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date: 15 November 2018

The Durability of Peace

Summary and Keywords

With wars—not just global, but civil wars and other domestic infightings—still being rampant in the modern world, scholars have begun to develop interest in identifying the conditions that can help establish a durable peace. Peace is a lack of conflict and freedom from fear of violence between social groups. Commonly understood as the absence of war or violent hostility, peace often involves compromise, and therefore is initiated with thoughtful active listening and communication to enhance and create genuine mutual understanding. The study of the durability of peace has greatly evolved through the years, and one of its implications is that recent empirical work on this topic has focused on civil war. Most of this study has been tailored in response to the model of war, a theory of armed conflict which presents war and peace as stages of a single process. Furthermore, this analysis on peace duration revolves around for main themes: the characteristics of conflict and conflict actors, belligerent-centered dynamics, the role of third parties, and the developments in the measurement, estimation, and the study of peace duration. Under the conceptions of peace, sustainable peace must be regarded as an important factor for the future of prosperity. Throughout the world, nurturing, empowerment, and communications are considered to be the crucial factors in creating and sustaining a durable peace.

Keywords: peace, wars, conflict, violence, civil war, third parties, peace duration, compromise


The costs associated with wars – the loss of lives, the destruction of infrastructure, and a host of other ills – have fostered an interest in identifying the conditions that can help countries establish a durable peace once a war has ended. Scholarship on this issue first appeared during the interwar period, particularly once it became apparent that World War I would not be the “war to end all wars” (Carr 1964). However, the emergence of the Cold War had the effect of deflecting attention away from this topic. The advent of nuclear weapons shifted consideration to the question of how to prevent wars from breaking out in the first place and away from issues related to conflict recurrence. Only with the end of the Cold War did attention once again turn to the durability of peace. Prompted by the realization that most armed hostilities in the post-World War II period have been civil wars, many of which have been serial conflicts, research attempting to account for the ability of some countries to establish a durable peace while others fight repeated wars has grown rapidly since the 1990s.

One implication of the manner in which the study of the durability of peace has evolved is that much of the recent empirical work on this topic has focused on civil war. A good deal of this research has been guided by or framed in response to the bargaining model of war, a general theory of armed conflict which conceives of war and peace as stages of a single process. We analyze this body of work in our chapter, assessing its relevance to the duration of the peace following both interstate and intrastate conflicts as well as focusing on commonalities and differences between these two types of conflict where the durability of the peace is concerned.

We structure our review of the scholarship on peace duration around four main themes. We begin with research that links characteristics related to the conflict and conflict actors to the prospect that the peace that follows will be long-lived. Next we examine literature that focuses on the obstacles or opportunities belligerent-centered dynamics pose for the durability of peace. We consider the ways bargaining dynamics, war outcomes, and settlement institutionalization shorten or prolong the post-conflict peace. We then shift to a focus on the role third parties play in influencing the duration of the peace through their actions as interveners, mediators, peacekeepers, and proponents of post-war policies. We conclude with a discussion of developments in measurement and means of estimating the duration of the peace.

Characteristics of Conflict and Conflict Actors

Numerous studies, including some of the earliest ones seeking to explain why peace has proved durable following some conflicts but not others, have focused on a range of characteristics related to the conflict. Although not all of these studies were structured as means of testing the logic of the bargaining model of war, works that sought to assess the costs associated with conflicts, the information with which wars provided actors, and the nature of the stakes involved in conflicts laid the groundwork for much of the research that has followed.

Nature of Conflicts

Many scholars have suggested that certain types of conflict are less likely to be followed by a long-lived peace than are others. Ethnic conflict in particular has received the bulk of attention in research on civil war recurrence, although scholars have been divided regarding the role they attribute to ethnicity in triggering a renewal of conflict. Kaufmann (1996) claims that ethnic wars harden identities and destroy the possibility for ethnic cooperation, thus making it impossible to restore civil politics in multi-ethnic states. Quinn et al. (2007) identify another mechanism linking ethnicity to the duration of the peace. They posit that ethnic markers lower the costs of remobilizing for renewed conflict by making it easier for groups to identify with potential supporters. Cederman (2010) proposes yet a third means by which ethnicity may serve to trigger repeated bouts of conflict, arguing that it is the exclusion of ethnic groups from access to state power that is associated with patterns of political violence.

Empirical studies support some of these propositions, but not others. Studies by Hartzell et al. (2001), Walter (2004), and Fortna (2008) find that ethnicity has no significant effect on the duration of the peace following civil wars. One shortcoming of these studies is that because they rely on ethno-demographic characteristics of ethnicity such as fractionalization and polarization as proxies, they do not directly tap into the concepts of ethnic cooperation and opportunity costs identified above. Recognizing this, scholars are now devoting increasing attention to developing data sets that allow for more specific testing of the mechanisms that have been identified as linking ethnicity with conflict renewal (Kirschner 2010). Using a cross-national data set with information on the power status of all politically relevant ethnic groups, for example, Cederman et al. (2010) found support for the hypothesis that excluding ethnic groups from state power has conflict-inducing effects.

