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date: 16 August 2017

Empirical Knowledge on Foreign Military Intervention

Summary and Keywords

While the empirical literature on foreign military intervention has made considerable progress identifying the causes and consequences of military intervention, we still have much to learn about the subject. Mixed and even contradictory results remain common in the literature, and cumulative knowledge has in many instances proven elusive. Arguably the two most prominent theoretical approaches in recent scholarship, the bargaining model and the rivalry approach, have provided important insight into the phenomenon. They would nonetheless benefit from further refinement. Common explanatory variables outside of these two approaches also require further theoretical and empirical development. The literature has recently begun to examine the impact that military intervention has on target societies as well, with particular attention being given to target state democratization, human rights development, and conflict resolution. Empirical research could shed additional light on all of these phenomena by developing more detailed theory and data on intervention targets. It would also profit from incorporating systematic knowledge on leaders’ proclivities to use military force into current theoretical models.

Keywords: foreign military intervention, use of military force, rivalry, bargaining model, democratization, human rights, civil conflict, empirical international relations theory

Introduction

Since the late 1960s, the empirical literature has attempted to analyze relatively well-defined conceptualizations of foreign military intervention to facilitate the development of theory and empirical knowledge on the subject. Rosenau (1969) was perhaps most influential in moving the literature in this direction by arguing that the field must distinguish relatively rare and impactful foreign policy behaviors like military intervention from activities that are part and parcel of the ordinary course of international affairs. He provided propaganda, ideological persuasion, and diplomatic intervention as examples of relatively continuous and common activities that must be differentiated from the use of interstate military force. A sizable empirical literature on foreign military intervention has subsequently developed over the decades. This article provides a summary of the progress made in bringing understanding to this complex phenomenon. It illustrates that while our grasp of the causes and consequences of foreign military intervention has become considerably firmer over the past two decades, much remains to be done to provide a truly comprehensive understanding of the subject.

Unfortunately, mixed and even contradictory outcomes remain commonplace in the literature, and the field has only recently begun to examine a number of important phenomena such as military intervention’s impact on human conditions in target societies. Scholars also use different data collections to test ideas about what might be broadly construed as foreign military intervention, such as the Militarized Interstate Dispute (MID), the International Military Intervention (IMI), and the Military Intervention by Powerful States (MIPS) data sets (Kisangani & Pickering, 2008; Palmer et al., 2015; Pearson & Baumann, 1993; Sullivan & Koch, 2009). Since these data sets capture different types of interstate military force, they often produce incompatible results. While the analysis of different components of the phenomenon using distinct measures is a strength that will allow us to broaden our understanding in the long run, it slows down progress toward the integrative cumulation (Zinnes, 1976) of knowledge on the subject in the short and the medium term.

The development of cumulative knowledge is also complicated by the fact that foreign military intervention is a product of multiple complex and dynamic social processes in both the international and domestic arenas. The phenomenon of military intervention has itself changed over the past half-century as well. Whereas foreign military intervention was once thought to be a breakdown in normal patterns of interstate interaction, over the past decade and a half it has become acceptable and legitimate in the context of humanitarian and civil conflicts (Olson Lounsbery et al., 2011). To complicate matters further, the impact that intervention can have on target societies is also varied and multilayered. Building detailed knowledge on a phenomenon that can take different forms and is one of many policy options from which leaders may choose is obviously a daunting task. As is detailed below, we think that despite the difficulties inherent in studying such a dynamic and varying phenomenon, the field has made considerable progress in tracing important empirical regularities associated with the use of foreign military force.

The next section reviews the two most notable theoretical frameworks that have advanced our understanding over recent decades and a broad array of additional structural factors that also help to explain the use of foreign military intervention. The third section of the article examines the various impacts that foreign military intervention can have on target societies. Specifically, we examine the effect that military intervention has on target state democratization, on human rights and living conditions in target societies, and on civil conflict. The final section concludes with suggestions for future research.

