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The Territorial Peace: Theory, Evidence, and Implications

Summary and Keywords

The territorial peace theory predicts that neighboring states with stable borders not only avoid conflict but that the removal of territorial threat facilitates the democratization process within those countries. The strongest and most controversial implication of this argument is that the observed peace between democracies (e.g., the democratic peace) is actually epiphenomenal or spurious to the removal of contentious territorial issues between contiguous states.

Building on observations within the international conflict literature, the territorial peace theory argues that disagreements over borders and other territorial issues are considerably more likely to lead to conflict than other types of issues because of their salience to both government elites and the domestic public. During crises in involving external territorial threats, opposition parties and the public turn to the government for protection and rally in support allowing the state to further centralize the regime and develop large standing armies which, in turn, can be wielded to repress the citizenry and maintain the status quo. Thus, states sharing unstable borders and experiencing high levels of territorial threat tend to become or remain autocratic as they are constantly defending their borders, centralizing their power, and maintaining their state control by repressing their citizenry. Conversely, in states with settled stable borders, they not only experience less conflict but ameliorating the territorial threat subsequently reduces government incentive to maintain a high level of centralization, thereby facilitating democratization. Thus, it predicts that both democracy and peace should form around stable borders and observe regional and temporal clusters. Empirical support for the theory has been consistently strong across a wide range of studies and researchers increasingly apply its arguments to explain a wide variety of different political phenomena.

Critics of the territorial peace cite some methodological and theoretical weaknesses. These critiques highlight difficulties replicating the results of early models of the territorial peace theory, point out empirical inconsistencies related to the effect of joint democracy on conflict onset, and cite several methodological and empirical issues. Defenders of the theory argue the theory has become more nuanced and more effectively operationalized over time and that these critiques may no longer be relevant. Finally, other critics charge that the use of large N statistics rather than comparative case studies detracts from the strength of the argument of the territorial peace. However, rather than framing the theories as competitors in opposition to one another, Andrew Owsiak contends that the disagreements between the democratic peace and territorial peace may be reconciled and demonstrates how the key factors from each theory compliment the other. His approach offers a promising pathway moving forward to further deepen our understanding of conflict onset, peace, and democratization.

Keywords: territorial peace, interstate conflict, democratic peace, interstate conflict, border stability, democratization, issue salience, border settlement, state centralization, empirical international relations theory


Over the last decade, the territorial peace argument has emerged as one of the most innovative new research programs in the area of empirical International Relations theory. Building from extensive empirical research linking territorial disputes to higher rates of interstate conflict, the territorial peace hypothesis predicts that neighboring states with stable borders not only avoid conflict but that the removal of territorial threat facilitates the democratization process within those countries. The strongest and most controversial implication of this argument is that the observed peace between democracies (e.g., the democratic peace) is actually epiphenomenal or spurious to the removal of contentious territorial issues between contiguous states. If the removal of territorial threats accounts for both peace between neighboring states and the subsequent development of democratic institutions, then the observed democratic peace is in actuality a territorial peace. Furthermore, the territorial peace predicts that both democracy and peace should cluster spatially and temporally as the borders of contiguous states stabilize and become settled (Gibler, 2007, 2012a).

Although the territorial peace argument was originally conceptualized to serve as an alternative to the democratic peace theory, this argument has been successfully applied and expanded into explaining a wide range of different political phenomena. The idea that the removal of territorial threat from the state subsequently reduces the incentive of governments to maintain a high level of centralization has obvious implications for both state development and democratization processes. Not surprisingly, the territorial peace argument offers explanations for both of these political phenomena as well as state repression. All told, the wide application of novel insights derived from the territorial peace argument is evidence of a progressive research program. That said, the territorial peace literature is not without important debates and disagreements over theory, measurement of core concepts, and empirical strategies. Nor has the territorial peace argument avoided criticism, particularly from scholars arguing on behalf of the democratic peace. However, these debates and critiques suggest a dynamic research area entering a critical stage of development as it transitions from an upstart to a mature theory within the field of empirical International Relations.

Although not an exhaustive overview of the territorial peace research program, this essay reviews key elements of the territorial peace argument by highlighting the novel insights, the extensive supporting empirical evidence, the debates and criticisms, and finally the theoretical and empirical challenges facing future researchers in this area. A primary concern of this review essay is to assess the remaining challenges facing the territorial peace argument, including the theorized causal mechanisms, measurement of core concepts, and building stronger theoretical links to the democratic peace literature. The road forward for territorial peace research needs to emphasize continued theory-building, more expansive data collection, and innovative empirical strategies designed to both isolate causal mechanisms and its implications.

