Jack Levy. Handbook of International Relations. Editor: Walter Carlsnaes, Thomas Risse, Beth A Simmons. Sage Publication. 2002.
Ever since Thucydides’s account of the Peloponnesian War over 2,400 years ago (Strassler, 1996), scholars from a wide range of disciplines have studied war in the hope of facilitating efforts to prevent its occurrence, reduce its frequency, or mitigate its consequences. Political science is absolutely central to this task. Clausewitz’s ( 1976) influential conceptualization of war as a ‘continuation of politics by other means,’ an instrument to advance political interests, suggests that war is intrinsically political, so that if we want to understand war we must understand why decision-makers choose military force rather than other means to achieve their desired ends.
The study of war in political science varies enormously in theoretical orientation, methodological approach, ontological assumptions and empirical domain, and there is now greater diversity in the study of war and in the international relations field as a whole than at any time in recent memory. The old ‘great debate’ between realists and liberals continues, but scholars have increasingly recognized significant variations within each contending paradigm, and new debates have arisen among rational choice theorists, constructivists and critical theorists. Scholars continue to use the influential levels-of-analysis framework (Singer, 1961) that emerged from Waltz’s (1959) three ‘images’ of international politics, but they have shifted from asking which level has the greatest causal impact to constructing multi-level theories and examining the interaction effects between variables at different levels. International relations scholars have also engaged in increasingly productive dialogues with economists, sociologists, psychologists and diplomatic historians (Elman and Elman, 2001). Finally, there has been a growing interest in the conditions of peace as well as the causes of war, and a growing belief that the study of war and the study of peace are inseparable.
In some respects all of these efforts have yielded little tangible progress. We have few law-like propositions, limited predictive capacities and no consensus as to what the causes of war are, what theories and what methodologies are most useful for identifying and validating those causes, what criteria are appropriate for evaluating competing theories, and whether it is possible to generalize about anything as complex and context-dependent as war. In other respects, however, we have made enormous progress. The field of international relations (and the sub-field of peace, war and security studies) is more rigorous in its theorizing, more sophisticated in the use of statistical methods, more theoretically and methodologically self-conscious in the use of qualitative methods, more willing to utilize multimethod research designs, and more willing to think critically about the meta-theoretical assumptions underlying theoretical and empirical research. A social science orientation is more entrenched than ever in the field, but the conception of social science has broadened. Those in the broad ‘mainstream’ engage in meta-theoretical debates with each other as well as with critical theorists about the logic of inference (King et al., 1994), the meaning of causality (Nicholson, 1996; Braumoeller and Goertz, 2000), the proper relationship between theory and evidence (Lichbach and Lebow, 2001), and the criteria for evaluating scientific progress (Elman and Elman, 2002). While some meta-theoretical discussions only paralyze theoretical and empirical research, these debates have enriched it.
I begin by examining some general trends in the study of war and peace. I then distinguish among three different things that we want to explain: the constant recurrence of war, variations in war and peace, and the origins of particular wars. I argue that these different questions or perspectives on war lead to different theoretical frameworks and different methodologies. I then examine some leading realist and liberal theories of the causes of war and conditions of peace, and conclude by emphasizing the utility of multimethod research programs.
Any essay of this kind must be selective. Given other reviews in the literature (Levy, 1989; Vasquez, 1993, 2000; Cashman, 1993; Doyle, 1997; Midlarsky, 2000), and given the division of labor in this volume, I focus in some detail on leading realist and liberal theories of the causes of war and the conditions for peace rather than attempting to cover the entire field. The most notable exclusions from this chapter are intra-state and Ethnonational wars, cognitive and psychological models, and feminist approaches, which are discussed in the chapters by Lars-Erik Cederman, Janice Stein and J. Ann Tickner, respectively, and an important new class of rationalist bargaining models, which are discussed in the chapter by Lilach Gilady and Bruce Russett. I make only brief references to constructivist approaches (Giddens, 1979; Katzenstein, 1996; Wendt, 1999), which are discussed in Part One of this volume.
General Trends in War and in the Study of War
International relations scholars generally define war as large-scale organized violence between politically defined groups (Bull, 1977: 184; Vasquez, 1993: 21–9). War has been a recurring and persistent pattern of behavior among peoples since the beginning of recorded history, but it has also varied enormously in its frequency and seriousness over time and space. The past five centuries of the modern system have witnessed an average of one great power war per decade, but the frequency of great power wars has declined significantly over time (Levy, 1983). We have experienced only three such wars in the twentieth century and arguably none in the period since the World War II and this constitutes the longest period of great power peace in 500 years. For many centuries war was disproportionately concentrated in the hands of the great powers in Europe (Wright, 1965) but the twentieth century, and the second half-century in particular, marked a significant shift in warfare from the major powers to minor powers, from Europe to other regions, and from inter-state warfare to intra-state wars (Levy et al., 2001).
The steady decline in the frequency of great power war has been accompanied by a steady increase in the severity of war (generally defined in terms of the number of battle-related deaths, either in absolute terms or relative to population). The rising severity of war is evidenced by the enormous destructiveness of the two world wars of the twentieth century, and, more generally, by the increasing severity of ‘general’ or ‘hegemonic’ wars, which have occurred at least once or twice a century since 1500 (Thompson, 1988; Levy, 1989). Inter-state wars have demonstrated no significant secular trend over the past century, while civil wars have increased in frequency. Vivid media images of ethnonational conflicts may have led to the impression that the severity of civil wars has continued to increase in the past half century or even in the past ten years, but deaths from civil wars have actually been declining since the Second World War and even since the end of the Cold War (Holsti, 1996; Wallensteen and Sollenberg, 1999; Marshall, 1999; Gurr, 2000).
The changing nature of warfare over the past century, and over the last two decades of that century in particular (Keegan, 1984; Bond, 1986; Van Creveld, 1991) has, along with new analytic developments, led to important changes in the way international relations scholars have approached the study of war. The nuclear revolution led to the analyses of the conditions for strategic stability and of the hypothesized obsolescence of war (Schelling, 1966; Jervis, 1989; Mueller, 1989; Mandelbaum, 1998–9), and the information revolution has led to discussions of the ‘revolution in military affairs’ (Biddle, 1998; Freedman, 1998). Political changes have been equally important. The end of the Cold War, the collapse of the Soviet Union, and the rise of ethnonational conflicts have contributed to a shift in focus away from a longstanding emphasis on the behavior of the great powers and on inter-state wars more generally, and toward intra-state wars.