Perhaps because territorial issues pose fewer measurement and identification problems than do those related to ethnicity, research on the role territory plays in conflict recurrence has produced generally consistent outcomes. Numerous studies of International conflict have found support for the proposition that disputes involving territorial issues have a high likelihood of recurring (Hensel 1994; Huth 1996; 2000; Grieco 2001). The peace following disputes of this nature is hypothesized to be short-lived because the salient nature of territorial issues increases the relative utility of conflict while lowering the relative utility of the status quo (Quackenbush 2010). Territory is found to produce a different effect in the case of enduring internal rivalries. Fuhrmann and Tir (2009) show that the peace is likely to be shorter after territorial conflict because control of territory allows rebel groups to regroup and resume fighting.

Conflict Features

The length and intensity of conflicts have also been hypothesized to have an impact on the durability of the peace. Wars provide belligerent groups with an opportunity to gather information about the enemy that may have been difficult to access prior to the conflict (Fearon 1995; Goemans 2000). The lengthier a conflict is the more information parties should be able to gather and the less uncertain they should be about the outcome of future encounters (Mason and Fett 1996). Based on that logic, protracted conflicts should be followed by a longer peace, a hypothesis that has been corroborated by several studies of peace durability following civil wars (Sambanis 2000; Walter 2004; Quinn et al. 2007; Fortna 2008; Mattes and Savun 2010).

Research on high-intensity conflicts suggests that this variable has a different effect on peace duration depending on whether the conflict is interstate or intrastate in nature. Focusing on interstate war, Werner (1999a) finds support for the argument that dyads that expect that the costs of renewed war will be high based on past battle deaths are less likely to use violence to renegotiate the terms of war-ending settlements and thus experience a longer peace. Studies of civil war by Doyle and Sambanis (2000), Hartzell and Hoddie (2007), and Mattes and Savun (2009) find that costly civil wars have a negative effect on the duration of the peace. They attribute this outcome to a heightened sense of hostility and insecurity which can lead adversaries to misinterpret one another's actions and thus trigger renewed war.

Although research on the duration and intensity of conflict suggests that adversaries gain information from the wars they fight, it does not make clear why interstate and civil war adversaries appear to interpret the information provided by high-cost wars differently. Is there some reason to believe that security concerns might be more pronounced among adversaries in a post-civil war context than following interstate wars? Do the audience costs produced by each type of conflict differ in their impact on support for each type of war by the relevant populations? These questions suggest that future research on the durability of the peace might benefit from pooling data on interstate and intrastate conflicts. Doing so might provide a means for learning whether the mechanisms that have been identified as having an impact on the duration of the peace operate in the same fashion following both types of conflicts (Lake 2003).

Attributes of Conflict Actors

A growing body of research has sought to determine whether conflict actors possess certain qualities that make them more likely to relapse into conflict. More specifically, domestic norms, institutions, and economic conditions have been identified as affecting, alternately, leaders' decisions to reinitiate conflict in the case of interstate wars and citizens' determination to (re)join rebel organizations in order to participate in a renewed round of civil war. Building on the democratic peace literature, a number of scholars have suggested that post-conflict peace should last longer the higher the democracy level of the least democratic actor in an interstate dyad. Democratic regimes have also been posited to extend the duration of the peace following civil wars as they provide citizens with a means of seeking to effect nonviolent change (Walter 2004). Higher levels of economic development and rapid rates of post-conflict growth have been hypothesized to help stabilize the peace by raising the opportunity costs of returning to war (Collier et al 2003; Walter 2004).

Many more empirical studies have sought to test the relationship between regime type, economic variables and the duration of the peace following intrastate wars than interstate conflicts. Among the latter, what research does exist supports the proposition that the post-dispute peace is longer the higher the level of joint democracy in a dyad (Senese and Quackenbush 2003; Quackenbush and Venteicher 2008). The picture is more mixed where the effects of democracy on post-civil war duration is concerned. Although Walter (2004) finds support for this proposition, many other scholars do not (Quinn et al. 2007; DeRouen and Bercovitch 2008; Fortna 2008; Morey 2009). One study concludes that although democracy has no significant effect on the peace, “severe autocracy appears to be highly successful in maintaining the post-conflict peace” (Collier et al. 2008: 470).

More support exists for the relationship between economic factors and post-civil war peace durability. Employing various economic indicators, Walter (2004), Doyle and Sambanis (2006), Quinn et al. (2007), Collier et al. (2008), and Hartzell (2009) find a positive association between better economic conditions and peace duration while Bigombe et al. (2000) and Collier et al. (2008) find that economies that grow faster after a civil war have a longer peace. DeRouen and Bercovitch (2008), Fortna (2008), and Morey (2009), on the other hand, find no association between post-war economic conditions and the longevity of the peace.

Belligerent-Centered Dynamics

Bargaining Dynamics

Conflict studies experienced a seismic shift that evolved into new understandings of the onset, duration and termination of war, ushered in by James Fearon's application of economic models of bargaining to the outbreak of war (Fearon 1995). Combined with Blainey's (1973) characterization of war and peace as parts of a single process, the focus on bargaining dynamics opened a new range of explanations for war termination and peace durability. From Fearon's characterization of war, scholars have identified several factors that capture belligerent expectations about the peace process and the decision to implement or “renegotiate” tacit or explicit contracts between parties with conflicting interests.