The rest of this essay is divided into several sections. It begins with an extensive review of the theory’s origins, its main theoretical arguments, and supporting empirical evidence highlighting the ways in which it has been extended to a wide range of questions including democratization, state development, rivalry, repression, and political behavior. The concluding sections highlight and evaluate some of the emerging theoretical and empirical critiques of the territorial peace, including the ongoing debate with the democratic peace scholars, and offer suggestions for the road forward in this research area.

The Origins of the Territorial Peace

The territorial peace theory largely evolved from two different research areas within empirical International Relations theory: the democratic peace and territorial theories of conflict. Although the argument has been extended into other areas over time, territorial peace theory, at its core, seeks to reconcile many of the findings from the observed democratic peace phenomena with insights derived from theories of territorial conflict and state development. Thus, to provide better insight into the main premises and claims of the territorial peace, it is important to review the theory’s empirical and theoretical origins within the democratic peace and territorial conflict literatures.

The democratic peace literature contends that democratic dyads are markedly less conflict-prone, observing that democratic dyads have never fought an interstate war and only rarely experience any level of militarized conflict (see Doyle, 1986; Bremer, 1992; Maoz & Russett, 1993; Russett & Oneal, 2001; Ghosn et al., 2004). Yet, despite the apparent pacific nature of democratic dyads, democracies are just as likely to engage in conflict with non-democracies as non-democracies are with each other (see Bueno de Mesquita et al., 1999). Thus, theories of the democratic peace explain why democracies are less threatening toward each other than non-democracies and have typically centered on two different mechanisms: norms or institutions. Normative theories of the democratic peace assert that democracies do not fight each other because of shared norms and mutual respect, which allow them to resolve disputes more peacefully (Maoz & Russett, 1993; Mitchell, 2002, 2012). Although there are several variants of institutional theories, they generally focus on how institutions within democracies constrain leaders’ behavior in qualitatively different ways than those in authoritarian regimes. These democratic institutions create strong incentives for leaders to avoid policy failure stemming from costly conflict (Bueno de Mesquita et al., 1999, 2003) and generate higher audience costs that more credibly signals their resolve to other international actors (see Fearon, 1994; Schultz, 1999, 2001; Slantchev, 2006). Therefore, democratic leaders will not only have greater incentive to fight harder during conflicts but will be more hesitant to risk potentially difficult conflicts and select weaker opponents (see Bueno de Mesquita et al., 1999, 2003; Reiter & Stam, 2002).

Empirical research on the democratic peace consistently demonstrates the link between democratic dyads and peaceful behavior. It also observes that the democratic peace is largely a regional phenomenon (also see Thompson, 1996), where democracies tend to cluster together and rarely fight near their home territory (Gleditsch, 2002). That is, the democratic peace tends to cluster both spatially and temporally. This finding is particularly notable since normative and institutional theories of the democratic peace offer little in the way of explaining this phenomenon offering an opportunity for alternative explanations. In the absence of clear answers derived from the leading theories of the democratic peace, Gleditsch and Ward (2006) conclude that regional external factors exert considerable influence on the prospects for democracy, which subsequently contributes to the clustering of democratic states. This observation would prove critically important in the development of the territorial peace argument.

While the democratic peace has long been criticized and questioned in a wide variety of ways, one of the strongest lines of critique argues that the observed democratic peace phenomenon may be spurious, or epiphenomenal to other factors, including regional external factors. Using case studies, Thompson (1996) made one of the first attempts with this general line of reasoning. Observing the emergence of clustered zones of peace, he asserted that states’ regional threat environments help incentivize the emergence of certain regime types rather than vice versa. More specifically, in regions where primacy is not clearly established and more conflict-prone, states are more likely to pursue a more aggressive foreign policy and centralize their regimes as a result. In peaceful regions where primacy was clearly established, democracy was able to flourish because states had less need to maintain high level centralization and more incentive to liberalize their regimes. Thus, Thompson argued that the causal arrow of the democratic peace may be wrong and that it is states’ conflict environment that drives regime type rather than the other way around (also see Rasler & Thompson, 2005). Gibler and Wolford (2006) make a similar claim and show how certain alliance behavior, which is reactive in part to a state’s external threat environment, leads to higher levels of democratization by mitigating the incentives for greater regime centralization through the reduction of external threats.1 Overall, this line of critique of the democratic peace strongly suggests that the causal relationship between regime type and conflict behavior requires further examination.