We have also witnessed a shift from a near-exclusive focus on the causes of war to a greater interest in the termination of war (King, 1997; Goemans, 2000). The analysis of the termination of war is now closely linked to analyses of the relationship between war and conditions for state-building and democratization (Zartman, 1995; Licklider, 1998), which has some parallels in the study of the role of war in state-building in early modern Europe (Mann, 1986; Tilly, 1990). This is related to the more general question of the conditions under which peace settlements are most likely to persist. Conflict theorists have traditionally defined peace as the absence of war (or perhaps the absence of militarized disputes) and have focused more on explaining war than explaining peace. Theorists have recently begun to give more attention to the conditions for peace (Wallensteen, 1984; Holsti, 1991; Vasquez, 1993: ch. 8), and some have begun to distinguish between peace and stable peace, or between cold peace and warm peace (Kacowitz and Bar-Siman-Tov, 2000; Miller, 2000).
The rise of regional and ethnonational wars has also contributed to increasing attention to the impact of domestic variables, long neglected by a field that has focused primarily on the role of structural systemic sources of conflict (Waltz, 1979) and secondarily on individual-level beliefs and misperceptions (Jervis, 1976; George, 1979; Holsti, 1989) and on bureaucratic/organizational factors (Allison and Zelikow, 1999). The new emphasis on domestic sources of conflict includes a variety of research programs, ranging from the democratic peace, diversionary theory, cultural approaches and signaling theories based on variations in the credibility of commitments across different regime types and institutional arrangements. There has also been a shift in the level (or unit) of phenomenon to be explained, as evidenced by the diminished attention to systemic patterns and greater attention to dyadic-level behavior. This shift derives in part from the explanatory power of the dyadic-level democratic peace hypothesis and the hope that its success could be duplicated elsewhere, the growing interest in international rivalries (Thompson, 1999; Diehl and Goertz, 2000) and from the emphasis on bargaining in rational choice theory (Fearon, 1995; Powell, 1999; Wagner, 2000).
The spread of ethnonational conflicts has also led to serious reconsideration of the best way to define and operationalize war. Traditional conceptualizations of warfare are grounded in Clausewitz’s ( 1976) emphasis on states and their organized armies, assume a sharp distinction between international and intranational conflict, and generally operationalize war in terms of a minimum threshold of 1,000 battle-deaths severity (Singer and Small, 1972). This conceptualization works well for most inter-state wars, but much less so for many contemporary conflicts, whether they be ‘low-intensity conflicts’ or ‘identity wars’ between communal groups. The recognition of this conceptual problem has led to the generation of several new data sets designed to better capture the new forms of armed conflict (Marshall, 1999; Wallensteen and Sollenberg, 1999; Gurr, 2000).
Three Perspectives on War
The earlier summary of patterns of warfare over time suggests two ways of looking at war. In one sense war has been a constant–a persistent, pervasive and recurring pattern of violent conflict between peoples since the beginning of recorded history. Some war is going on somewhere almost all of the time, particularly if we include internal as well as external wars. At the same time, however, war has varied enormously in terms of its frequency, severity, location, participants, and social and political consequences. Some scholars focus on the first theme and treat war as a non-zero constant in international politics, while others treat war as a variable. Still others focus neither on the general phenomenon of war nor variations in wars but instead on particular wars.
These three perspectives on the question of what causes war are clearly related, yet they generate fundamental differences in the kinds of methodologies, research designs, conceptions of causation and more general epistemological orientations that scholars bring to bear on the question of what causes war. Explanations for war as a constant (based on ‘human nature’ or international anarchy, for example) cannot explain either variations in war and peace or the outbreak of particular wars; explanations for particular wars cannot be generalized to explanations for variations in war and peace without empirical tests of those generalizations against a broader empirical domain; and the idea of using explanations for variations in war and peace to explain individual wars–by subsuming explanations for particular events under ‘covering laws’ (Hempel, 1942)–is something that most historians reject (Dray, 1957).
While political scientists give some attention to each of these questions, the most influential work in the discipline focuses on the second question, explaining variations in war and peace. This orientation is more pronounced in the United States, where positivistic social science is most dominant, than in Europe or elsewhere (Hill, 1985; Waever, 1998). True, many political scientists focus on explaining particular wars, not only as a vehicle for constructing or testing more general propositions about war, but sometimes also for the primary purpose of understanding those wars as an end in itself, as illustrated by studies of the First World War or the Cold War. Many of these studies–especially those that use well-developed theoretical frameworks to guide historical case studies–have made important contributions to our understanding of individual wars or rivalries. Given prevailing social science norms and current reward structures in the discipline, however, this is not the kind of work that has the greatest impact on the study of international relations, at least in the United States (Levy, 2001).
Realist Theories of War
The realist tradition has long dominated the study of war, and includes intellectual descendants of Machiavelli, Hobbes and Rousseau (Doyle, 1997). All share a common set of assumptions: the key actors in world politics are sovereign states who act rationally to advance their security, power and wealth in an anarchic international system, defined as the absence of a legitimate governmental authority to regulate disputes and enforce agreements between states or other actors. Given uncertainties regarding the current and future intentions of the adversary, political leaders focus on short-term security needs, adopt worst-case thinking, engage in a struggle for power and security, and utilize coercive threats to advance their interests, influence the adversary and maintain their reputations.
Following Waltz’s (1979) development of neorealism (or structural realism), most contemporary realists begin with the proposition that international anarchy is an important permissive condition for war. For Rousseau and other system-level theorists, ‘wars occur because there is nothing to prevent them’ (Waltz, 1959: 232). While realist arguments based on anarchy provide an important alternative to explanations for the recurrence of war based on human nature, anarchy is generally treated as a structural constant (except for rare instances of non-anarchic systems such as the Roman Empire) and consequently it cannot account for variations in war and peace (Milner, 1991). Waltz recognizes this and concedes that ‘Rousseau’s analysis explains the recurrence of war without explaining any particular war’ (1959: 232), and that ‘although neorealist theory does not explain why particular wars are fought, it does explain war’s dismal recurrence through the millennia’ (1988: 620). Ironically, then, Waltz’s (1959) compelling critique of ‘first image’ theories on the grounds that one cannot explain a variable with a constant applies to the neorealist focus on anarchy as well.