Explaining the Use of Force and Military Intervention

Two major approaches to conflict and the use of force have framed much empirical research over the past two decades: the interstate rivalry framework and the bargaining model of war (Mitchell, 2017, p. 9; Prins, 2010). The rivalry approach maintains that a relatively small number of states account for the bulk of the forceful interactions in the international system, and these actors use force against one another in part because of a history of past conflict (Diehl & Goertz, 2000; Thompson & Dreyer, 2012). Past tensions produce a climate of mistrust and fear, which often prevents attempts at conflict resolution and lays the groundwork for future conflict. States involved in intense rivalries may internalize the rivalry to such a degree that the issues that produced the rivalry may recede in public discourse in comparison to the zero-sum competition that characterizes the rivalry itself. Hawkish leaders may rise to power and be rewarded in this context, perpetuating the rivalry and exacerbating security concerns among the participants (Colaresi, 2004). The outcome is a vicious cycle of repeated interstate violence that can be difficult to break free from. Studies have produced compelling empirical evidence that earlier disputes between rivals make later disputes more likely and that a growing number of disputes increases the likelihood of an escalation to war (Colaresi & Thompson, 2002; Hensel, 1994; Leng, 1983). There is also ample evidence that rivalry makes a number of different international processes—such as territorial disputes, arms races, and diversionary proclivities—more dangerous (Gibler, Rider, & Hutchison, 2005; Huth, 1996; Mitchell & Prins, 2004). While this vein of research has undoubtedly been rich, some facets of the rivalry process require further investigation. The processes involved in the emergence, evolution, and termination of rivalries could benefit from additional theoretical and empirical refinement (Rasler, Thompson, & Ganguly, 2013; Rider & Owsiak, 2015; Valeriano, 2013). As Thompson (2015) maintains, the field would also benefit from analyses that look beyond dyads to triads and more complex interactions within interlinked rivalry fields (see Valeriano & Powers, 2016).

The bargaining model has grown more prevalent than the rivalry approach over the past decade.1 In the bargaining model, low-level uses of force are thought to be rational attempts to obtain information about opponents. Force may be necessary because leaders have strategic reasons to bluff about their military capabilities and resolve, and costly action—such as deploying soldiers on foreign military missions—may compel opponents to reveal information about their strategic competencies and determination (Fearon, 1994; Schultz, 2001). It also sends an unmistakable signal about the actor’s resolve, which may reduce uncertainty and advance the bargaining process. When this type of costly action and other interactions fail to “facilitate the convergence of expectations,” however, the probability that the dispute will escalate to war increases (Reiter, 2003, p. 32). A great deal of empirical scholarship has lent credence to the bargaining model over recent years. Given the complexity of the bargaining process, however, it seems likely that empirical knowledge will grow only through the combination of large N quantitative analysis and detailed case studies (Reiter, 2003). Shirkey (2016) provides an example of the fine-grained qualitative work that will help to fill in gaps in empirical knowledge on the bargaining framework. It is also likely that the bargaining and rivalry approaches complement and inform one another in interesting ways (e.g., Langlois & Langlois, 2012).

These two broad theoretical frameworks do not, of course, capture the totality of our knowledge on the use of foreign military force. Empirical research has illuminated a wide variety of structural factors that may operate independently from bargaining and rivalry but may also inform each of these approaches. These structural factors are often studied in isolation or as components of middle-range theories that focus on specific types of foreign military force. They are frequently bound by scope conditions that are the subject of debate. As Thompson (2003) and many others have noted, the probability that interstate force will be used tends to rise as the number of such structural factors increases in a given context. Thompson likens the presence of a growing number of structural factors to pouring ever-larger amounts of lighter fluid on a stack of firewood. The addition of the lighter fluid does not by itself ensure that the firewood will burst into flames, but the odds of a blaze increase substantially. As he puts it, “the general nature of such [structural] arguments is that given this highly combustible set of ingredients (whatever they may be), the probability of a conflagration is higher than if the firewood is wet or unsoaked in kerosene” (Thompson, 2003, p. 455).

The remainder of this section outlines some notable structural factors that help to explain the use of interstate military force. Note that this survey focuses on the causes of traditional, strategic military interventions rather than the more recent phenomenon of normatively acceptable peace-building or humanitarian interventions. The latter form of intervention will be touched upon in a later section on intervention into civil conflicts. It is perhaps easiest to examine these structural factors by categorizing them into different levels of analysis.

Domestic-Level Factors

Two structural factors at the domestic level have received considerable attention in the empirical literature: the impact of domestic institutions and diversionary incentives. Research on both has grown more refined over the past decade. A sharper, but still incomplete, image of the impact that domestic institutional arrangements and diversionary pressures have on decisions to use force has gradually emerged.