At the same time that research on the democratic peace began taking off within the field of empirical International Relations, territorial conflict theory was also being developed and tested in a similar, if parallel, fashion. Starting with Vasquez’s (1993) classic meta-analysis of international conflict research and Bremer’s (1992) seminal article using a multivariate, dyadic-level of analysis of international conflict patterns, these studies clearly identified territorial issues as a leading cause of conflict in the international system and laid the foundation for what would eventually become a territorial theory of conflict. Central to this theory is the idea that territorial issues are qualitatively different from other potential security threats to the state and governments are more likely to respond to territorial threats with aggressive power politics (Vasquez, 1993, 2009). Governments and domestic publics tend to value territory over other issues because of its instrumental and symbolic importance (Carter, 2010; Hensel & Mitchell, 2005; Toft, 2006; Vasquez, 2009; Tir, 2010; Manekin et al., 2015). Thus, territorial conflict theory argues that territorial issues are more salient to both government elites and their respective domestic audiences than other types of issues (see Huth, 1996; Senese, 2005; Vasquez, 2009; Gibler, 2010, 2012a; Wright & Diehl, 2016).

The central argument that territorial issues are a leading cause of international conflict due in part to the high level of salience of these issues to government decision makers and the greater public is consistently supported across a wide range of empirical studies. Quantitative studies on international conflict have produced a preponderance of empirical evidence consistently demonstrating that, more than any other type of issue, disputes over territory are more likely to result in interstate war and other conflict (Kocs, 1995; Hensel, 1996, 1998; Huth & Allee, 2002; Senese & Vasquez, 2005, 2008; Colaresi et al., 2007; Kahler, 2006; Vasquez & Valeriano, 2010; Toft, 2014) as well as rivalry (Tir & Diehl, 2002) and arms races (Rider, 2009, 2013). International conflicts over territorial issues also tend to be more severe in terms of both duration and fatalities (Senese, 1996; Tir & Diehl, 2002). Similarly, an increasing number of studies demonstrate that territorial issues are more salient to domestic societies than other issues by linking territorial conflict to changes in individual-level attitudes and behaviors, such as identity, tolerance, trust, political participation, and government support (see Hutchison & Gibler, 2007; Hutchison, 2011a, 2011b; Gibler et al., 2012; Miller, 2013, 2016; Manekin et al., 2015; Tir & Singh, 2015; Tanaka, 2016; Hutchison & Johnson, 2017). In many of these studies, they observe that the effect of territorial conflict on individual-level attitudes is qualitatively different than other types of international conflict and threat. All in all, territorial conflict theory offers a strong case that not only are territorial issues a leading source of conflict in the international system but that the desire to maintain a state’s territorial integrity is often a major driving force shaping government choices.

Territorial Peace Theory

Building on the logic and insights of the territorial conflict literature, territorial peace theory provides an alternative logic to explain why democracies do not engage in conflict with one another but are no less conflict-prone toward non-democracies. The primary originator of the theory, Douglas Gibler, questions the relationship between joint democracy and peace in his foundational study, stating that “[d]emocracy and peace might both be symptoms—not causes—of the removal of territorial issues between neighbors, and in this sense empirical law of democratic peace might be spurious” (Gibler, 2007, p. 509). He argues that as contiguous states stabilize their shared borders they are less likely to experience future conflict, which leads to more peaceful relations. By removing a major source of external threat, these states have less need for highly centralized regimes to facilitate aggressive foreign policies (i.e., power politics) that maintain their territorial integrity, opening the door for regime liberalization and eventual democratization (Gibler, 2007, 2012a, 2012b, 2014).

In a subsequent work, Gibler (2012a) outlines and empirically evaluates his fully specified theory of the territorial peace that is based on three critical components. First, because of the salience of territorial issues to both the government and its domestic audience, credible threats from neighboring states over the state’s delineated borders exert a strong influence over government policy, particularly in the area of foreign policy. In the face of territorial threat, governments are more likely to engage in realpolitik behavior marked by an aggressive foreign policy directed at their neighbors making conflict more likely (also see Vasquez, 1993, 2009).

Second, this external threat environment coupled with an aggressive policy strategy not only makes militarized conflict more likely but also incentivizes militarization supported by a more centralized regime structure. As Gibler (2007, 2012a) points out, this connection between militarization and state development is powerful and well-known, as earlier notable works by Hintze (1975) and Tilly (1990) make clear. Gibler’s theory, however, connects this basic insight to the democratization process by building from Boix’s (2003) argument that repression costs within society is often determinative of regime type with low repression costs being more conducive for emergence and maintenance of authoritarian regimes. He asserts that because repression is easier and less costly in the militarized states that develop in reaction to high levels of territorial threat, it effectively inhibits democratization in those societies until those threats are ameliorated (Gibler, 2007, 2012a).