If constants (or variables with limited variation) cannot explain variations in war and peace, can they explain the constant recurrence of war? The problem here is that it is not clear how to empirically test explanations of the constant recurrence of war (whether based on anarchy, human nature, or perhaps the patriarchal and gendered nature of all human societies) or to adjudicate among competing theories. If a permissive condition allows war to happen but does not make war happen, then whether wars occur or not, or the frequency with which they occur, has no bearing on the validity of the theoretical argument. In addition, because there can be more than one necessary condition for an outcome, and because there is no logical basis for preferring one necessary condition over another, it is difficult to assess the relative validity of contending explanations for the recurrence of war unless they generate additional testable implications besides the recurrence of war. This suggests that for a theory of the recurrence of war to be meaningful and useful, it would have to generate a series of implications or predictions about variations in war and peace.
In an important sense, this is what realist theory has done. Anarchy is not a causal variable but instead an analytical primitive or point of departure, from which a theoretical system is constructed and hypotheses are generated. It is these hypotheses, not anarchy, that carry the explanatory power in neorealism, and what they attempt to explain is not ‘war’s dismal recurrence through the millennia,’ but the conditions under which war is most likely to occur.
The core realist proposition is that variations in the distribution of power, and polarity in particular, help to explain variations in the frequency of war and other important aspects of international behavior (Waltz, 1988: 620). As many critics have argued, however, the distribution of power alone does not explain enough of the variance in war and peace across time and space, much less broader changes in international systems, and in this sense war is underdetermined in neorealist theory (Keohane, 1986; Ruggie, 1998).
Realists have increasingly come to acknowledge this limitation, and have begun to incorporate other variables–including the offensive–defensive balance (Lynn-Jones, 1995; Van Evera, 1999) and even domestic institutional and cultural variables (Van Evera, 1990; Snyder, 1991)–in an attempt to explain more of the variation in international conflict. Whether these additional variables explain enough variation for realism to constitute an empirically adequate theory, and whether the addition of domestic variables is consistent with the ‘hard core’ assumptions of the realist research program, thus constituting a ‘theoretically progressive problem shift’ (Lakatos, 1970), are subjects of ongoing debate (Vasquez and Elman, 2002).
Paths to War in Realist Theory
Realists often distinguish between status quo and revisionist states and use this distinction as the basis for identifying two alternative paths to war. One path involves a direct conflict of interests between two states and calculations by at least one side that this conflict is better resolved by war than by peace. Ancient imperial conquests like those of Alexander, or more modern cases of aggression by Hitler’s Germany in the Second World War and Saddam’s Iraq in 1990, come immediately to mind, though each of these cases is usually more complex than first appears.
A second path to war involves two or more security-seeking states, each of whom is more interested in maintaining its current position than in extending its influence. The explanation of the conditions and processes through which such security-seeking states can still end up in war is a distinctive contribution of realist theory. Because the absence of a higher authority requires states to depend on their own actions (or perhaps those of allies) for their security, because of the uncertainty regarding the present and future intentions of other states, and because of the dire consequences of the failure to be prepared for possible predatory actions of others, states take certain actions (armaments, alliances, deterrent threats, etc.) to protect themselves against current and future threats. Because of the inherent uncertainty regarding the intentions underlying the actions of others, the offensive as well as defensive potential of most weapons systems, and the tendency toward worst case analysis in the context of extreme uncertainty, even defensively motivated actions are perceived as threats to the security of others (the ‘security dilemma’). The threatened state responds with measures to protect itself, these are perceived as threatening by the other, and the ensuing action–reaction cycle often leads to conflict spirals that can escalate to war (Jervis, 1978; Glaser, 1997).
Realists debate how compelling anarchic structures and the security dilemma actually are, or how often status quo states without expansionist motivations get locked in conflict spirals that end up in war. ‘Offensive realists’ argue that the international system is so hostile and unforgiving that uncertainty about the future intentions of the adversary combined with extreme worst-case analysis lead states to adopt offensive strategies, which often lead to war (Mearsheimer, 1990, 2001; Zakaria, 1992; Labs, 1997). ‘Defensive realists,’ on the other hand, argue that security is not as scarce as offensive realists suggest, that international anarchy does not in itself force states into conflict and war, and that moderate behavior and defensive strategies work to provide security. If states behave aggressively, it is not because of systemic pressures but instead because of domestic pressures and pathologies. Defensive realists argue that war will not arise in a world of purely security-seeking states in the absence of domestically induced revisionist goals or extreme misperceptions of external threats (Snyder, 1991; Schweller, 1996; Glaser, 1997; Van Evera, 1999).
One possible causal mechanism in the ‘pure security’ path to war involves a ‘pre-emptive strike’ by a state motivated only by the fear that its adversary is about to attack and by the belief that if war is inevitable it is better to initiate it. Reiter (1995) makes a persuasive case that such wars rarely happen, and identifies only the First World War and the 1967 Arab–Israeli War as cases of preemptive wars. The First World War has traditionally been interpreted as one that leaders did not seek but that was the inadvertent result of a combination of blunders, misperceptions, miscalculations and overreactions that spiraled out of control (Tuchman, 1962), but an important line of argument suggests that Germany did in fact seek war in 1914, just not a world war involving British intervention (Fischer, 1975; Levy, 1990/1; Van Evera, 1999).
The hypothesis of a German preference for some kind of war in 1914, even a war against Russia and France, if true, would not settle the question of predation versus security fears, or revisionist versus status quo motivations. If a state faces a rising adversary, anticipates a ‘power transition’ and initiates a ‘preventive war’ to maintain the status quo (a standard interpretation of Germany in 1914, given the rising power of Russia), or if it initiates war to recover territory lost in a previous war and re-establish the earlier status quo, is this predation or security-seeking? Thus the analytical distinction between predation and security-seeking is not always clear, and in fact the two can mutually reinforce each other. As Snyder and Jervis (1999: 21) argue, ‘the security dilemma gives rise to predators, and predation intensifies the security dilemma.’
A pure ‘preventive war’–one motivated only by the anticipation of a negative power shift and the fear of its consequences–is another path through which anarchic structures alone might induce war. Although the anticipation of negative power shifts often plays a significant role in the processes leading to war, whether the combination of power shifts and uncertainty about the future is ever jointly sufficient for war, apart from more specific conflicts of interests, is open to question. The clearest case of military action induced almost exclusively by a negative power shift was Israel’s 1981 attack against the Iraqi nuclear reactor. Even here, however, if Peres rather than Begin had been in power, or if Egypt rather than Iraq was building a nuclear reactor, it is unlikely that Israel would have responded militarily (Perlmutter, 1982: 35; Levy and Gochal, 2000). As Kydd (1997: 48) argues, ‘preventive wars sparked by fears about the future motivations of currently benign states almost never happen.’ The preventive motivation leads to war only in conjunction with existing hostilities or conflicts of interest, and most wars involve some combination of security-seeking and predatory motivations.