Fearon (1995: 379) presents a simple puzzle: “wars are costly but nonetheless wars recur.” Challenging existing explanations derived from dominant paradigms, Fearon presents a simple formalization of an interaction between states to demonstrate why a bargain always exists that is more efficient than playing the costly lottery of war, a demonstration supported by three crucial assumptions: (1) that states have complete information; (2) that they interact once; and (3) that the stakes are perfectly divisible. He derives three explanations for war by relaxing each assumption in turn. First, he notes that states do not have complete information about factors that influence expectations about the war (capabilities, resolve, costs, etc.) and so cannot locate the ex ante bargaining range. Given that this information determines the location of the bargaining range, states have great “incentive to misrepresent” those factors in an effort to shift the bargaining range in their favor. Second, Fearon makes clear that although an interaction that occurs only once eliminates the possibility of shifting factors over time, states interact constantly. Fearon argues that if the interaction is repeated, then the bargain struck in the present may not be desirable as factors change over time. This creates incentives to alter the deal down the line, a dynamic that Fearon calls a credible commitment problem. States may anticipate this impending change and attack in the present rather than wait for a certain future in which the deal will move further from their preferred division of stakes. Finally, Fearon argues that not all stakes are perfectly divisible, so a range of bargains may not exist within the reservation values set by both states. From these explanations scholars have extrapolated arguments for why wars end and why peace survives in some cases, but not in others.

The information problem made its way into the literature on war termination even before Fearon's model appeared in print in the form of questions regarding how states at war decided to end them. The Rubinstein (1982) bargaining framework of alternating offers serves as the basis for more recent, formal models that demonstrate how states use fighting to reveal information about capabilities and resolve (several scholars, including Clausewitz (1976), Pillar (1983) and Blainey (1973), make these arguments informally). If a lack of information about capabilities and resolve is the cause of fighting, the convergence of those expectations is likely the cause of negotiated settlement and an end to the war. These arguments suggest that settlement terms are endogenous to the conflict, reflecting battlefield conditions such as cost accumulations and relative capabilities (Powell 1999; Werner 1999b; Goemans 2000; Filson and Werner 2002; Slantchev 2003). As long as those expectations persist, war will not resume because the actors will not anticipate any new information that might change the deal (Kecskemeti 1958; Wittman 1979; Pillar 1983).

Werner (1999a) tests the information-expectations relationship in one of the first uses of hazard analysis to assess factors explaining peace duration. She measures changes that might influence the convergence and divergence of expectations in three ways: relative power; leadership changes; and expected military costs, and for each, she finds consistent support across three models for the effect of changes in relative power. Others have considered whether the war ended in stalemate or victory as a proxy measurement for convergent expectations, predicting that victory helps peace endure because it leaves the actors in no doubt as to who possesses the greater capability, while stalemate offers no guarantees regarding who might win the next encounter (Maoz 1984; Zartman 1989; 1995; Wagner 1993; 1994; Licklider 1995; Walter 1997; 2002; Stinnett and Diehl 2001; Fortna 2003a; 2004c).

Another approach focuses on the commitment problem to explain why peace fails, an issue that has received considerable attention in the civil conflict literature. The commitment problem arises because belligerents cannot credibly commit to maintaining the current deal when a change in the future creates an advantage for one of the adversaries that the advantaged belligerent will exploit. The focus is not on incomplete information but instead on the repeated interactions and natural changes that take place over shorter or longer periods of time or even as a result of the war and settlement (Fearon 1995). This last scenario is the most frequently used to explain peace durability. Barbara Walter (1997; 2002) focuses on commitment problems as the primary obstacle to resolving civil conflicts. She argues that in civil conflicts, settlements usually require rebel groups to disarm while the government retains its military forces (Wagner (1993) makes a similar argument to explain the failure of negotiated settlements). Given that the political settlement is a reflection of the conditions when the rebels were armed, disarmament creates the exact commitment problem that Walter argues encourages a resumption of violence. The government cannot commit to maintaining the settlement since it gains the upper hand as the rebels relinquish weapons. She tests this theory indirectly by determining whether her proposed solutions, third-party security guarantees and power-sharing agreements, are associated with successful implementation.

Fortna (2004c) argues that commitment problems are not as severe for interstate conflicts because independent nations are not expected to disarm completely, meaning they are able to maintain the forces that helped create the existing settlement. Werner's (1999a) test of enforcement as a problem in interstate conflict resolution also suggests that short-term incentives to cheat are not consistently and systematically related to peace duration. Reiter (2009), however, considers several factors that could lead to renewed fighting in interstate conflict; these, he argues, explain why states choose to pursue absolute war rather than accept a peace that will be subject to commitment problems. His propositions suggest that whether one observes limited or absolute war depends on what belligerents expect the peace to look like and whether the commitment problems created by rapid or gradual changes are sufficient to justify fighting in the present to avoid a worse deal later. His tests consider whether third party insurance and transfer of goods are more or less effective than increasing war costs and diminishing hopes for victory.

Few studies have considered the indivisibility issue because many scholars have questioned whether any issue is truly indivisible given that the options of issue linkage and side payments can help to facilitate a deal. Hensel and Mitchell (2005) consider this question by undertaking a substantial data collection process (Issue Correlates of War). Testing tangible and intangible territorial salience with both the incidence of settlement and militarized disputes, they find mixed results for the expectation that intangible salience (indivisibility) leads to more militarized disputes and novel and unexpected results that show that intangible disputes are more likely to lead to settlement than those that are tangible. Both Fortna (2004c) and Walter (2002) test divisibility issues in their analyses of interstate and civil wars, with conflicting results. Fortna shows that conflicts in which a state's existence is at stake are much more likely to resume, while Walter's measure of divisibility appear unrelated to implementation.