Finally, the salience of territorial threat alters the domestic bargaining space by encumbering domestic opposition. Because territorial issues are so salient to elites and the domestic public, territorial disputes often trigger rally-‘round-the-flag effects (Hutchison & Gibler, 2007; Tir, 2010; Hutchison, 2011a, 2011b; Gibler et al., 2012; Hutchison & Johnson, 2017). Furthermore, these rally effects triggered by territorial disputes foster attitudes and behaviors that are either antithetical to key democratic values, such as intolerance (Hutchison & Gibler, 2007), decreased political trust (Hutchison, 2011b; Hutchison & Johnson, 2017), and support for authoritarian leadership (Miller, 2016), which further inhibits democratization. Finally, during these crises, opposition groups and the public unify and support the regime and their national identity (Gibler, 2010; Gibler et al., 2012; Hutchison, 2011a, 2011b). This cycle not only further institutionalizes regime centralization but may also actually incentivize further hostilities (Gibler, 2012b).

All told, the territorial peace is not simply a theory about conflict within contiguous dyads but rather, as Gibler (2012b, p. 230) contends, “an explanation of the domestic politics of the state under duress.” The main implications of the theory are far-reaching. First, it provides an alternative explanation for the observed regime-type-conflict relationship, which should manifest in other conflict-related behavior and events. It also accounts for why we see regional clusters of democracy and peace (see Gleditsch, 2002; Gleditsch & Ward, 2006). Here the focus is on the role of contentious territorial and border disputes rather than regional primacy (Thompson, 1996), generalized external threat (Rasler & Thompson, 2005), or alliances (Gibler & Wolford, 2006). Additionally, it offers novel insights into the state development process and the important role that external threat environments play in shaping the incentives of domestic actors, particularly democratization. Lastly, the territorial peace theory implies that democracies should face a qualitatively different set of security challenges than autocracies, especially in terms of the type and severity of territorial issues they encounter; a phenomenon first observed by Mitchell and Prins (1999).

Empirical Evidence Supporting the Territorial Peace

Empirical support for the theory has been consistently strong across a wide range of studies despite some of the implicit methodological challenges in testing propositions of the territorial peace. One of these challenges is properly modeling the sequencing inherent within the theory while the other is constructing an appropriate measure of latent territorial threat—a critical task given its role as the primary causal mechanism within the theory. In fact, the subsequent studies which focused on better specifying this concept have driven much of the progress and innovation within this research area. Of course, Gibler (2007) acknowledges this difficulty in his original study, noting that his indicators of border stability, used as a latent measure of territorial threat, cannot include an absence of conflict so as to avoid a tautology. In his empirical analysis, he employs three proxies for border stability, similar colonial legacy, rugged terrain across the border, and the presence of cross-border ethnic groups, to test his contentions that: 1) stable borders should increase the likelihood of joint democracy within the dyad, and 2) stable borders should also decrease the likelihood of a militarized interstate dispute (MID) within the dyad. His analyses offer support for his hypotheses by observing that border stability predicts both the emergence of joint democracy within a dyad as well as a decrease in the likelihood of conflict. However, more importantly, his model shows that the presence of joint democracy is not a statistically significant predictor of conflict onset once the effect of border stability is accounted for within a dyad.

Gibler continued to refine both the general theory and empirical tests of those propositions in subsequent studies. Focusing on how the removal of territorial threat facilitates the democratization process by reducing the incentive for countries to maintain high levels of militarization and regime centralization, Gibler and Tir (2010) examine the effect of peaceful territorial transfers within contiguous dyads. Building on earlier work by Tir (2006) linking territorial transfers to subsequent peace, they argue that peaceful territorial transfers represent evidence of a positive territorial peace (as opposed to simply the absence of conflict). They find strong evidence linking these transfers to subsequent reductions in state militarization and an increased likelihood of democratization within the dyad. Not only does this study offer additional supporting evidence for the territorial peace by further demonstrating the key relationships, it also better approximates territorial threat (or its removal) within the dyad by using peaceful and violent territorial transfers as indicators. Gibler (2012a) reproduces the general substantive findings of his 2007 article using these updated measures of border stability and once again shows that border stability predicts both joint democracy and a decrease in the likelihood of conflict onset. In a similar vein, Gibler and Braithwaite (2013) use regional conflict hotspots as a proxy for territorial threat and find once again that dyadic peace is sustained by the absence of territorial conflict within a dyad’s approximate region. They also suggest that this dynamic can account for the observed clusters of democracies and peace. Taken together, these studies offer strong empirical support for the main propositions of the territorial peace while demonstrating that the findings are also robust to changes in measurement of key concepts.