Varying domestic political institutions have long been claimed to either impede or facilitate leaders’ decisions to use military force. The most well-known variant of these claims is democratic peace theory, which some controversially argue holds at the monadic as well as the dyadic level (Boehmer, 2008; Pickering, 2002; but see Quakenbush & Rudy, 2009). Others have found evidence of an autocratic peace (Peceny & Beer, 2003), leading to a blossoming of studies attempting to assess the potentially varying conflictual tendencies of distinct democratic and non-democratic regime types. Within democracies, studies have analyzed whether presidential or varieties of parliamentary systems tend to have a greater proclivity to use military force abroad (Ireland & Gartner, 2001; Prins & Sprecher, 1999; Reiter & Tillman, 2002). Among others, Morgan and Campbell (1991) hypothesize that presidential systems are more constrained than parliamentary systems because the authority to dispatch troops is shared across the legislative and executive branches in the former, while it resides in a single branch of government in the latter. Maoz and Russett (1993) conversely maintain that presidential systems are less constrained because they are not as dependent upon support from the legislature. While it is too early to claim that a consensus has been reached on the subject, much current evidence suggests that coalition governments use military force abroad less frequently than majoritarian or presidential systems (Brulé & Williams, 2009; Leblang & Chan, 2003; Palmer, London, & Regan, 2004). Leaders may be less likely to be held responsible for policy failures when the lines of accountability become blurred, as they often are in coalition governments. Domestic political pressures may subsequently be less likely to compel leaders in coalition governments to use foreign military force abroad (Brulé & Williams, 2009; Pickering & Kisangani, 2011). Coalition partners can also check the policy ambitions of dominant parties in the government, vetoing missions to dispatch troops overseas (Leblang & Chan, 2003).

Coalition governments, however, take many different forms, and scholarship has recently given increasing attention to the conflict propensities of the various types (Beasley & Kaarbo, 2014). To date, evidence suggests that coalitions with critical junior parties tend to be more peaceful than other types of coalition governments, largely because of the veto that junior parties hold over significant policy decisions (Auerswald, 1999).2 Coalition governments with critical junior parties may only be pushed toward more aggressive and conflictual foreign policies when the junior party is located on the far right of the ideological spectrum (Clare, 2010). In contrast, coalition governments with sizable parliamentary majorities may be more prone to conflictual foreign policies for a variety of reasons. The presence of a large number of parties may produce governmental instability, which can compel leaders to use aggressive foreign policy to help to shore up support for their continued rule (Hagan, 1993; Prins & Sprecher, 1999). Large coalitions may also facture, resulting in logrolling behaviors, which can justify extreme policies such as the use of interstate military force (Beasley & Kaarbo, 2014; Snyder, 1991).

The propensity of non-democratic governments to use foreign military intervention has not been analyzed in similar detail, but analysis of the subject has increased over the past decade. In general, the current evidence suggests that autocratic political systems that more closely resemble democracies are more cooperative and less prone to use military force abroad than counterparts that are more dictatorial (Lai & Slater, 2006; Mattes & Rodriguez, 2014; Weeks, 2012). Single-party regimes and military regimes—where leaders have some degree of accountability to party members, senior officers, or other elites—seem to use military force abroad less frequently than personalist regimes. Personalist regimes, in contrast, are not only prone to use military force abroad, they also tend to pursue reckless foreign policies because there are few reliable checks on leaders’ power. There may be one exception to this general tendency. Even though personalist leaders are more likely to use foreign military force in general, they may be less likely than single-party and military regimes to dispatch soldiers abroad in response to domestic political or economic pressures (Pickering & Kisangani, 2010; but see Lai & Slater, 2006). The use of force in response to domestic pressures is commonly called diversionary force, since it is assumed that leaders dispatch soldiers abroad in this context with the intention of diverting the attention of politically relevant groups away from domestic problems. Since domestic groups are not essential to personalist leaders’ continued rule, they have little reason to pursue such diversionary policies.