The contribution of bargaining dynamics to the study of peace durability after both civil and interstate wars is significant. As a framework, bargaining dynamics focus explanations for peace durability on belligerent expectations to understand how they reach settlements and why peace survives in some cases but not others. An additional benefit of the bargaining framework is that it works well for understanding conflict between and within states. Scholars of interstate conflict characterize war as renegotiation between states, while civil conflict scholars view civil war as renegotiation between domestic groups and government. Wagner provides a detailed characterization of both the international system and the states within that system as an elaborate web of bargains among “predators” and between “predators” and “prey,” each of which is subject to violent renegotiation. David Lake (2003) also recognizes the applicability of bargaining dynamics for both categories of war. Differences remain, particularly with respect to the practical issues of settlement and implementation, but the framework is flexible enough to encompass both. Scholars have also pointed out weaknesses in the bargaining framework, particularly the assumption that actors are not always risk averse or risk neutral and that the model tells us nothing about where deals are likely to be located within the contract zone. These issues are generally unaddressed in the peace duration literature, but have been shown to have potential implications for war duration and termination (Schelling 1960; Princen 1992; Goemans 2000; O'Neill 2001; Wagner 2007).

War Outcomes

Employing a bargaining framework, a number of scholars have sought to test the proposition that war outcomes – military victory, negotiated settlements, and stalemates – have an influence on the duration of peace. Changing trends in civil war outcomes since the end of the Cold War have drawn increased attention to this issue, with scholars and policymakers speculating about the implication the growing number of wars ending in negotiated settlements will have for the longevity of the peace. Whereas some civil war analysts have expressed concern about this development, arguing that negotiated settlements fail to promote convergent expectations among adversaries (Luttwak 1999; Walter 2009; Toft 2010), others have posited that negotiated agreements can be designed in such a way as to reduce uncertainty and stabilize expectations among belligerents (Hartzell and Hoddie 2007; Mattes and Savun 2009; 2010).

Empirical tests of the effect war outcomes have on the duration of the peace following interstate conflicts and civil wars have produced somewhat mixed results. Maoz (1984) and Grieco (2001) demonstrate that disputes that end in a decisive military victory by one member of an interstate dyad are most likely to experience a durable peace. Research by Hensel (1994) supports this result, although he also finds that negotiated settlements or compromises were followed by a stable peace, albeit of shorter duration. Studies by Werner (1999a) and Quackenbush and Venteicher (2008), on the other hand, conclude that war outcomes are unrelated to the duration of the peace. The literature on periods of peace following civil war yields an even more confusing picture. Licklider (1995) finds evidence that peace lasts longer following military victories than negotiated settlements. Studies by Doyle and Sambanis (2006), Quinn et al. (2007), and Fortna (2008) indicate that both military victories and negotiated settlements decrease the risk of conflict breaking out again. Finally, Walter (2004) finds no relation between war outcome and peace duration.

The belief that decisive victories provide belligerents with critical information which leads to a convergence of their expectations lies at the heart of the hypothesis that conflicts ending in military victory are the ones most likely to be followed by a durable peace. Recent scholarship suggests, however, that military victory may prove flawed as a proxy measurement for convergent expectations. Werner and Yuen (2005) show that third-party pressure on belligerents can lead to “unnatural” ceasefires followed by a short-lived peace. Opponents who believe that a war outcome, including a military victory, was only arrived at due to the interference of third-party actors are unlikely to have expectations that converge. As a result, they can be expected to fight again in an effort to collect more accurate information regarding each other's capabilities. Hartzell (2009) investigates the claim advanced by Wagner (1993), Licklider (1995) and Walter (2009), among others, that military victories produce convergent expectations among civil war adversaries by destroying the organizational structures of all but the victorious faction. She finds that slightly more than 40 percent of the wars that ended in military victory saw the organizational structures of factions other than the victor preserved, a result that casts doubt on the extent to which military victories produce the type of stability-inducing expectations that have been attributed to them.

Another potential reason the research on civil wars has produced such varying results may have to do with the time periods covered by the various data sets. Licklider's study, for example, encompasses the years 1945 to 1993, thus missing the large number of conflicts that have been concluded by negotiation since the end of the Cold War. Indeed, studies focusing only on intrastate conflicts terminated since then have found that those that ended in negotiated settlements have been followed by longer periods of peace than those brought to an end by military victories (Human Security Centre 2005; UNDP 2008). This trend poses a challenge to the conventional wisdom regarding the relationship between war outcomes and peace durability which scholarship has yet fully to address.

Future research on war outcomes could benefit from further disaggregation of the concepts “military victory,” “negotiated settlement,” and “stalemate.” Scholars might also examine ways in which the nature of each of these types of war outcomes, as well as the environments in which they have been arrived at, may have changed over time. Efforts should also be made to develop proxies that more accurately capture the mechanisms associated with war fighting and war termination that the bargaining model suggests are linked to the duration of the peace.