Although these subsequent studies improve on the key measures of border stability to indicate the level of territorial threat within a dyad, they still rely on indirect proxies of the core concept (Rasler & Thompson, 2011). Given that borders are largely fixed and change infrequently, these proxies can be problematic when so much of the theory is built around the timing and sequencing of certain events, such as the removal of territorial threat and subsequent democratization within the dyad. Owsiak (2012) contends that while border stability provides a rough approximation of territorial threat, it does not make the distinction between stability and mutual agreement of border delineation. He argues that border settlement agreements are a superior indicator of the level of territorial threat within a dyad because they “reflect neighboring states’ respect for the territorial limits of one another’s sovereign jurisdiction” (Owsiak, 2012, p. 54). Using a new data set of all border settlement agreements between contiguous stats, Owsiak (2012) tests some of Gibler’s main propositions and demonstrates across all of his models that conflict onset is less likely in dyads after they reach a border settlement agreement.2 Furthermore, his analyses reveal that border settlement agreements have an additional pacifying effect by observing that even in those cases where these contiguous dyads engage in MIDs over other issues, they rarely escalate those conflicts to a state of war. However, Owsiak’s analyses fail to fully mirror Gibler’s (2007) findings as here he only finds partial support for the contention that joint democracy no longer exerts a pacifying effect on conflict onset once the territorial threat is removed. In his models, joint democracy still predicts international conflict across all of the models except the one examining the 1816–1945 time period.

Using border settlement agreements as an indicator of territorial threat within contiguous dyads marks an important progression in this research area for several reasons. First, it more closely mirrors the hypothesized causal dynamics within the territorial peace argument. Second, it further demonstrates the continued robustness of the original empirical findings while generating additional novel insights. In Owsiak (2013) and Owsiak and Vasquez (2016), border settlement agreements are used to predict the emergence and the timing of democratization within the dyad, while Clay and Owsiak (2016) observe that border agreements diffuse spatially. Considered together, these results not only reinforce a link between removing territorial threats and democratization but may also account, in part, for the observed democratic clustering. Overall, the subsequent research conducted by Gibler, Owsiak, and coauthors provide a consistent and robust empirical foundation supporting the main propositions of the territorial peace.

Extending the Territorial Peace Argument

The empirical support for the territorial peace extends well beyond the original propositions as researchers successfully apply this argument to explain a wide variety of different political phenomena. In many ways, this is the main strength of the theory and the research program. One of the important implications of the territorial peace argument is that because the removal of territorial threat encourages the emergence of democracy, the distribution of security issues facing democracies will be censored. In other words, democracies face a qualitatively different set of security challenges than non-democracies; a set that, for the most part, may be less consequential because they do not include salient territorial issues (see Gibler & Hutchison, 2013; Gibler, 2014). This censored set of security challenges facing democracies means that they often have more leeway in choosing which conflicts to pursue and/or escalate based on other factors rather than from public pressure due to the salience of the underlying issue of the dispute like, for example, territory. Indeed, this argument aligns well with Mitchell and Prins (1999), who showed that democracies rarely face external challenges to their principal territories.

This notion that democracies face a qualitatively different set of security challenges provides further insight into other studies which draw inspiration from the main ideas outlined in the territorial peace argument. Subsequent research by Gibler and coauthors consistently show that the supposed advantages enjoyed by democracies in dispute outcomes, negotiating settlements, and generating audience costs are largely illusionary once the issue type of the dispute is accounted for and modeled within the analyses (see Gibler & Miller, 2013; Miller & Gibler, 2011; Gibler & Hutchison, 2013). Democracies are able to enjoy these advantages not due to some inherent feature of their regime type but because they are largely protected from direct threats to their homeland since their borders are stable or settled. As Gibler and Hutchison (2013, p. 883) note, “democracies will rarely contend over [an] issue that is most consistently salient to domestic populations.” Removing this territorial threat allows these countries to be more selective of which disputes to escalate, choosing winnable conflicts and avoiding potential losses. Thus, it is hardly surprising that, given this relative autonomy, these countries are more likely to win their wars or negotiate settlements. Taken together, these results subvert the idea that regime type alone accounts for these democratic advantages and offers additional empirical evidence in support of the territorial peace argument.