Balance of Power and Hegemonic Realism
Another important but underemphasized debate within realist theory is between ‘balance of power realism’ and ‘hegemonic realism’ (Levy, 1994). Balance of power approaches include both classical theories as reformulated by Morgenthau (1948), Gulick (1955), Claude (1962) and Aron (1973), and the more systematic structural realism of Waltz (1979). Hegemonic realism includes power transition theories, hegemonic stability theory (Gilpin, 1981; Keohane, 1984), and long cycle theories (Thompson, 1988; Rasler and Thompson, 1994).
Balance of power theories posit that the avoidance of hegemony is the primary goal of states (or at least of the great powers) and that the maintenance of an equilibrium of power in the system is an essential means to that end. The theory predicts that states, and particularly great powers, will build up their arms and form alliances to balance against those who constitute the primary threats to their interests and particularly against any state which threatens to secure a hegemonic position over the system. Balance of power theorists argue that the balancing mechanism almost always works successfully to avoid hegemony, either because potential hegemons are deterred by their anticipation of a military coalition forming against them or because they are defeated in war after deterrence fails.
Hegemonic theories share realist assumptions but de-emphasize the importance of anarchy while emphasizing system management within a hierarchical order. The most influential hegemonic theory is power transition theory (Organski and Kugler, 1980; Kugler and Lemke, 1996). Hegemons commonly arise and use their strength to create a set of political and economic structures and norms of behavior that enhances the stability of the system at the same time that it advances their own security. Differential rates of growth lead to the rise and fall of hegemons (Organski and Kugler, 1980; Gilpin, 1981; Kennedy, 1987), and the probability of a major war is greatest at the point when the declining leader is being overtaken by the rising challenger. Either the challenger initiates a war to bring its benefits from the system into line with its rising military power, or the declining leader initiates a preventive war to block the rising challenger while the opportunity is still available.
Because balance of power theory posits that concentrations of power are destabilizing and that hegemony never occurs, while power transition theory posits that hegemony frequently occurs and is stabilizing, the two appear to be diametrically opposed. Indeed, power transition theory grew directly out of Organski’s critique of balance of power theory (Organski and Kugler, 1980). What is rarely recognized, however, is that most applications of the two theories define key concepts differently. Most balance of power theories have a strong Eurocentric bias and implicitly conceive of power in terms of land-based military power and of hegemony in terms of hegemony over Europe. They predict balancing coalitions against threats of European hegemony (the Habsburgs under Charles V, France under Louis XIV and then Napoleon, Germany under Wilhelm and then Hitler). They do not necessarily predict balancing against rich naval powers that have small armies and that are geographically separated from the European continent (nineteenth-century Britain or the twentieth/twenty-first-century United States), which pose little direct threat to the territorial integrity of other great powers (Levy, 1994, 2002; Mearsheimer, 2001). Most applications of hegemonic theories generally define hegemony in terms of dominance in global finance, trade and naval power, and indeed most versions of ‘hegemonic stability theory’ (Keohane, 1984) are theories of the stability of the international political economy and say little about war and peace.
Given these different conceptions of the nature of the system and of the basis of power in the system, it is possible that both theories are correct. It is conceivable, for example, that the European system has been most stable under an equilibrium of military power and that hegemonies rarely if ever form, whereas the global system is most stable in the presence of a single dominant economic and naval power (which occurs frequently). These two systems interact, of course, but exactly how they interact has been undertheorized. The most destabilizing situation would be one characterized by the combination of the diffusion of global power (and particularly an impending global power transition) with the increasing concentration of power in Europe (Rasler and Thompson, 1994). In fact, many of history’s ‘hegemonic wars’ fit this pattern: the two world wars of this century, the French Revolutionary and Napoleonic wars (1792–1815), and the wars of Louis XIV (1672–1713)–the first two pairs following the decline of Britain’s global dominance and the third following the decline of Dutch global economic supremacy. Although scholarship has been shifting away from systemic-level analyses, studies of the interaction of the global system with the dominant continental system–and of other nested systems as well–is a particularly fruitful area for further research.
Liberal Theories of War and Peace
Liberals have always questioned the realist perspective on international politics and argued that under certain domestic and international conditions and with appropriate state strategies the violent-prone character of world politics can be ameliorated and levels of warfare significantly reduced. Although the liberal theory of the international political economy is fairly well-developed, until recently there was no coherent liberal theory of peace and war. With the development of the democratic peace research program, renewed interest in the hypothesis that economic interdependence promotes peace, the development of a theory of international institutions, and preliminary attempts to combine these into a single integrated theory, we now have the outlines of a liberal theory of peace and war. For some (Russett and Oneal, 2001) this represents the systematization and empirical testing of Kant’s ( 1949) conception of perpetual peace based on democratic institutions, free trade, and international law and institutions.
Here I focus on the hypotheses that economic interdependence and democratic institutions each promotes peace. I leave aside theories of the peaceful effects of law and institutions. Institutionalists have constructed theories about the effects of international law and institutions on cooperation between states, particularly in the international political economy and on environmental issues (Keohane, 1984; Archibugi, 1995; Keohane and Martin, 2002; see also Chapters 10 and 28 in this volume). They have also applied institutionalist approaches to collective security systems (Kupchan and Kupchan, 1991), regional security communities (Deutsch et al., 1957) and alliances (Haftendorn et al., 1999). Some have taken a constructivist approach to the study of institutions, alliances and security communities and emphasized the role of norms and collective identity (Schroeder, 1994; Risse-Kappen, 1996; Adler and Barnett, 1998). But analyses of the impact of institutions on war and peace are still in an early stage of development, raise important analytic and historical questions, and have yet to be empirically tested (Betts, 1992; Mearsheimer, 1994/5; Keohane and Martin, 1995).
Economic Interdependence and Peace
The idea that trade and other forms of economic interdependence promote peace was a central theme in nineteenth-century liberal economic theory, and was expressed most famously by Norman Angell when he argued in The Great Illusion (1912) that the economic costs of a great power war would be so devastating that such a war was unthinkable. Angell’s argument was discredited within two years by a very thinkable world war, but was resurrected after the Second World War as a cornerstone of American liberal internationalist ideology. It is now the basis for optimistic (but qualified) forecasts about the beneficial effects of globalization on international security (Friedman, 1999).
