave great emphasis to the value of hard power, and placed no faith in perpetual peace. Most Realists would embrace these ideas, as well as others developed by Clausewitz.
The Three Strands of IR Realism
Butto fully understand howClausewitzs ideasrelate toIR Realism,it is necessary to consider where they fit within thethreekeystrands ofthe Realist enterprise: Defensive, Offensive, and Neoclassical Realism. However, a close consideration of Clausewitzs main ideas renders attempts to place them in an IR Realist context a difficult proposition. One primary reason Clausewitz fits awkwardly in an IR Realist context is that the goals and approach found in the writings of modern IR Realists differ in significant ways from those found in On War. At its core, IR is an effort to explain, and sometimes predict, how the international system operates. The focus of On War, however, is just that: war. The book is largely unconcerned with international relations, diplomacy, or foreign policy.But if we nevertheless attempt to situate Clausewitz within the confines of IR Realism, he is most closely connected to Neoclassical Realism, as we shall see.
The Three Levels of IR Analysis and the Key Differences between Them
One primary goal of IR in general is to identify and explain the sources of war. Basic to this goal are the three levels of analysis of IR: the individual, the state, and the international system. The first level of analysis focuses on the individual. It posits that the leader of the state sets the tone for that states conduct. It concentrates on the personality, perceptions, and worldview of the leader. If, for example, a Napoleon or Hitler controls the state, aggression is a likely outcome. The second level of analysis, the state, suggests that something intrinsic to the state itself is a cause of war, irrespective of its political leadership. Here the focus is on things such as state structure, bureaucracy, the nature of the political system, or ideology. Examples of states prone to war would include those suffused with nationalism, such as Revolutionary France, or militarism, such as the pre-World War II Empire of Japan. Along this line, some IR scholars argue that democratic states are less war-prone than authoritarian states. The third level of analysis considers the international system. Here the principal elements that govern the system include anarchy, the number of great military powers, the pattern of military alliances, levels and diffusion of technology, and economic patterns. According to proponents of this level of analysis (i.e., Structural Realists), states dwell in a grim international environment that forces them to compete for security, which is often notable by its absence. Because all states are essentially trapped in this system, they are forced to behave in similar ways.
Defensive, Offensive, and Neoclassical Realists differ over how states interpret the opportunities and limitations presented by the system, as well as over what value to place on power relations. All Realists agree that states seek power as a way to ensure their survival. But Defensive and Offensive Realists disagree about how much power states seek to acquire and to what end.
Defensive Realists, such as the very influential Kenneth Waltz, argue that a states main goal is survival; hence states seek only sufficient power, not maximum power. Defensive Realists argue that we invariably live in a defense-dominated world and that starting wars typically does not pay. Offensive Realism is most closely associated with John Mearsheimer, another important and influential IR theorist. In contrast to Waltz, Mearsheimer believes that states perpetually seek to gain an advantage over their rivals. He maintains that states try to maximize power in order to be the hegemon in the system, and that military policies, strategies, doctrine, budgets, and alliances are thus geared to that end. Neoclassical Realism, by contrast, concentrates on the interplay between the international system and the internal dynamics of states.Neoclassical Realism, the main proponents of which are William Wohlforth, Thomas J. Christensen, and Gideon Rose,affirms that a states actions in the international system result from three sets of variables. First, intervening systemic variables, such as the distribution of power among states. Second, cognitive variables, such as the perception and misperception of systemic pressures, other states intentions, or threats. Third, domestic variables, such as state institutions, elites, and other actors within society.
According to Neoclassical Realists, the interaction of these three sets of variables establishes the room for maneuver that policymakers have in foreign policy. Neoclassical Realists contend that a countrys foreign policy is governed by its rank in the international system and its relative power potential. They also believe that the influence of material power on foreign policy is indirect and mediated through domestic factors.