Settlement Design

The question of whether or not the design of war-ending settlements has an impact on the durability of the peace has been the source of considerable debate. On one side of the debate are scholars who claim that different types of settlement provisions extend the duration of the peace by mitigating commitment and information problems (Fortna 2003a; 2003b; 2004a; 2004b; 2004c; Mattes 2008; Mattes and Savun 2009; 2010). On the other side of the debate stand researchers who maintain that agreement design has no real effect on the stability of the peace. According to this school of thought, the settlement provisions in question fail to dissuade or inhibit belligerents who want to renegotiate the terms of peace from using force to do so. In this view it is not commitment problems that are responsible for the failure of the peace but problems related to distribution and uncertainty (Werner 1999a; Werner and Yuen 2005).

One of the first studies to find support for the peace-extending effects of cease-fires designed to end interstate conflict was carried out by Fortna (2003a). Strong peace agreements consisting of several measures such as buffer zones and peacekeeping monitors, she demonstrates, help overcome post-conflict commitment problems and foster a durable peace by raising the costs to belligerents of renewed conflict, reducing uncertainty, and preventing accidental settlement violations. The peace-enhancing effects of measures that increase the costs of a return to conflict are confirmed by Mattes (2008) for interstate conflicts and by Mattes and Savun (2009) for civil wars. Mattes and Savun (2010) also establish that civil war settlements can be designed in such a manner as to mitigate information asymmetries and that doing so helps extend the duration of the peace.

Studies by Werner and Yuen (2005) and Lo et al. (2008) find little empirical support for the hypothesis that strong agreements lengthen the duration of the peace. The results of these studies do provide some support, on the other hand, for the argument that factors favoring renegotiation of agreements prompt actors to return to war. Changes in actors' relative capabilities and/or information gleaned from battles before the war's end were found to make it more likely that at least one of the belligerents preferred a return to war over a peace based on previously agreed-upon terms.

The difficulties associated with preventing actors from unilaterally defecting from a peace agreement have generated interest in the design of self-enforcing agreements. Two mechanisms have been identified as potential means of dissuading or impeding the stronger party to an agreement from renegotiating the terms through the use of force. Using formal models, Schwarz and Sonin show that a self-enforcing peace agreement is feasible if it is structured as a sequence of concessions by the weaker party to the stronger. Realizing that forbearance will ensure that it receives further payments, the stronger party should desist from returning to war. Measures that balance power – political, military, territorial, and economic – between governments and rebel groups following a civil war have also shown promise as a means of structuring self-enforcing agreements. Provisions that provide weaker actors with veto power at the political center or that integrate the government and rebel group's coercive forces, for example, have been posited to help check the ability of stronger actors to use their power to overturn the agreement. Empirical results provide support for the peace-prolonging effects of individual measures of power sharing (Hartzell et al. 2001; Mattes and Savun 2009) as well as the combinations of these measures (Hartzell and Hoddie 2007).

Interest has also grown in the possibility of designing settlements in such a manner as to encourage states peacefully to renegotiate the terms of an agreement. If factors that alter a state's probability of winning a war or decrease the costs to it of fighting lead it to want to revise the terms of a settlement, why wouldn't an opponent who is aware of these changes accept the dissatisfied state's demands? Mattes (2008) investigates this question. She posits that uncertainty may inhibit the peaceful renegotiation of agreement; although both parties may know that change has occurred, they may be unable to calculate its impact on the probability of winning or on each side's cost of conflict. Accordingly, she argues, agreements that contain provisions that reduce uncertainty should increase the likelihood that states will peacefully renegotiate the terms of a settlement. Mattes' results support her prediction; the inclusion of uncertainty-reducing provisions such as hotlines or third-party monitoring in conflict management agreements makes it more likely states will peacefully revise these agreements.

The design or contents of post-conflict settlements are most likely not randomly distributed across post-conflict societies. Leaders that have a preference for peace, for example, may be more likely to include some of the types of measures discussed above in the war-ending agreements to which they are a party. Scholars have demonstrated a growing awareness of the implications endogeneity may have for their analyses of settlement design, although they have yet to employ any of the matching, two-stage models, or instrumental variable analysis techniques that have been developed to address this issue. In the interim, efforts such as Cunningham's (2011) to identify the factors that determine the contents of peace agreements may help to advance scholarship on the question of the independent effect settlement design has on peace durability.

The Role of Third Parties

The difficulties of maintaining peace often involve more than just the direct belligerents in a conflict. As a solution to the many problems associated with keeping the peace, third parties have taken a prominent role in a variety of functions that have consequences for peace durability. From fighting in the actual conflict, as NATO did in Bosnia, to coercing negotiations, and to providing post-agreement functions of security, monitoring, and verification, a variety of third-party actors are often called upon to help peace survive. How effective are these actors? Which problems do they solve, and which do they create? These questions drive current research on the influence third parties have on peace duration.

Military Intervention

Military intervention comes in many forms that have implications for peace durability. The fundamental distinction often drawn in the literature is between interventions that influence the war, and therefore the settlement, and interventions that are designed to support a settlement already reached between the belligerents. The former type of intervention is the topic of this section, while the latter is dealt with in the peacekeeping section that follows.