In all regime type analyses, attention must be paid to the regime classifications employed. As Vreeland (2008), Wilson (2014), and others have pointed out, there can be significant variation across different regime categorizations (Cheibub et al., 2010; Geddes, 2003; Geddes, Wright, & Frantz, 2012; Hadenius & Teorell, 2007; Jaggers & Gurr, 1995). Wilson (2014) provides a good illustration in his examination of discrete classifications of autocracies. While there is consensus that autocracies can profitably be catalogued as personalist, party-based (single-party regimes), or military-led (military regimes or juntas), the most widely used data collections offer different conceptualizations and measurements of these distinct regime types. Analysts would be well served by keeping such variations in mind and utilizing the classification best suited to evaluate their theoretical questions. Differences across classifications will also obviously hamper cumulation, but there seems to be growing agreement in the field that a handful of regime classifications have the greatest utility. If this is the case, cumulative knowledge will grow.

A related but different approach to regime classification cuts across regime type categories. The selectorate theory analyzes the size of two societal groups to understand institutional constraints on leaders (Bueno de Mesquita, Smith, Siverson, & Morrow, 2003). These two groups consist of the winning coalition (W), which is the small circle of elites whose support is essential for the leader to retain power, and the selectorate (S), which makes up the portion of the populace who have a role in selecting the leader. Selectorate theory posits that when W is small relative to S, leaders need only to funnel private goods to members of the W to retain power. When W is large relative to S, as it is in democracies, leaders must provide public goods to the broader population to satisfy the constituencies that help them to retain power. Political systems that come closer to approaching the latter should tend to use force abroad sparingly, because leaders in these systems are held accountable for foreign policy fiascoes. One exception may be foreign military force used against substantially weaker foes, where the outcome is practically certain. In contrast, when W/S ratio is small, leaders have fewer domestic restraints on the use of force abroad, and thus they may deploy troops with greater frequency. Selectorate theory has been used as the foundation for a remarkable number of studies and has helped to shed considerable light on the use of foreign military force. However, as scholars have increasingly honed regime classifications and distinctions, they have also pointed out limitations in selectorate theory (Ezrow & Frantz, 2011; Kennedy, 2009; Weeks, 2012). Some assumptions in the theory may not be empirically valid, suggesting the need for further refinement moving forward.

We would be remiss in this summary if we did not point out that some question how important domestic political institutions are for understanding the use of interstate force. One notable school of thought argues that domestic economic structures, such as capitalist economies and/or contract intensive economies, have greater explanatory power than political arrangements. The evidence for such claims is compelling (Gartzke, 2007; Mousseau, 2013; for a contrary view see Choi, 2011; Dafoe, Oneal, & Russett, 2013), but it seems likely that some combination of domestic political and economic structures will offer the most explanatory purchase moving forward (Hegre, 2014).

Substantial research has also been done on the second domestic structural factor: the diversionary incentive to use foreign military force. Work by Ostrom and Job (1986), James (1987), Levy (1989), James and Oneal (1991), and Morgan and Bickers (1992) rekindled interest in the subject, and since the early 2000s research on the topic has burgeoned. Empirical support for diversionary behavior was contested for some time (Gowa, 1998; Meernik & Waterman, 1996; Moore & Lanoue, 2003), but the bulk of the evidence now seems to suggest that diversionary military force is in fact used under certain conditions (Levy & Mabe, 2004; Oakes, 2012). The question that remains is what these conditions are. Early attention focused on regime type, with results appearing to converge on patterns similar to that for use of force in general (Brulé & Williams, 2009; Kisangani & Pickering, 2011; Lai & Slater, 2006; Pickering & Kisangani, 2005).

More recent work suggests that other domestic considerations, such as a state’s extractive capacity (Oakes, 2012) and its ethnic composition (Haynes, 2016a; see also Carment, James, & Taydas, 2006), also shape the propensity to use diversionary force. U.S.-focused studies demonstrate the impact of partisanship and congressional–executive relationships. Results suggest that Republicans and Democrats respond to different macro-economic cues, with the former prone to use force when unemployment increases and the latter doing so when inflation rises (Brulé & Hwang, 2010; Fordham, 1998; Howell & Pevehouse, 2005). Characteristics of the target state may also impact decisions to use diversionary military force. Haynes (2016b), for example, finds that leaders target states with comparable capabilities when opting for diversionary force to send an unmistakable signal to domestic audiences about their political competence (see also Jung, 2014).3 Mitchell and Prins (2004), Mitchell and Thyne (2010), and Tir (2010) suggest that states are prone to use diversionary force against interstate rivals and countries with which they have unsettled contentious or territorial issues.