Clausewitz and the Three Strands of Realism
How do the three strands of Realism relate to Clausewitz, if at all? Structural Realism generally seems alien to Clausewitzs thought. He says nothing about the international system; his main concern is with the European state system. However, one could argue that Clausewitz embraced aspects of both Offensive and Defensive Realism. For example, Clausewitz fought against Napoleonic France and witnessed Napoleons defeat of the Prussian Army at Jena. He was clearly impressed by the power of the attack and the decisive battle. He accorded primacy to unrestrained violence in the conduct of war. Clausewitz deeply admired Napoleon, and his concept of the great captain is clear evidence of his emphasis on the individual, the great man, and thus on the first level of IR analysis.
Clausewitz most famous maxim, War is nothing but the continuation of policy with other means,  indicates that war results from a deliberate political choice made by the state, and this aligns with the second level of IR analysis (the state). One state that fueled much of the analysis in On War, France, was led by Napoleon, whom Clausewitz deemed the ultimate military commander. Clausewitz thus exhibits some affinity with both the first and second level of IR analysis. However, Napoleons catastrophic defeat in the Russian campaign forced Clausewitz to reassess his beliefs. How could a military strategy (i.e., taking the offensive) that Clausewitz championed lead to such different outcomes over the course of less than a decade? He began to see the defensive as the stronger form of warfare, as well as the limits to the unrestrained application of military force. Clausewitz wrote that defensive is a stronger form of fighting than attack. Waterloo marked another turning point in Clausewitzs thinking. Napoleon had again relied on the maximum application of military force, but it resulted in his defeat. This outcome forced Clausewitz to recognize that achieving a political objectivethe ultimate aim of wartook precedence over purely military aims. After Waterloo, Clausewitz had to rethink his view of war. He noted that If war is part of policy, then policy will determine its character.
Clausewitz recognized early on the great power of nationalism, and how it could affect the conduct of war. Unlike other contemporary writers on military affairs, he saw that the reason Napoleons armies dominated the battlefield stemmed not from new tactics or technology (although these did play a role in French success), but because of a profound change in French society. The French Revolution had fundamentally changed the French state, turning subjects into citizens who were willing to fight and die for the new republic. France had thus tapped into the energy of its people and created the nation in arms, which gave it the ability it to dominate Europe militarily. He argued that his own state, Prussia, needed to do the same if it was to catch up to France. In addition, his emphasis on nationalism as a tremendous force multiplier underscores his strong belief in the importance of the state, which accords with the second level of IR analysis. This all links him to Neoclassical Realism.
Why Clausewitz is a Neoclassical Realist and Not a Structural Realist
One can make a strong case that Clausewitz is closest to Neoclassical Realism. Neoclassical Realism, unlike Structural Realism, focuses more on the first and second levels of analysis (the individual and the state). Clausewitzs thoughts in Books 1 and 8 are quite consistent with the first and second strands of IR analysis. In addition, Realists of all types highlight the importance of military power. For Clausewitz, the army itself is usually the center of gravity. Finally, Neoclassical Realists believe the outcome of a war is a function of politics and people as well as military power. This is in line with what Clausewitz tells us in On War.
In specific areas, Clausewitzs ideas are congenial to all three camps of IR Realists. His ideas about the utility and the legitimacy of hard power, the inevitability of conflict, and several of his other beliefs support this point. But while some elements of Clausewitzs writing make him amenable to Defensive, Offensive, and Neoclassical Realists, he is closer to the latter camp. Clausewitzs writings demonstrate an appreciation for what can go wrong in warso much so that we cant accurately call him an Offensive Realist. He argues for the strength of the defensive over the offensive and supports this contention with several examples of what can go wrong when on the attack.
There is an important similarity between Clausewitz and Defensive Realists, in that he strongly believes in the superiority of defense. In his telling, there are few quick and decisive victories. Wars are slugfests; it is hard to see how war pays off in Clausewitzs description of it. This understanding fits neatly with the Defensive Realists view that we live in a defense-dominated world, and that initiating wars of conquest, more often than not, simply does not pay.