Forceful military intervention and its effects on peace durability have only recently been connected in the conflict literature. In keeping with Blainey's characterization of war and peace as a continuous process, the research on alliance reliability and war duration give us some sense of how third parties might be expected to change belligerent expectations about initiating or continuing a war. For example, the alliance literature spends a great deal of effort trying to determine whether formal or informal alliance pacts serve as an effective deterrence mechanism (Huth and Russett 1988; Morrow 1994; Smith 1995; 1998; Fearon 1997; Huth 1998; Leeds et al. 2000; Leeds 2003). Others rely only on the potential for intervention, whether codified or implied, and its effects on aggressor demands (Werner 2000; Yuen 2009) Many scholars also consider whether intervention once war has begun makes any difference to war outcomes and duration (Regan 1996; 1998; 2002; Balch-Lindsay and Enterline 2000; Elbadawi and Sambanis 2000; Collier et al. 2004).

Few studies, however, have directly considered what the implications of military intervention are for peace durability. Werner and Yuen (2005) hypothesize that military intervention in conflict is likely to have a negative impact on peace duration. First, third party efforts will skew the balance of capabilities such that the original belligerents cannot update their information about each other to produce a settlement in a multilateral setting that would be certain to hold in a bilateral setting. In other words, the influence of the intervener is factored into the settlement terms creating the conditions for (violent) renegotiation if the intervener attempts to extract itself from the deal. This scenario is reflected in NATO's bombing efforts in Bosnia and its restrictions on natural gas exports to Serbia in an attempt to force Milosevic to negotiate (Holbrooke 1998). Additionally, interveners may attempt to impose their own preferred settlement that does not reflect the new information gained by fighting (such as returning to the pre-war status quo), which creates the conditions for settlement failure again if the belligerents' expectations do not support such a division. Lo et al. (2008) consider a more severe version of intervention, foreign imposed regime change, and find that it increases the duration of peace. This form of intervention accomplishes this by fundamentally transforming or removing from power any belligerent who has an interest in disrupting a peace agreement and thus forcing pacifism into the new governmental structure.


Mediation is another form of intervention that scholars have considered in understanding peace durability. Many studies of mediation have emphasized the roles mediators play in negotiations. These vary widely from less intrusive methods such as hosting and facilitation to more involved efforts that include sharing information among parties, locating settlement terms and offering outside inducements (or penalties) to help secure agreement among the negotiating parties (Beardsley et al. 2006). Princen (1992) offers a detailed dissection of how mediators influence bargaining, noting that one of the biggest roles they play is to help solve the problem of choosing a settlement among the many possible deals that exist within the bargaining range. Based on this logic, some scholars argue that mediators create more robust agreements that have a lower likelihood of defection and failure (Miall 1992; Zartman 1995). Others claim that mediators must also bring with them promises of benefits for implementation or threats of punishment for defection in order to encourage the peace (Smith and Stam 2003).

Empirically, the patterns of mediation suggest some positive short-term effects. In the longer term, however, the effects look very much like those of military intervention, particularly when using the techniques recommended by Smith and Stam (2003). Beardsley et al. (2006) show that many different forms of mediation can help produce an agreement and increase trust and cooperation to reduce crisis tensions. Fortna (2004c) finds that mediation is effective at getting agreements in place. However, she finds no effect for implicit promises from mediators assisting with the long-term survival of peace and notes that explicit promises are so few that it is difficult to draw inferences from the result. Werner (1999a) also finds no systematic relationship between peace duration and mediation. Beardsley (2008) provides an argument for these results. He suggests that mediators are good at getting agreement in the short term, especially when using positive and negative inducements, but they weaken the long-term prospects for peace because mediator interest and involvement wanes as the peace continues. As a result, mediators stop offering the benefits (or punishments) once used to maintain peace. His tests demonstrate support for the conflicting mediation effects with respect to war termination and peace duration.

To assess the effects of mediation, however, scholars must also consider that the presence of mediators is not a random event but instead may be systematically related to features of the conflict itself and the desires of potential mediators (Grieg 2005). Gartner and Bercovitch (2006) argue that mediators take the hard cases. This means there is an underlying difficulty for the survival of peace, a point which they demonstrate through their quantitative analysis of the Balkans.

Peacekeeping and Peacebuilding

Post-conflict intervention is a technique that is used in both civil and interstate conflicts to try to help maintain peace and stability. Several studies have devoted attention to a variety of factors that affect whether peacekeeping and peacebuilding help ensure a durable peace. The theoretical and policy dimensions of peacekeeping have created a vibrant and diverse literature surrounding this form of intervention and its effects on peace duration. Scholars and policymakers in the 1990s focused on cases in which the UN mission outcomes were at variance with the organization's stated goals. More recently, a more systematic and theoretically driven set of studies has attempted to untangle the conditions under which peacekeeping is more or less likely to succeed (for a more extensive review of the peacekeeping literature as it relates to issues beyond the scope of this chapter, see Fortna and Howard 2008).

Following the end of the Cold War, the United Nations began to take on a more active role in conflict management and abatement. After several discouraging outcomes in the early 1990s (e.g., Somalia and Rwanda) some scholars argued that the United Nations was achieving, at best, short-term goals like reducing hostility and preventing outside actors from encouraging continuation of the conflict. Other scholars claimed that intervention was actually making the situation worse by creating incentives that led conflicts to persist and that it would be better to see one party defeated and the fighting ended (Haas 1986, Luttwak 1999). Diehl et al. (1996) find in an event history (duration) model that peacekeeping had no effect on interstate crisis outcomes, despite a finer gradient of peacekeeping behavior in international crises. Fortna (2004a) finds a different result. She demonstrates with a duration analysis of ceasefires that the presence of peacekeepers in interstate conflicts is associated with longer peace, controlling for various conflict-relevant factors.