Insights from the territorial peace argument have also been extended to research on international rivalry and state repression. Owsiak and Rider (2013) find that border settlement is positively associated with the termination of long-standing international rivalries. In addition to removing territorial threat, border settlement eliminates another major source of external threat and reason to maintain a highly militarized society, thereby providing further opportunity for democratization to occur (also see Rider & Owsiak, 2015). Similarly, Gibler (2007, 2010, 2012a) argues that territorial threat is one of the main impetuses for continued state centralization and repression. When a state is threatened at home it becomes more centralized in response to external threat, particularly to territory, for several distinct reasons. First, these threats to the homeland of a state are salient to those citizens of the state who often rally to support the government. Given this domestic climate in the aftermath of a territorial crisis, opposition leaders are under great pressure to promote the security of the state (Hutchison & Gibler, 2007; Tir, 2010; Hutchison, 2011a, 2011b; Gibler et al., 2012; Miller, 2013, 2016). Second, these territories can only be defended and occupied by standing armies that can later be used as a tool by state leaders to repress the citizenry following the conclusion of conflict. Gibler (2012a) states that large standing armies formed during territorial conflicts will often morph into repressive forces in their wake. Political elites will use such forces to reinforce their regimes and mobilize them against any group that tries to challenge the status quo. Third, bargaining during territorial conflict is altered as the leaders’ political opposition joins in the promotion of state security efforts (Gibler, 2010, 2012a). State leaders capitalize on the rally effect generated by the issue salience of external threats and centralize with the approval of its citizenry under the guise of defending the state, even in non-democracies (Gibler, 2010). Conflict with rivals is particularly salient for the individual, as the external threat of the rival in a dispute over territory makes the citizenry more obliged to support the centralization of the state by allowing the imposition of censorship and restrictive policies (Lu & Thies, 2013). Thus, the termination of rivalry due to border settlement should also affect the subsequent level of state repression.

Wright (2014) further examines this association between territorial conflict and domestic repression. Building on Poe et al.’s (1999) observation that international conflict is responsible for increasing government repression scores by 44% over 10-year periods, he finds that democracies increase their repressiveness as the conflict-years become more deadly while autocratic states are more repressive at the onset of conflict and become less repressive as the conflict intensity increases. Wright argues that this difference is due to the mobilization and preoccupation of the repressive military forces that are no longer available to police domestically. This has important implications for territorial peace research, particularly for the hypothesized effect of territorial threat on democratization. Not only does territorial conflict have a direct impact on state repression, but the intensity of the conflict and the regime type of the affected states may also determine the repressive response by a state engaged in this form of conflict. Gibler (2012a) suggests that states will centralize and repress regardless of regime type whereas Wright’s findings suggest otherwise.

Critiques of the Territorial Peace

Given some of the controversial claims made within the territorial peace literature, it is perhaps not surprising that it has elicited several prominent critiques focusing primarily on empirical issues. To date, Park and Colaresi (2014) present the strongest and most comprehensive empirical critique of the territorial peace argument, which stems from their difficulties in replicating Gibler’s (2007) original results. Their criticism raises serious questions over the argument’s validity, particularly with regard to its controversial claim that the relationship between democracy and peace is spurious. Using the coding rules outlined in Gibler (2007) in an attempt to replicate his original data set, Park and Colaresi (2014) find that joint democracy still does predict a decrease in the probability of MID onset—a finding contrary to that which would be expected from data replicated by the territorial peace’s foundational data set and supportive of democratic peace theory.3 They find two key problems in their attempt to replicate Gibler’s (2007) data. The first problem stems from an omission of the constituent terms in modeling an interaction effect within the analysis of joint democracy and militarized disputes within dyads. The second concern relates to Gibler’s assumption that “democracy scores are independent across dyads in a given year even with a common member” (Park & Colaresi, 2014, pp. 118–119). Within the confines of their replication attempt, they estimate the impact of both stable borders and joint democracy and find that joint democracy continues to predict conflict onset. On the issue of explaining joint democracy within a dyad, they find that stable borders are an inconsistent predictor. In fact, their analyses reveal that border stability variables provide evidence supporting the territorial peace in only a handful of instances and their results still strongly support the propositions of the democratic peace despite accounting for stable borders. Overall, Park and Colaresi were largely unable to replicate most of the Gibler’s (2007) core claims and offer evidence that his original study suffers from significant methodological flaws.

Gibler (2014) responds to some of the criticisms laid out by Park and Colaresi (2014) by arguing that since his original 2007 article on the territorial peace, the theory has become more nuanced to reflect the enormous impact that contiguity has on conflict onset. He argues that Park and Colaresi’s replication confirms that the stable borders mechanism does not apply to non-contiguous states. In his response to Park and Colaresi, he also uses a replication of his original data set as it applies to contiguous dyads and finds strong support for his original argument, suggesting that the democratic peace is epiphenomenal (also see Gibler, 2012a). Additionally, he references his earlier co-authored study demonstrating how territorial “hot spots” created by clusters of recent conflicts eliminate joint democracy’s mitigating impact on the likelihood of militarized armed conflict (Gibler & Braithwaite, 2013). This, according to Gibler, confirms that a prerequisite of border stability must exist prior to peace. He also points to several related studies that support other propositions of the territorial peace argument.