Scholars have also begun to recognize that the relationship between domestic pressures and external force is nonlinear. Domestic pressures that are minimal will rarely compel leaders to use force abroad for diversion, but those that are sufficiently strong may prevent decisive foreign policy action and may even result in the removal of the leader. Levy (1989) theorized about this curvilinear relationship some time ago, but it has only recently been modeled and supported empirically (Pickering & Kisangani, 2005). Scholarship has also begun to recognize that humanitarian and other non-hostile military interventions can be used for diversionary purposes (Kisangani & Pickering, 2007) and that other forms of foreign policy behavior may have diversionary intent (Li, James, & Drury, 2009). In addition, it is now understood that that target states are important for more than their innate characteristics: their behavior is also significant. The strategic conflict avoidance argument, which has some empirical support, suggests that leaders in potential target states take action to reduce the likelihood they will be attacked (Clark, 2003; Clark, Fordham, & Nordstrom, 2011; Fordham, 2005). They halt provocative actions and attempt to minimize diplomatic friction with the potentially diversionary adversary. In this regard, strategic conflict avoidance can be considered one aspect of the larger strategic phenomenon of action and reaction between pairs or among groups of states.

Knowledge on diversionary motivations to use military force abroad has advanced in two further ways. First, research has begun to study reciprocal effects between diversionary force and the domestic factors that prompt it. These studies answer the question of whether or not diversionary force works. In other words, does it have any impact on the domestic political conditions that leaders hope to improve though diversionary action? As recent studies suggest, it is far from clear that diversionary force works in the way leaders anticipate (DeRouen, 2000; Kisangani & Pickering, 2009; Oakes, 2012; Pickering & Kisangani, 2010). These early results hold out some hope that leaders may eventually come to understand that diversionary force may not often generate domestic benefits. This type of foreign policy learning is, however, less relevant if leaders only divert when their backs are up against the wall and they are gambling for resurrection, as one recent study suggests is common (Haynes, 2016b; see also Goemans, 2000). Second, recent scholarship has begun to take seriously the fact that leaders tend to have multiple policy options available to respond to domestic pressures. An exhaustive understanding of decisions to use diversionary force must take these alternatives into account and develop theoretical and empirical knowledge that indicates when military force will be chosen over other options. Enterline and Gleditsch (2000) and Oakes (2012) have begun to analyze such substitutable policy options, but this component of diversionary research is in its infancy.

Interstate Factors

The primary structural conditions that lead to the use of force at the interstate level are so ubiquitous that they hardly need elaboration (i.e., Mitchell, 2017). In fact, their influence on decisions to use force has been discussed for such an extended period of time that they are commonly included as control variables in models testing theories on the use of force, even when solid consensus has yet to be reached on their impact. We provide a basic overview of proximity, territorial disputes, capabilities, alliances, economic interdependence, intergovernmental organization (IGO) membership, and colonial history in this section.

Proximity and territorial concerns are the most well-established structural conditions influencing decisions to use force. Contiguity provides both opportunity and risk and greatly reduces limitations on power projection capabilities (Boulding, 1962). As the number of land borders a state has increases, the likelihood that it will use force rises as well. Souva and Prins (2006), for example, find that each additional land border increases MID onsets by nearly 11% (for a contrary view, see Pearson, 1974). Disputed territory also substantially increases the odds that military force will be used (Diehl & Goertz, 1988; Hensel, 2001; Vasquez, 1995). In addition, it plays a significant role in the development of rivalries (Rider & Owsiak, 2015) and increases the odds of an escalation to war (Vasquez & Henehan, 2001). When combined with other structural causes, such as alliances or rivalry, territoriality further constrains leaders’ ability to pursue accommodative strategies and increases the likelihood that military force will be used (Colaresi & Thompson, 2005; Rasler & Thompson, 2006; Senese & Vasquez, 2005). As territory becomes more valuable—because of the presence of natural resources, religious sites, or historical homeland claims—the chance that force will be used rises further (Hensel & Mitchell, 2005; Toft, 2003). Conversely, legal border agreements significantly reduce the probability that force will be employed between states (Owsiak, 2012). Regime type may influence the use of force over territorial issues as well, but the empirical evidence on this proposition remains mixed (James, Park, & Choi, 2006; Park & James, 2015).