Many studies of peacekeeping effectiveness have attempted to separate the effects of peacekeeping efforts from other influences that may be related to the presence and function of peacekeepers. One of the most challenging aspects of evaluating peacekeeping is determining what aspects of peacekeeping missions are the most strongly associated with longer spells of peace. As the literature has developed, scholars have gone from simple dichotomous measures of peacekeeping/no peacekeeping to examining factors like troop numbers, “types” of peacekeeping (i.e., traditional/observer missions, interpositional and enforcement missions), and characteristics of mandates that reflect different levels of investment in a given conflict (Diehl et al. 1996; “Brahimi Report” United Nations 2000; Doyle and Sambanis 2000; 2006; Hartzell et al. 2001; Fortna 2003b; 2004a; 2008).

Fortna's (2008) work is representative of the increasing sophistication of peacekeeping studies. Her analysis seeks not only to demonstrate that peacekeeping is associated with longer periods of peace, but also shows the individual effects of the different mechanisms that alter belligerent behavior. Fortna divides the much-debated causal effects of peacekeeping into four categories. First, she notes, peacekeeping missions can alter the belligerents' strategic calculations, generally making the peace more attractive than fighting by raising the costs of cheating and providing legitimacy and financial incentives to encourage cooperation. Second, Fortna argues that peacekeepers provide a monitoring/signaling role that reduces belligerents' fear and uncertainty. Third, peacekeeping forces can prevent or identify accidental defection, ensuring that peace continues despite the occasional violence that may occur due to spoiler groups or inadvertent defections (Doyle and Sambanis 2006). Finally, peacekeepers can also assist with post-conflict political development.

Doyle and Sambanis (2006) distinguish peacekeeping from peacebuilding. Peacekeeping refers to fulfilling a mandate to assist with security concerns and implementation. Peacebuilding, on the other hand, deals principally with the efforts that occur after the mandate has ended, including developing stronger political institutions and economic opportunities to create a sustainable peace. It is the latter focus on the long-term efforts of international organizations (capacity) to build a stable society in which peace takes hold that distinguishes Doyle and Sambanis's work from Fortna's. Efforts to understand how peace can survive for the long term face particular methodological challenges. Establishing the enduring effects of peacekeeping is particularly difficult since it is a “treatment” that is applied, and eventually ends (in most cases), but which may have long-term effects that are misattributed to other factors. Doyle and Sambanis address this issue by examining conflicts for a full two years after the “peace stimulus,” meaning after a settlement, victory, or peacekeeping mission has ended. Fortna uses both time-constant models of peace duration (was there ever a mission and how long did peace last) and coding that denotes whether the mission is current or had already been concluded.

Several scholars have recognized that peacekeeping missions are not applied at random. Debate centers on whether missions are awarded to cases in which peacekeeping is “easy” or “hard,” a difference that has implications for peacekeeping success and peace duration analysis. Gilligan and Stedman (2003) consider where peacekeepers are likely to be sent. They find that the United Nations is less likely to send missions to civil conflicts in which a government has a strong army, although they also find that higher casualties make peacekeeping more likely. Many scholars have interpreted their findings as suggesting that the United Nations avoids difficult cases. Fortna (2008) addresses this question directly by generating a predicted measure of the degree of peacekeeping difficulty. Using a set of civil conflicts prior to the Cold War when peacekeeping was rare as a baseline estimate, she concludes that the United Nations sends missions to the hard cases. Employing matching techniques to account for selection effects, Gilligan and Sergenti (2008) also find that the United Nations sends peacekeepers to the “hard” cases, although in the cases of ongoing wars the United Nations appears to send peacekeepers to “easy” cases. Howard (2008) argues that if, as the evidence above suggests, peacekeepers are going to the most difficult cases, their efforts are even more noteworthy since the general expectation for a hard case is failure.

Finally, if we are to understand the effects of peacekeeping on peace duration, it is important to note that the practices of the United Nations and other peacekeeping organizations can evolve over time. A burgeoning literature devoted to institutional learning and development considers changes in the United Nations over time (e.g., Barnett and Finnemore 1999; Howard 2008). Systematic analysis of the United Nations requires tracing these changes in behavior, as well as the sources of these changes, over time.

Democratization and Liberalization “From Above”

Third-party actors have sought to promote political and economic liberalization in a variety of post-conflict countries as a means of stabilizing the peace (Paris 2004; del Castillo 2008). These activities have been particularly marked since the end of the Cold War – although as US efforts to establish liberal, capitalist democracies in Germany and Japan following World War II indicate, they are not without precedent. How well have these externally guided or mandated initiatives in democratization and economic liberalization served to reduce the risks of war recurrence? Somewhat surprisingly, few empirical studies have sought to investigate this question.