Liberal theorists advance a number of interrelated theoretical arguments in support of this proposition, but the greatest emphasis is on the economic deterrence argument: because trade generates economic benefits for both parties, the anticipation that war will disrupt trade and lead to a loss or reduction of the gains from trade deters political leaders from taking actions that are likely to lead to war against key trading partners (Polachek, 1980; Oneal and Russett, 1999).
Liberals also advance domestic-level causal arguments in support of the trade-promotes-peace hypothesis (Veblen,  1966; Schumpeter,  1951; Aron, 1958). Trade increases prosperity, and prosperity lessens the domestic problems that sometimes lead to war, either through external scapegoating by élites to solidify their domestic political support, or through pressures for protectionism that can lead to countermeasures, increase hostilities and trigger conflict spirals. As Wilson (1978: 150) argues, ‘economic depression [is] particularly favorable to war hysteria.’ Prosperity can also generate a culture of acquisitiveness that dampens the martial spirit and diverts resources away from the military sector, as reflected in the view of the nineteenth century that ‘Men were too busy growing rich to have time for war’ (Blainey, 1988: 10).
Some researchers argue that trade alters the domestic balance of power within states by increasing the influence of groups who benefit from trade and who have a vested interest in maintaining a peaceful environment for trade (Rogowski, 1989), and others suggest, from a sociological perspective, that trade increases contact, communication, familiarity and understanding, which in turn reduce the hostilities and misperceptions that contribute to war (Deutsch et al., 1957). Finally, some argue that while trade promotes peace, the link is indirect: trade promotes prosperity, prosperity promotes democracy and democracy promotes peace (Weede, 1995).
The commercial liberal hypothesis suffers from a number of analytic problems. The argument that leaders’ fears of the economic costs of war deters them from taking actions that might lead to war attempts to explain a dyadic outcome (peace) with state-level variables (foreign policy preferences) and ignores strategic interaction. It is possible that if a dispute arises between trading partners, each of whom prefers peace, both will refrain from belligerent actions in order to preserve the benefits of trade. It is also possible that one side might interpret the other’s conciliatory actions as a lack of resolve and lead it to believe that it can exploit the adversary’s fear of war by standing firm and thereby improving its own strategic or economic position. In the absence of additional information about expectations regarding the economic benefits of trade, the impact of war on trade and each side’s risk orientation and domestic sensitivity to those costs, the outcome–and hence the impact of economic interdependence on peace within a dyad–is theoretically indeterminate (Morrow, 1999; Gartzke et al., 2001). The neglect of the impact of inter-state bargaining is a serious deficiency of most empirical research on the trade-promotes-peace hypothesis.
Strategies of coercion rather than cooperation with the trading partner are more likely if one side believes that it is more resolved than the other, whether because of differing risk orientations or perhaps different sensitivities to the domestic economic and political costs of a cutoff of trade. It is also more likely if economic interdependence is asymmetrical rather than symmetrical (Hirschman,  1980; Barbieri, 1996), in which case the least dependent state may be tempted to resort to economic coercion to exploit the adversary’s vulnerabilities and influence its behavior relating to security as well as economic issues.
The potential for exploitation of the weak by the strong in a situation of asymmetrical interdependence is the basis of the argument, advanced by both realists and Marxist-Leninists, that interdependence, and particularly asymmetrical interdependence, increases rather than decreases the probability of militarized conflict. While some realists concede that symmetrical interdependence may create mutual incentives to maintain the peace (Barbieri and Schneider, 1999), even symmetrical interdependence is no guarantee of restraint if the two sides have different risk orientations and/or different sensitivities to domestic costs. Just as analysts of crisis bargaining have begun to incorporate the risk orientations and domestic cost-sensitivity of political leaders into their models (Fearon, 1995; Wagner, 2000), those who study the political economy of war and peace must do the same.
Another basis for the realist argument that trade can increase rather than decrease the likelihood of militarized conflict involves relative gains concerns. Realists argue that political leaders are less influenced by the possibility of gains from trade in an absolute sense than by the fear that the adversary will gain more from trade and convert those gains into further gains, political influence and military power (Grieco, 1990; Gowa, 1994). Realists are not always clear, however, about the precise causal mechanisms leading from relative gains to war. To the extent that relative rather than absolute gains are important, they should have a greater impact on decisions to engage in trade (particularly with adversaries) than on the likelihood of conflict once trade is under way. If states are extremely worried about a particular adversary making relative gains they will have minimal trade with that adversary, and if states are already trading with each other they have presumably already discounted relative gains concerns. If diplomatic relations between trading partners begin to deteriorate, however, relative gains concerns may lead states to cut back on trade, which may exacerbate existing tensions and contribute to a conflict spiral.
Not all realists argue that trade significantly increases the likelihood of war. Some concede that trade and other forms of economic interdependence might have pacifying effects, but argue that these effects are negligible relative to the effects of military and diplomatic considerations (Buzan, 1984; Levy, 1989: 261–2). Other realists acknowledge that periods of trade might be peaceful, but question the causal impact of trade on peace. They argue that the causal arrow often points in the opposite direction: it is peace that creates the conditions under which trade flourishes, as Blainey (1988) argues with respect to nineteenth-century Europe. The more general argument is that politics determines trade, or that ‘trade follows the flag,’ rather than trade shaping politics (Pollins, 1989; Gowa, 1994).
It is also possible that the inference that trade promotes peace is spurious, because the conditions that facilitate trade simultaneously promote peace. States with common interests tend to trade with each other (Morrow et al., 1998) and also to be less inclined to fight, so the association between trade and peace may be explained in part or in full by the commonality of interests. Similarly, there is more trade between allies than between adversaries (Gowa, 1994), and allies are less likely to go to war with each other (Ray, 1990), so alliances may account for part of the association between trade and peace. Democratic dyads trade more than other pairs of states (Mansfield et al., 2000), so that studies of trade and peace must parcel out the effects of the democratic peace (Russett and Oneal, 2001). Finally, hegemonic stability theorists argue that one of the primary conditions facilitating trade is the existence of a liberal economic hegemon able and willing to maintain a stable political economy, and they strongly imply that liberal economic hegemony also promotes peace (Gilpin, 1981; Keohane, 1984), so the link between trade and peace might be spurious and explained primarily by economic hegemony.