Measures of national military capabilities, as captured in Correlates of War CINC (Composite Index of National Capability) scores, are also often presumed to influence decisions to use military force. Leaders are commonly thought to use military force against weaker countries in order to increase the probability of mission success and to reduce the chance of reciprocation. Empirical evidence on this supposition is, however, inconsistent at best (Chiba, Martinez Machain, & Reed, 2014; Choi & James, 2016; Clark, Fordham, & Nordstrom, 2016; Fordham, 2004). A large number of studies suggest that disputes between states with roughly symmetrical capabilities have a higher probability of escalation (Lemke, 2002; Moul, 2003; Thompson, 1988), but contradictory findings remain (Bennett & Stam, 2004). Rapid changes in capabilities also influence the probability that military force will be used (Bell & Johnson, 2015).

Mixed findings have also been common in the literature on alliances. Defensive alliances may deter attackers (Fang, Johnson, & Leeds, 2014), and alliances that settle territorial claims may solidify peace (Gibler & Vasquez, 1998). Alternatively, offensive and neutrality pacts can increase the chance that interstate force will be used (Leeds, 2003) and may even increase conflict probabilities by involving more potential war participants (Leeds, 2005). All of these relationships seem to be marked by substantial temporal variation (Kang, 2012), suggesting the need for further analysis. We need to better understand why specific types of pacts have an impact during certain historical periods and not others.

Economic interdependence is another interstate cause that is often posited to influence the use of force. Again, results are inconsistent, with some studies showing that interdependence reduces the chance that force will be used (Kinne, 2012; Russett & O’Neal, 2001), particularly when intra-industry trade constitutes a large proportion of the interdependent relationship (Peterson & Thies, 2012). Other studies find that trade partners use force against one another in asymmetric relationships and that the relationship is conditional (Gartzke & Westerwinter, 2016). More recent work suggests that export similarity may lead to conflict regardless of interdependence levels (Chatagnier & Kavakli, 2015), and that the issues at stake may moderate the impact interdependence has on conflict. Rising levels of interdependence may reduce the probability that military force is used over territorial issues, but it may have little impact on decisions to use force over existential issues, such as regime survival or core regime policies (Bell & Long, 2016).

Common membership in IGOs is also included in many analyses. Similar to alliances and trade relations, shared IGO membership may reinforce the bargaining approach by providing a transmission belt for shared information that reduces the propensity for states to use force. The impact of IGO membership is, however, contested. While scholars such as Russett, Oneal, and Davis (1998) and Crescenzi (2007) find that IGO membership decreases conflict propensities, other studies find that it increases such propensities or has little effect (Boehmer, Gartzke, & Nordstrom, 2004; Fausett & Volgy, 2010; Shannon, Morey, & Boehmke, 2010). Recent work suggests that scope conditions are important in this relationship, as they are for most of the influences examined in this article. Anderson, Mitchell, and Schilling (2016) demonstrate that IGO membership had a conflict-reducing effect during the Cold Wa, but a conflict-increasing effect afterward. The type of IGOs involved also matters.

Shared history may also lead one country to intervene into another. Former colonial powers seem significantly more likely to use military force in their former colonies than in other countries (Pickering & Peceny, 2006). In fact, Chacha and Stojek (2016) demonstrate that colonial ties have substantial explanatory power even when controlling for bilateral trade, shared language, and other commonalities between the intervener and the target (see also Lemke & Regan, 2004).

International or System-Level Factors

Over recent decades relatively little attention has been paid to system-level factors that might explain patterns in the use of military force. System-level theories and empirical tests were prevalent through the 1980s, but since then the empirical literature has tended to focus on dyadic theories and tests of interstate and domestic structural factors. Also, system-level theories such as power transition theory, long cycle theory, and power cycles pay scant attention to low-scale uses of military force. They instead tend to focus on system-changing events, particularly full-scale wars among great powers in the international system. The distribution of power in the international system highlighted by these theories is nonetheless an important context for understanding the overall levels of military force used during a specific time period. They thus offer a baseline for understanding the probability that force will be used in a given period. Pollins (1996), for example, uses the long-cycle approach to demonstrate that militarized disputes are significantly less frequent during periods of power concentration in the international system than during periods of power deconcentration. Although they use a different analytical framework, Hebron, James, and Rudy (2007) find similar results (but see Bennett & Stam, 2004; Reuveny & Thompson, 2002). Lemke (2002) maintains that power hierarchies at the regional level can help to explain uses of force, but Bennett and Stam (2004) dispute this result.