Existing work on democratization and economic liberalization suggests that these processes may well be destabilizing. States in the early phases of transitions to democracy have been found to have a higher likelihood than other states of becoming involved in war (Mansfield and Snyder 2005), for example, and are at an increased risk of civil war and other forms of instability (Hegre and Fjelde 2010; Hewitt 2010). Countries that adopt IMF economic liberalization programs have been found to have a higher risk of experiencing civil war onset (Hartzell et al. 2010). None of these studies, however, speaks directly to the issue of the impact that externally sponsored programs of democratization and liberalization have on post-conflict peace duration. Given the convergence of two trends – the number of countries experiencing repeated incidents of civil war and the increased involvement of the international community in promoting democratization and liberalization – over the course of the past two decades, this is a topic that merits additional research.

Measurement, Estimation, and the Study of Peace Duration

Quantitative studies of war and peace face a number of challenges stemming from missing data, difficult data generation processes and unusual dependent variables. These have obliged scholars to search for more appropriate estimation techniques that can help to ensure the best possible analysis and interpretation of findings.

Operationalizing and Estimating Peace

The most direct way of measuring peace durability is to use the “spells” of peace, or duration analysis, to determine what factors make peace more likely to survive. These studies use event history models (otherwise known as duration, survival, or hazard models) in which the dependent variable is the time a given case remains in the data set before an event removes the case from the data set. In these studies, the event that removes cases is renewed fighting. Peace duration as a measure of peace durability began to be used in the mid to late 1990s when duration analysis techniques began to be applied to studies of war duration (Diehl et al. 1996; Werner 1999a; Fortna 2003a; 2003b; 2004a; 2004b; 2004c; 2008; Werner and Yuen 2005; Mattes and Savun 2009).

Duration analysis has many substantive benefits for the study of peace durability, but the techniques also have limitations and complications that should be considered. For example, differences between estimation techniques that are parametric, semi-parametric or non-parametric affect the efficiency and interpretation of results. Additionally, some duration models use an accelerated failure time approach while others use a proportional hazards assumption. These modeling choices, which represent two different ways covariates are expected to change the likelihood of an event at a given time, produce opposite outputs that require careful discussion and interpretation. Furthermore, the analyst must decide whether to use time-varying covariates or time-constant covariates in the analysis, a choice that creates potential data availability issues (see Hosmer and Lemeshow (1999) for a comprehensive discussion of duration analysis). Nonetheless, with careful attention to detail, duration analysis has produced very useful studies of peace duration.

Other scholars have chosen simpler methods of operationalizing peace duration. Some use a dichotomous measure denoting war termination or peace success based on a given number of years since the end of the conflict. For example, Doyle and Sambanis (2000) code peacebuilding as successful if there has been no renewed fighting two years after the war ended, Doyle and Sambanis (2006) considers peacebuilding a success if the peace is maintained two years after the peace “stimulus.” Licklider (1995) codes peace as durable if civil war does not break out for a period of 5 years following the negotiated settlement or victory that terminated the original conflict. Walter (2002) captures not just success or failure but also how far into the settlement process combatants ventured by using an ordered, categorical variable that ranges from no negotiations to successful implementation. Regan (1996) defines intervention success based on whether peace lasts for six months after fighting ends.

Use of dichotomous or discrete measures defined by a set time period such as those described above has both advantages and disadvantages. These analyses generally consider the static conditions of the war, intervention or outcome to explain whether a conflict reaches a certain temporal threshold of peace or not. The standard criticism is that the time limit chosen is somewhat arbitrary, although scholars who use them go to great lengths to justify the threshold in question. The advantage of discrete time modeling is a simple, straightforward data collection process and analysis. Furthermore, discrete measures require the use of simple maximum likelihood techniques for estimation, such as logistic regression or ordered/categorical estimation techniques. Techniques such as nested logistic regression are also being employed to capture the sequential nature of conflict resolution and peace. Each of these techniques adds to our understanding of peace durability by addressing different kinds of outcomes and providing multiple ways to capture the concept of peace.

Common Data Sets

Data collection has helped to advance the study of conflict significantly in the past two decades. For analyses of peace durability, most scholars begin with a universe of cases defined by the existence of conflict and build their dependent and independent variables onto these cases. Below are some of the most commonly used data sets in the peace durability literature.

Correlates of War

The Correlates of War (COW) project is an ongoing data collection effort that determines state membership in the system and records the start and end dates, among other factors, for interstate, intrastate and non-state wars. War is defined as a conflict resulting in at least 1000 combatant deaths in a year. The original data set is discussed extensively in Small and Singer (1982) and includes periodic updates. Available at

Militarized Interstate Disputes

The Militarized Interstate Disputes (MID) data code conflict at levels lower than the COW threshold for interstate war, providing an ordered coding of the hostility level of a given dispute. The data are available per dispute and per disputant within a dispute (Jones et al. 1996). Available at

International Crisis Behavior

The International Crisis Behavior (ICB) data set codes international crisis events and several dimensions of the crisis, including the value of the issue, levels of violence, systemic activity in response to the crisis and the start and end dates of the crisis. Inclusion in this data set requires at least one entity to perceive a threat that suggests the possibility of military exchange or requires a timely response. Further description of the variables can be found in Brecher and Wilkenfeld (2000). Available at

Peace Durability

Several scholars have built data sets specifically testing peace durability from the base data sets described above. These data sets focus specifically on tests for peace durability and represent a small sample of the efforts that have continued to grow as the study of peace durability has become more nuanced and sophisticated.


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