Another reason to hesitate before assigning Clausewitz to a particular IR camp is that he and IR theorists differ over what they are intending to accomplish. While IR theorists are attempting to explain the behavior of states in the international system, Clausewitz is primarily concerned with answering the most fundamental of all strategic questions: how to compel the enemy to submit to your will. The differing methods employed and objectives sought by Clausewitz and IR scholars make comparisons between them somewhat difficult.
What about foreign policy? Kenneth Waltz argues that Neorealism (Structural Realism) is not a theory of foreign policy. Rather, it is a theory of how the interaction of states plays out within the international system. For his part, Clausewitz does not promote a theory of foreign policy. However, he advocated war against France in 1806, and criticized his own government for not using its foreign policy to gain allies for that war. For Clausewitz, war is an instrument of foreign policy, though admittedly he says relatively little about this.
Attempting to place Clausewitz within the framework of IR Defensive, Offensive, or Neoclassical Realism is an uncertain proposition at best. He wrote about war and not how the international system operated. But of the three strands of Realism, he is much more in line with Neoclassical Realism. Beyond that, he is more aligned with Defensive than Offensive Realism.
 Carl Philipp Gottfried von Clausewitz, Vom Kriege, 3 vols. (Berlin: Ferdinand Dmmler, 183234). Although several versions of On War have appeared over the years, the English-speaking world has largely coalesced behind this version: On War, Michael E. Howard and Peter Paret, eds. & trans. (Princeton University Press, 1976/1984).
 Michael I. Handel, Corbett, Clausewitz, and Sun Tzu,Naval War College Review Vol. 53, No. 4 (2000), 110-124.
 Jacob W. Kipp, Lenin and Clausewitz: The Militarization of Marxism, 1914-1921, Military Affairs, Vol. 49, No. 4 (1985), 184-191.
 Mao Tse-tung, On Guerrilla Warfare, Samuel B Griffith II, ed. & trans., 2nd revised (University of Illinois Press, 2000)
 Harry G. Summers, On Strategy: A Critical Analysis of the Vietnam War (Presidio Press, 1982).
 As Powell writes in his memoirs, after he entered the National War College in 1975, he readOn Warfor the first time. He was bowled over. It was, he says, like a beam of light from the past, still illuminating present-day military quandaries. See Colin Power with Joseph E. Persico, My American Journey (Ballantine Books, 2003), 207.
 According to two well-known IR scholars, Theory is the lodestone in the field of International Relations (IR). Its theorists are the fields most famous and prestigious scholars. See John J. Mearsheimer and Stephen M. Walt, Leaving theory behind: Why simplistic hypothesis testing is bad for International Relations, European Journal of International Relations, Vol. 19, No. 3 (2013), 428.
 Jeffrey W. Legro and Andrew Moravcsik, Is Anybody Still a Realist? International Security, Vol. 24, No. 2 (Fall 1999), 5.
 In Clausewitzs words, No one can deny a nation the right to fight for its interests with all its strength. See Carl von Clausewitz, Trans. Peter Paret andDaniel Moran, Historical and Political Writings (Princeton University Press, 2016), p. 239.
 This stands in contrast to Immanuel Kant, who argued that one should apply the principles of the European Enlightenment to government. Kant put forth a set of six preliminary articles including treaty-writing, abolishing standing armies, when to take on national debt, the conditions for foreign intervention, and adopting rules of war that could collectively reduce the likelihood of war. See Immanuel Kant, To Perpetual Peace: A Philosophical Sketch, 1795, in Immanuel Kant, Perpetual Peace and Other Essays, trans. Ted Humphrey (Hacket Publishing, 1983), 107-143.
 Defensive and Offensive Realism are varieties of Neorealism, which is also known as Structural Realism. In fact, many Neorealists prefer the term Structural Realism, because they believe that the structure of the international system (the distribution of power among the states that comprise the system) is the main driver of state behavior. I will use the term Structural Realism going forward.