All told, research linking system-level dynamics to the use of force remains limited and incomplete. Given the periodicity that is apparent when analyzing many interstate and domestic causal factors, a renewed focus on system-level approaches that shed light on historical dynamics seems worthwhile.

Military Intervention Outcomes

Historically, the empirical literature has tended to focus on the factors that explain the use of military force rather than the impact that such force has on target countries (Thompson, 1993). This has changed in recent years, although interestingly most studies on the consequences of military force use data that measures military intervention specifically (IMI, MIPS; Regan, 2000) rather than nearly synonymous phenomena like militarized force (MIDs). Perhaps the dyadic structure of militarized dispute data leads scholars to view it as most valuable for understanding reciprocation and escalation processes between countries, while the monadic design of much military intervention data allows for more careful examination of within country impacts. The largest volume of research on intervention consequences examines three areas: democratization, quality of life and human rights, and civil war duration and outcomes.

Democratization

Some early studies suggested that U.S. military intervention helps to democratize target countries (Hermann & Kegley, 1998; Meernik, 1996). Peceny (1999) added nuance to these results by demonstrating that not all U.S. military interventions are correlated with political liberalization in target countries. His findings indicate that U.S. military interventions that have political liberalization as a primary policy goal do, however, improve the chances that the target country will become a democracy (with a Polity score of 6 or above) within three years. Pickering and Peceny (2006) expanded this line of research by comparing U.S., British, French, and UN military intervention and democratization outcomes. Their quantitative results suggest that only U.S. interventions are associated with democratization. They examine each of these cases of democratization in detail and find that the countries involved hardly constitute full-blown, stable democracies. Rather, they are fragile democracies that do not remain democratic for long.

Later empirical research punched further holes in the U.S. military intervention–target state democratization thesis. Walker and Pearson (2007) re-examined the cases analyzed by Peceny (1999) and found that the governments that emerged in the wake of U.S. military interventions were, at best, illiberal democracies. Although most of these countries held marginally fair elections, they did not have many of the broader characteristics of democracy such as civil rights and independent judiciaries. Gleditsch, Christiansen, and Hegre (2007) provided arguably the most comprehensive examination of democratic military intervention and target state democratization. They analyzed all democratic interveners (again, countries with a Polity score of 6 or above) in comparison to autocratic interveners. They find that democracies tend to intervene in poor or resource-dependent countries with limited prospects for democratization. When political liberalization does occur, the maximum outcome is typically movement up to mixed regime status. In other words, the resulting regimes are neither fully democratic nor fully autocratic.4

A recent study suggests that the military interventions that developing countries launch may actually be more helpful to their political development than external interventions by democracies, no matter how well intentioned. Kisangani and Pickering (2014) demonstrate that in the context of border fixity (Atzili, 2011), foreign military interventions by developing countries can enhance state power by increasing the government’s ability to extract tax and non-tax revenues. Transnational rebels and insurgents are often influential in parts of the world where border fixity is pronounced, and foreign military interventions that target non-state foes that operate across borders can strengthen the government’s relative domestic political position considerably. It also allows the government to increase control and expand taxing authority over troubled border lands. Government forces may be able to retake lucrative mines, forests, and other natural resource deposits from rebels during such military missions as well. By helping governments to simultaneously assert control over territory and build up their coffers, low-scale military force used across borders may be important for state-building in the developing world. Kisangani and Pickering’s (2014) quantitative empirical findings suggest that foreign military intervention does indeed have a statistically significant, positive impact on government income over the long run.