Park and James (2015) also examine the relationship between democratic dyads when a salient territorial issue is the source of conflict. While their findings show that territorial issues result in more MIDs even between democratic dyads, they also support the conclusion that the effects of joint democracy on territorial and non-territorial dyads promote the maintenance of peace between the two states. They show that highly salient territorial issues still enhance the likelihood of militarized armed conflict among jointly democratic dyads. However, these findings challenge the territorial peace argument that joint democracy and peace are spurious. Given the high salience of territorial issues, they argue that these findings are actually a testament to the power of democratic institutions in reducing the likelihood of conflict. They further contend that the territorial peace’s large N quantitative approach increases difficulty and may create problematic data samples. Instead, they propose that it would be more efficient to use Mill’s comparative methods of analysis to better understand why events occur in relation to MID onset. They conclude that further study of a marriage between the variables of the democratic peace and the territorial peace may offer a better understanding of international conflict processes.

As suggested by Park and James (2015), there certainly exists room for both the variables of the territorial peace and the democratic peace to coexist. One theory does not necessarily need to invalidate the other but instead they may provide complementary pieces that can assist in better examining international conflict. Owsiak (2016) takes this very approach, arguing that the competing theories’ each provide viable mechanisms that can be integrated to explain interstate peace between dyads. He further demonstrates empirically that the democratic peace and territorial peace may operate within mostly separate domains. For non-contiguous dyads, the democratic peace offers the best explanation for observed conflict patterns while the territorial peace better accounts for the conflict behavior in contiguous dyads. He concludes that democratic norms and institutional constraints can be valuable pieces of a territorial explanation of conflict.

While the direct criticisms of the territorial peace focus mostly on empirical issues, there are also several problems and issues related to its theoretical foundations. First, the general theoretical premise of the territorial peace argument that a single set of conditions (operationalized as either border stability or settled borders) shapes states’ internal and external behavior may be overly reductionist.4 Given the complexity of domestic and international politics, it is more likely that the removal of territorial threats works in conjunction with a myriad of other factors in shaping state internal politics and foreign policy behavior. For instance, there may be alternative factors involved in the creation of circumstances where states choose to quit worrying about borders, including the termination of rivalry, rapid economic development, and pressure from international or regional hegemons (e.g., NATO or the Warsaw Pact). In fact, it is the largely overlooked role of hegemonic pressure in shaping a host of related phenomena in the 20th century, including patterns of peace and the distribution of regime types, which form the basis of McDonald’s (2015) recent critique of the democratic peace literature.

Although directed toward the democratic peace literature, several of McDonald’s (2015) critiques are also largely applicable to the territorial peace literature, most notably his argument that the observed democratic peace phenomenon is spurious to the great power bargains or hierarchies that followed the major 20th century conflicts (WWI, WWII, the Cold War). He contends that assumptions of statistical independence in the standard research designs favored in both democratic and territorial peace research (dyad-year) are inherently biased toward finding correlations because “great powers utilize these hierarchical relationships to shape the domestic institutional structure and foreign policy choices of subordinates states” (McDonald, 2015, p. 558). In short, McDonald’s argument strongly suggests that great power hierarchies were largely responsible for facilitating both democracy and the settlement of many contentious borders for the purpose of ensuring peace among its members and, thus, accounting for both the territorial and democratic peace findings.

Lastly, the territorial peace argument fails to offer much in the way of a comprehensive theory of democratization. More specifically, it lacks causal mechanisms that directly link the absence of territorial threats to the complex democratization process within states. As presently constructed, the territorial peace argument that an absence of territorial threat creates a permissive environment for each state to reduce state centralization still remains largely unconnected to the leading causal explanations of democratization. Given that the link between border stability and subsequent democratization lies at the heart of the territorial peace, the general lack of a causal story remains its biggest theoretical weakness.

Future Directions for the Territorial Peace

The territorial peace theory offers several important insights on the complicated relationship between territorial threat, militarized conflict, and democracy, and represents an innovative approach within the broader discipline. By successfully connecting theories of territorial conflict and state development to the observed patterns of conflict behavior associated with the democratic peace, this theory is able to make provocative claims backed by a plethora of supporting empirical evidence. Although a relatively new theory, the territorial peace has been successfully extended into explaining other political phenomena beyond just conflict behavior and democratization. However, more work refining the territorial peace remains as it continues the transition from a series of arguments and ideas to an established theory within international relations.