A similar problem arises in analyses of the relative efficiency of commerce and conquest as strategies for accumulating wealth (Rosecrance, 1986). Although many formulations of the trade-promotes-peace hypothesis implicitly assume that trade is always more efficient than military coercion in expanding markets and investment opportunities and in promoting state wealth, realists are almost certainly right that this assumption is historically contingent rather than universal. Trade may be economically efficient and peace-promoting in the Western world in the contemporary era, at least for advanced industrial states, because the foundations of wealth and power have historically shifted from territory to industrialization and now to knowledge-based forms of production, and because the economic value of territorial conquest has diminished while the military and diplomatic costs of territorial conquest have significantly increased (Van Evera, 1990). In many historical eras, however, military force has been a useful instrument to promote state wealth as well as power. In the mercantilist era of the seventeenth and eighteenth centuries, for example, war was a profitable enterprise for both merchants and kings. War also increased the political influence of those merchants who benefited from war; which generated additional political support for war (Howard, 1976; Wilson, 1978).
How much causal weight to attribute to trade in this equation is a more difficult analytic question. If underlying conditions make conquest more efficient than trade as a strategy for acquiring wealth, those conditions will simultaneously decrease trade and increase the use of force, while conditions under which trade is more efficient may simultaneously and independently promote peace. If this is true, trade has no independent impact on conflict.
Whether the deterrent effects of the gains from trade outweigh the potentially destabilizing effects of economic asymmetries and economic competitions, whether the latter escalate to trade wars and militarized conflicts, and whether the magnitude of these economic effects is outweighed by strategic considerations are ultimately empirical questions that analysts have only recently begun to analyze systematically. Although there is a growing consensus that trade is associated with peace, both at the dyadic (Polachek, 1980; Oneal and Russett, 1999; Russett and Oneal, 2001) and systemic levels (Mansfield, 1994), some find that trade is associated with war (Barbieri, 1996). Because few of these studies have dealt with possible endogeneity effects and explored the simultaneous impact of trade on conflict and conflict on trade (Reuveny and Kang, 1996), because of questions regarding the sensitivity of these relationships to the precise operationalization of interdependence (Barbieri and Schneider, 1999), and because a number of prominent historical cases (the First World War, for example) appear to run counter to the liberal hypothesis (Copeland, 1996; Ripsman and Blanchard, 1996/7; Papayoanou, 1999), the current evidence in support of the liberal economic theory of peace must be treated as provisional. Further research needs to focus as much on the conditions under which trade promotes peace as on the aggregate relationship between trade and peace.
One important informational condition relates to the beliefs of political leaders and of key economic actors regarding the likely impact of war on trade, both in the short term and in the long term. There is ample evidence that trade often continues even after the outbreak of war (Barbieri and Levy, 1999), though the frequency of trading with the enemy, in what kinds of goods, and with what impact on the economy and the war effort has yet to be established. If leaders anticipate that war will not significantly suppress trade, their economic incentives to avoid war will be diminished. There is also a strategic dimension: for bargaining purposes leaders may have incentives to threaten to cut off trade in the event of war. Once war occurs, however, those same leaders may have incentives to allow their firms to continue to trade, either for the good of the economy as a whole or to gain the political and economic support of key domestic groups. The anticipation of this undercuts the credibility of earlier threats to cut off trade. This credibility depends on alliance ties as well as other variables (Dorussen, 1999).
Most explanations of the trade-promotes-peace hypothesis focus on the dyadic level. Scholars have overlooked the systemic context of dyadic trade in general and diplomatic alignments in particular. Trade between A and B may deter a dyadic war between A and B. If A is aligned with C and B threatens C, however, A’s trade ties with B may prevent A from attempting to deter B’s attack on C, which may actually increase the probability of a war between B and C. This is one causal mechanism through which high levels of economic interdependence contributed to the First World War: the British failure to make a formal commitment to join France and Russia if they were attacked by Germany was a critical factor leading to German aggression (Fischer, 1975; Levy, 1990/1), and Britain’s failure to do so derived in part from her strong economic ties with Germany (Papayoanou, 1999).
This brief overview suggests the relationship between interdependence and peace is shaped by factors associated with both liberal and realist international theories. A satisfactory theory of interdependence and conflict theory must incorporate ‘liberal’ concerns about the opportunity costs of the loss of trade, the influence of domestic actors who have an interest in maintaining and expanding trade and the political power to influence state decisions, and the constraints imposed on firms by state structures and actions. Such a theory must also incorporate ‘realist’ concerns about the strategic consequences of trade at both the dyadic and systemic levels, but the theory must be sensitive to whether these considerations affect the level and kinds of trade between countries or the impact of that trade on the likelihood of war. More generally, empirical research designs must do more to reflect the complex causal linkages among trade, security and war that theorists have begun to identify. They must also give more attention to the dependent variable and distinguish between war and militarized conflict more generally. It is possible that trade ties might have a pacifying effect on war but not on militarized disputes.
The Democratic Peace
Liberals have long argued that democracies are more peaceful than are other states, but the ‘democratic peace’ did not become a coherent and quite visible research program in international relations until a number of studies in the mid-1980s offered systematic and compelling evidence that democracies rarely if ever go to war with each other (Ray, 1995; Doyle, 1997). Researchers then demonstrated that this empirical regularity cannot be explained by the geographic separation of democratic states, by extensive trade among democratic dyads, by the role of American hegemonic power in suppressing potential conflicts between democracies in the period since the Second World War, or by other economic or geopolitical factors correlated with democracy (Ray, 1995, 2000; Doyle, 1997; Maoz, 1997; Oneal and Russett, 1999; Russett and Starr, 2000; Russett and Oneal, 2001).
Although some still argue that the hypothesized impact of democracy on peace is spurious or that causality runs from peace to democracy rather than from democracy to peace (Brown et al., 1996; Gowa, 1999), there is a growing consensus that the pacifying effects of joint democracy are real. While some say that it goes too far to claim that the absence of war between democracies ‘comes as close as anything we have to an empirical law in international relations’ (Levy, 1989: 270), no one has identified a stronger empirical regularity, and many make the law-like claim that joint democracy is a sufficient condition for peace (Gleditsch, 1995; Chan, 1997; Russett and Starr, 2000; Braumoeller and Goertz, 2000).