 Gideon Rose coined the term Neoclassical Realism. See Neoclassical Realism and Theories of Foreign Policy, World Politics, Vol. 51, No. 1 (October 1998), pp. 144172.
 Kenneth Waltz, Man, the State, and War: A Theoretical Analysis, Revised edition (Columbia University Press, 2001).
 More specifically, it is argued that democratic states are less aggressive toward other democracies, but can be hostile when dealing with authoritarian states. This is known as the democratic peace theory.
 Anarchy is not synonymous with chaos. Instead, it simply means that there is no global referee. States are thus forced to fend for themselves.
 Neoclassical Realists incorporate all three level of analysis in their writings.
 Kenneth Waltz, Theory of International Politics (McGraw Hill, 1979), 126.
 Mearsheimer, The Tragedy of Great Power Politics, 21.
 Rose, Neoclassical realism and theories of foreign policy, 146.
 Note that in German, there is only one word for policy and politics: politik. Thus when reading On War, one could potentially substitute one word for the other.
 Andreas Herberg-Rothe, Clausewitzs Puzzle: The Political Theory of War (University Press, 2007), 29.
 Clausewitz, On War, 69. The original German is: Wir behaupten dagegen, der Krieg ist nichts als eine Fortsetzung des politischen Verkehrs mit Einmischung anderer Mittel. However, the word mit can be translated into English as either with or by.
 Clausewitz, On War, 84.
 Clausewitz referred to Waterloo as a dbacle. See Clausewitz, On War, 235. For a discussion of why Waterloo altered Clausewitzs thinking, see Andreas Herberg-Rothe, Clausewitzs Puzzle: The Political Theory of War (Oxford University Press, 2007), 2-3, 36-38, 79-85.
 Clausewitz, On War, 606.
 These would include the great Swiss strategist Antoine-Henri, Baron Jomini (17791869), who authored a number of works on strategy that were highly influential at the military academies and staff colleges that proliferated during his lifetime. Clausewitz leveled a number of criticisms at Jominis theories. See John Shy, Jomini, in Peter Paret,Makers of Modern Strategy: From Machiavelli to the Nuclear Age (Princeton University Press, 1984); and Christopher Bassford, Jomini and Clausewitz: Their Interaction. Paper presented to the 24th Meeting of the Consortium on Revolutionary Europe at Georgia State University, 26 February 1993. Proceedings of the Consortium on Revolutionary Europe, XX (1992). Tallahassee, FL: Florida State University, 1994. Another prominent contemporary was Dietrich Heinrich von Blow (17571807), a Prussian soldier and military writer who wrote The Spirit of the Modern System of War Trans. C. Malorti de Martemont (London: T. Egerton, 1806). Clausewitz also criticized the theories proposed in this book.
 In fact, at the outset of the French Revolution, most analysts saw the event as precipitating Frances decline. See T.W.C. Blanning, The Origins of the French Revolutionary Wars (Routledge, 2017).
 Peter Paret, Clausewitz, in Makers of Modern Strategy, 190.
 Clausewitz, On War, Book 8.
 Peter Paret, Clausewitz and the State: The Man, His Theories, and His Times(Princeton University Press, 1985).
 According to Clausewitz, the center of gravity (Schwerpunkt) is always found where the mass is concentrated most densely. It presents the most effective target for a blow. Clausewitz, On War, 485.
 Clausewitz asserts that one never knows in advance the result of a war, writing that warfare is the realm of chance and probability. He also discounts the value of intelligence at the tactical level: Many intelligence reports in war are contradictory; even more are false, and most are uncertain. Thus the entire enterprise is fraught with uncertainty. See Clausewitz, On War, 89, 117.
 War is thus an act of force to compel the enemy to do our will. Clausewitz, On War, 775.
 Kenneth N. Waltz,International Politics is Not Foreign Policy,Security Studies,Vol. 6, No. 1 (1996),54-57. See also Rose, Neoclassical Realism and Theories of Foreign Policy.
 Paret, Clausewitz, 190-91.