One of the primary challenges facing the territorial peace theory is trying to reconcile and integrate these novel insights with the democratic peace literature. In their defense of the democratic peace, Park and Colaresi (2014) raise legitimate questions surrounding the proposition that removing territorial threats subverts the effect of joint democracy on conflict onset. Furthermore, this inability to replicate coupled with only partial support for this claim in subsequent work by Owsiak (2012) suggests that the territorial peace theory requires additional refinement to fully support these claims. Owsiak (2016) correctly notes, however, that the current debate between the two theories is unnecessarily antagonistic and competitive rather than collaborative. He argues the “debate is misplaced and masks avenues for future theoretical advancement” (Owsiak, 2016, p. 5). And while much of the additional empirical evidence supporting the claims of the territorial peace is consistent and robust, there remain phenomena that the democratic peace can account for but the territorial peace cannot, and vice versa. In seeking to integrate the two arguments, Owsiak not only provides greater understanding of this dynamic by suggesting that the relationship between the two arguments is conditional but also provides a basic framework to follow reconciliation and integration. Future researchers in this area need to build on Owsiak’s initial framework of domain-specific explanations and continue to focus on identifying fault lines of these domains as well as overlap.

As the territorial peace theory moves forward into more of a “normal science” phase, researchers should focus on offering greater understanding of several important facets of the theory. First, there needs to be further development in specifying the causal mechanisms for each component of the theory. To date, most of the empirical evidence is based on observational, cross-sectional data which can often present difficulty in establishing an identification strategy. Future research needs to think carefully and creatively about demonstrating these causal relationships, particularly the path from reduced territorial threat to subsequent democratization. Indeed, the timing and sequencing of events and processes are critical elements of this theory, so special attention needs to be given to these challenges in future quantitative studies. Democratization is a complex process involving a wide array of important factors, and future research may require looking beyond the confines of existing international relations research. The observed empirical patterns described earlier suggest that it is likely that this process is context-driven. This is an area in which Owsiak’s framework of thinking about different domains may be critical, as even Gibler (2014) admits that his theory works best as an explanation for contiguous dyads. Indeed, it may be best to think of stable or settled borders simply as a permissive condition that varies across dyads and time. In the end, however, if these approaches offer clearer answers on how the removal of territorial threat specifically affects this process, they will provide more validity to the core tenets of the existing theory.

Although current research has taken the territorial peace theory far, the road forward requires researchers to strongly consider alternative empirical strategies, such as matching, quasi-experimental approaches, and qualitative case studies that focus on isolating the identified causal mechanisms. Each of these approaches should add important insights for our understanding of the territorial peace. For example, using matching techniques allows researchers to better isolate and compare the causal effects of territorial threat and joint democracy on conflict onset. As discussed above, Park and James (2015) also suggest employing a case study approach to purchase better leverage through the timing and sequencing of events. Finally, certain regions of the world may offer several natural experiment opportunities to examine the effect of border stability on conflict and development. Africa, for instance, is a potentially rich source of comparative analysis. Given the potentially confounding role that the African Union plays in limiting border changes, this region may provide us with clear answers to some of the debates with the territorial peace literature, particularly to the question of whether it is stable borders or settled borders that are sufficient in accounting for democracy and peace.

As the international system deals with new political realities and anticipates future challenges, the insights derived from the territorial peace theory may be critical in helping policymakers across the globe. Already the world is dealing with recent and potentially dangerous territorial disputes in the South China Sea and Eurasia, and future challenges lay on the horizon as the international community will confront new territorial issues stemming from environmental degradation and a changing global order. The ways in which the territorial peace theory could help inform and guide those future policymakers may very well depend on those answers yet to be discovered.


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                                                                                                                                                                                  (1.) The capitalist peace theory makes a similar argument that the democratic peace is spurious to other factors. In this case, dyads with two capitalist economies will not only be more peaceful but also account for the subsequent emergence of democracy (Gartzke, 2007; see Mousseau, 2000, 2013, 2016 for a similar contractualist peace argument).

                                                                                                                                                                                  (2.) Rasler and Thompson (2011) make similar arguments and use a similar measure of border settlement although their analyses were limited to Europe only.

                                                                                                                                                                                  (3.) Gibler’s (2007) original data set was not available for use in Park and Colaresi’s (2014) replications.

                                                                                                                                                                                  (4.) Of course, the territorial peace argument is hardly alone as a theory subject to this line of criticism. For example, other leading alternative explanations for the democratic peace such as the related but distinct capitalist (Gartzke. 2007) and contractualist peace arguments (Mousseau, 2000, 2013, 2016) are similarly reliant on a single set of conditions used to explain states’ internal and external behavior. Furthermore, McDonald (2015) argues the democratic peace theory itself is overly simplistic in explaining a complex series of phenomena and behavior at the international level.