Research has uncovered other patterns involving the behavior of democratic states with respect to war and peace–though none as strong as the near absence of war between democracies. At a minimum, any explanation for the dyadic peace between democracies must not contradict these other patterns, and ideally it should explain them. Most analysts have found that, contrary to Kant, democracies are not significantly more peaceful than other kinds of states. Democracies are as likely as authoritarian states to get involved in wars; they frequently fight imperial wars; in wars between democracies and autocracies they are more likely to be the initiators than targets; and they occasionally use covert action against each other (Ray, 1995, 2000; Bennett and Stam, 1998; Russett and Oneal, 2001). In addition, democratic-authoritarian dyads are more war-prone than are pure authoritarian dyads.
Some recent research suggests than democracies may be more peaceful than authoritarian states after all (Rummel, 1995; Benoit, 1996; Ray, 2000). The differences are only modest, however, and this is unlikely to change with future research. Consequently, any explanation for the democratic peace that implies that democracies are significantly more peaceful than other states, in terms of the frequency of their involvement in wars or in their tendency to initiate wars, will probably not be viable.
An explanation for the democratic peace must also be consistent with evidence that democracies almost never end up on opposing sides in multilateral wars, win a disproportionate number of the wars they fight, suffer fewer casualties in the wars they initiate (Reiter and Stam, forthcoming), and engage in more peaceful processes of conflict resolution when they get into disputes with other democracies (Dixon, 1994). Although some argue that states involved in transitions to democracy are more likely to end up in war than are other states (Mansfield and Snyder, 1995), most evidence suggest that democratizing states are not more warlike (Ward and Gleditsch, 1998; Russett and Oneal, 2001) and that democracies do not fight shorter wars than other states do (Reiter and Stam, forthcoming). It is important to distinguish, however, between the early stages of transitions away from authoritarian rule and later stages when democratic institutions have begun to consolidate. Recent evidence suggests that war is significantly more likely only in these early transitional stages (Mansfield and Snyder, forthcoming).
The growing consensus that democracies rarely if ever fight each other is not matched by any agreement as to how best to explain this strong empirical regularity. Theorizing about the democratic peace is in its early stages, and new theories will undoubtedly be proposed, but at the present time there are several alternative explanations.
The ‘democratic culture and norms’ model (Owen, 1997; Russett and Oneal, 2001) suggests that democratic societies are inherently averse to war because (as Kant argued) citizens will not vote to send themselves off to war. In addition, democracies share norms of bounded political competition and peaceful resolution of disputes; and these internal democratic norms are extended to relations between democratic states. Democracies shed norms of peaceful conflict resolution in relations with non-democratic states, however, because they fear being exploited.
The plausibility of the normative model of the democratic peace is undercut by the fact that such norms have not precluded democratic states from initiating imperial wars against weaker opponents despite the absence of any threat of exploitation by the latter, or from fighting wars against autocracies with an intensity disproportionate to any plausible security threat. These concerns lead some to supplement a democratic culture argument with a constructivist emphasis on shared identity and perceived distinction between self and other (Risse-Kappen, 1995), which provides a more plausible explanation for democratic hostility toward culturally dissimilar, non-democratic states. Yet democracies do engage in covert action against each other (James and Mitchell, 1995), and they occasionally use low levels of military force against each other (Bueno de Mesquita and Lalman, 1992), which is not consistent with the idea of a shared identity of democratic states.
The ‘institutional constraints model’ emphasizes checks and balances, the dispersion of power and the role of a free press. These institutions preclude political leaders from taking unilateral military action, ensure an open public debate and require leaders to secure a broad base of public support before adopting risky policies. As a result, leaders are risk-averse with respect to decisions for war and can take forceful actions only in response to serious immediate threats (Morgan and Campbell, 1991; Siverson, 1995). Although the institutional constraints model provides a plausible explanation for the relative absence of wars between democracies, like the democratic norms model it fails to explain why democracies frequently fight imperial wars despite the absence of serious threats. It also fails to explain why democracies get involved in wars just as frequently as do non-democratic states. In addition, most versions of the institutional model assume that leaders have more warlike preferences than do their publics, which is why leaders need to be constrained. This is not always true, and in fact belligerent publics sometimes push their leaders into wars those leaders prefer to avoid (the United States in the Spanish-American War, for example), and politically insecure leaders engage in diversionary action in order to trigger ‘rally round the flag’ effects that bolster their domestic political support (Levy, 1989; Smith, 1996; Gelpi, 1997).
Many of these anomalies are explained by Bueno de Mesquita et al. (1999) in an alternative institutional explanation of the democratic peace based on a game-theoretic model that more fully incorporates strategic interaction between democracies and their adversaries. The model emphasizes political survival as the primary goal of political leaders. It suggests that the political survival of leaders with larger winning coalitions (usually characteristic of democracies) depends on successful public policies, whereas the political survival of leaders with smaller winning coalitions (authoritarian states) depends on their ability to satisfy their core supporters through the distribution of private goods. This implies that democratic leaders are more sensitive to the outcome of wars than are authoritarian leaders, which in fact is consistent with the finding that democratic leaders are more likely than their authoritarian counterparts to be removed from office after an unsuccessful war (Bueno de Mesquita and Siverson, 1995). Because of the political benefits of successful wars and the political costs of unsuccessful wars, democratic leaders tend to initiate only those wars they are confident of winning and, once in war, to devote enormous resources to win those wars. Autocratic leaders devote fewer resources to war because the costs of failure in war are less and because they need those resources to distribute to their key supporters at home.
In a war between democracies, both sides would invest enormously in the war effort, which would result in a war that is economically costly to both sides as well as politically costly to the loser. Democratic leaders understand this and have strong incentives to seek a negotiated peace rather than to fight, which explains the dyadic democratic peace. The model also accounts for the fact that democracies frequently get involved in wars as well as other empirical regularities regarding democratic war behavior. Because democratic leaders benefit from successful wars, especially those involving low casualties, they will not hesitate to initiate imperial wars and wars against weaker autocracies. The model also explains why strong democracies sometimes initiate low levels of force against a much weaker democracy (few domestic political risks), why the target capitulates immediately (anticipating that leaders in the stronger state have strong incentives to win the war), and thus why militarized disputes between democracies do not escalate to war.
In the Bueno de Mesquita et al. (1999) model, the willingness of democracies to invest heavily in the war effort makes them unattractive targets of aggression, but autocrats also take greater gambles in war because the outcome of war has less of an impact on their political survival. Autocracies will initiate wars against weak democracies but rarely against strong democracies. If ex ante military capabilities are approximately equal, the likelihood of a democratic-autocratic war depends on the specific values of key variables in the model. Democratic leaders will consider war if they believe that their greater investment in the war effort guarantees victory, while autocratic leaders’ greater willingness to gamble might lead them to consider war if the democratic effort advantage is modest.
Schultz (1998) provides an alternative explanation of the democratic peace, one based on a signaling game and the transparency of democratic institutions and processes. The basic argument is that because a free press guarantees transparency and because the political opposition has different incentives than the government and some influence over the government, democracies are better able than non-democracies to send credible signals of their resolve in crises, and this reduces the dangers of crisis escalation due to misperceptions.
More specifically, the transparency of the democratic process makes it obvious whether democratic political leaders involved in international crises have the support of the political opposition and the public in an international crisis. In the absence of domestic support the government cannot stand firm in a crisis because it cannot implement its threats, and the adversary knows this and adopts a harder line in crisis bargaining. Democratic leaders anticipate their adversary’s resolve and refrain from getting involved in crises in the first place.
If leaders expect public support, however, they will initiate disputes knowing they will be able to stand firm if the adversary resists, and the anticipation of this leads the adversary to behave more cautiously. As a result, crises involving democratic states are less likely to be characterized by misperceptions regarding the adversary’s resolve and less likely to escalate to war because of misperceptions. This is critical because misperceptions based on private information and incentives to misrepresent that information play a central role in the outbreak of war (Jervis, 1976; Blainey, 1988; Fearon, 1995). In jointly democratic dyads, misperceptions are reduced even further, though whether this reduction is enough to account for the near-absence of wars between democracies is problematic.
The democratic peace research program has evolved from the description of empirical regularities to controlling for spurious influences, exploring anomalous cases and constructing models to explain observed regularities. Another important development is the effort to use these models to generate a new set of predictions about a wide range of other types of behavior (conflict resolution, intervention, covert action, the conduct and outcome of war, perceptions of the adversaries, etc.), and to subject these predictions to empirical test (Russett and Starr, 2000). This last step is a particularly welcome one, as the generation and empirical confirmation of new testable implications is a critical requirement of a progressive research program (Lakatos, 1970; King et al., 1994).
One distinctive feature of the democratic peace research program is that it has engaged scholars from several different research communities sharing rather different methodological orientations–large-nstatistical methods, small-n case study methods, and formal modeling. Each has made a distinctive contribution to our understanding of the security policies and strategic interaction of democratic states, and the democratic peace research program has benefitted enormously from the combination of their efforts.
Quantitative methods were indispensable in establishing the empirical regularity that propelled the research program, demonstrating that the extraordinarily strong association between joint democracy and peace was not spurious, and identifying other empirical regularities that constrain any theoretical explanation of the democratic peace. Qualitative case studies were indispensable for exploring potentially anomalous cases, helping to resolve debates about whether states did or did not satisfy the definitional requirements for democracy, and assessing whether the inference of a causal connection between joint democracy and peace was valid or spurious in that particular case. Finally, applications of formal modeling helped in exploring possible causal paths leading from joint democracy to peace, incorporating a theory of strategic interaction as well as the domestic sources of foreign policy, and in the process generating some very plausible theoretical explanations of the democratic peace and associated empirical regularities.
Each of these methodological approaches has been essential in the evolution of the democratic peace research program, and their synergistic effects have added much to our understanding of the pacifying effects of joint democracy. Although the literature on the democratic peace was driven largely by evidence in its early stages (the 1980s)–by the unprecedented level of empirical support for the dyadic democratic peace proposition–it is now driven by a genuine dialectic of theory and evidence. This particular temporal sequence of approaches (quantitative, case study and formal) may not follow the textbook model of theory, hypotheses and empirical test, but it has unquestionably been effective in the accumulation of knowledge about the democratic peace. It is also a useful reminder that different methodologies can be combined in different sequences and that there are multiple paths to knowledge about the empirical world.
Having emphasized the advantages of the multimethod character of research on the democratic peace, let me end by noting that multimethod approaches are beginning to benefit other research communities as well. The analysis of the relationship between interdependence and conflict is a good example. In response to the very general theoretical argument that trade promotes peace, scholars attempted to operationalize this hypothesis and test it statistically over a large number of cases (Barbieri, 1996; Russett and Oneal, 2001), and at the same time others explored the trade-promotes-peace hypothesis through more intensive case studies of individual cases (Ripsman and Blanchard, 1996/7). Other scholars followed with the application of insights from formal models to help identify logical problems in early theories of commercial liberalism and to suggest new causal linkages (Morrow, 1999; Dorussen, 1999), and case studies in turn have been helpful in illuminating some of these derived causal linkages (Papayoanou, 1999). Some of the testable implications of hypotheses on trade and peace are best examined by large-n statistical methods, while other implications are more easily examined through case study methods. One example of the latter involves hypotheses about the impact of leaders’ expectations regarding the duration and severity of an anticipated war and their expectations of the impact of war on trade.
Just as multimethod approaches have potentially significant benefits, the reluctance of scholars to cross methodological divisions and build on the work of those in other research communities is a serious limitation of some research programs. One example is the old realist debate on the relative stability of bipolar and multipolar systems. Although neorealists rely heavily on polarity as a key explanatory variable (Waltz, 1979; Mearsheimer, 1990), their analyses are primarily deductive in character and not informed by systematic empirical investigation. They overgeneralize from the Cold War experience, where bipolarity is confounded with the existence of nuclear weapons and other key variables, and fail to demonstrate the validity of their arguments with respect to earlier historical eras.
Although multipolarity is more common than bipolarity, there are surely enough instances of the latter to warrant empirical investigation, but there has been no serious effort to do this. Neorealists also ignore a number of quantitative studies that generally show no significant and systematic relationship between polarity and war (Sabrosky, 1985; Kegley and Raymond, 1994). In addition, formal modeling perspectives question the logical links between the basic assumptions of neorealist theory and the hypothesis of bipolar stability (Bueno de Mesquita, 2002). The opportunity costs of failing to build upon important research on polarity in different research communities is particularly serious because the bipolar stability hypothesis is one of the central testable hypotheses in the neorealist research program.
For the most part, however, scholars have increasingly come to recognize the benefits of a multimethod orientation towards research and to inform their own work with the insights and findings of scholars working in other research communities. This methodological pluralism is yet another reason for optimism about progress in the study of war and peace and of international relations more generally.