Human Rights and Quality of Life

Peksen (2011, 2012) has presented convincing evidence that external military interventions do not tend to improve human rights or women’s rights in target countries. Even if the intervener is a liberal democracy or an international actor like the UN, human rights abuses such as extrajudicial killings and torture tend to accelerate during and after the intervention. Similarly, U.S. military interventions seem to undermine women’s rights in target states, perhaps because they contribute to instability in the target country. However, interventions by intergovernmental organizations like the UN appear to help to advance women’s rights, perhaps because these organizations insist on improvement as a condition for their involvement.

Military intervention’s impact on human rights abuses in target countries may be most pronounced when the situation is dire. Dispatching troops seems to be able to slow or stop the horror of government sponsored mass killing, commonly called politicide. Krain (2005) demonstrates that military interventions against the offending government can halt the killing. Kathman and Wood (2011) find that impartial interventions can also halt mass killing if the intervener is committed to staying for an extended period to demonstrate resolve. Kuperman (2008), however, has advanced a moral hazard argument that suggests that these findings may be misleading. Kuperman maintains that the possibility of external military intervention may encourage or accelerate rebellions, with the end result being an increase in the probability of atrocities. Kydd and Straus (2013) challenge this thesis by demonstrating that logical outcomes other than the moral hazard pathway highlighted by Kuperman may follow a credible signal of an impending military intervention. DeMeritt (2015) builds on Krain and Kathman and Wood to provide empirical evidence that also contradicts this moral hazard claim. She finds that interventions before the killing begins do not increase instability and violence. Rather, they prevent the outbreak of politicide, and interventions during atrocities reduce death tolls. Murdie and Peksen (2013) provide insight into why external actors may act to stop such internal violence. They show that the probability that outside actors will intervene increases when international human rights organizations shine the light of publicity on unfolding tragedies.

Military interventions may also have a long-term impact on broad indices of human well-being. While Pickering and Kisangani (2006) find no relationship between foreign military intervention and physical quality of life indicators (PQOL) in developing countries, they return to the subject in a later study with more refined methods (Kisangani & Pickering, 2017). They find that in certain situations, external military intervention can improve life expectancy outcomes in target states, but it tends to have a negative short-term impact and a positive long-term impact on infant mortality and literacy rates. Perhaps the instability engendered by the military intervention initially suppresses human quality-of-life measures, but the increased flow of capital and aid that tends to follow Western and other military interventions may lead to improvements in education and health care over the long term. Interventions from rival states, however, tend to have negative impacts on PQOL no matter the measurement context (Lounsbery, Pearson, & Talentino, 2011). Rival military actions may cause greater destruction or instability and certainly cannot be expected to improve conditions in the target country.

Interestingly, even though intervention has a mixed history of improving living conditions and human rights in target states in the long run, humanitarian intervention appears to be one of the few reasons the U.S. public will support military intervention (Jentleson & Britton, 1998) and one of the main impetuses behind U.S. uses of military force since 1945 (Choi, 2011; Choi & James, 2016; although see Fordham, 2008, for a contrary view).

Civil Conflict

Military intervention into an ongoing civil war poses a distinct set of challenges. Before examining the impact that external military intervention may have on civil war, it seems wise to review some of the reasons third parties choose to intervene in civil conflicts. The evidence to date suggests that many of the same influences that impact decisions to use military intervention as a policy tool outside of the context of civil war also impact the decision to send troops into active zones of civil conflict. Neighboring states, allies, rivals, and former colonial powers are prone to intervene in the midst of civil wars (Findley & Teo, 2006; Fordham, 2008; Lemke & Regan, 2004). Bilateral trade volume also influences civil war intervention on the side of the government (Stojek & Chacha, 2015), and the probability that a state will intervene increases significantly when a rival state has dispatched troops into the country (Daxecker, 2011; Findley & Teo, 2006). The impact of rivals may, however, apply only to the Cold War and not the post–Cold War period (Mullenbach & Matthews, 2008). Transnational ethnic affinities also increase the likelihood of external intervention (Carment & James, 2000; Mullenbach & Matthews, 2008; Pearson, 1974; Saideman, 2012). Even transnational linkages between marginal ethnic groups in the intervener and target may prompt intervention (Nome, 2013). Religious ties also seem to be important predictors of external military intervention into ongoing civil conflicts. Intervening states tend to have populations that have similar religious affinities as targeted groups in civil war states (Fox, 2001), and third-party military intervention seems to be undertaken on behalf of religious minorities more often than any other type of minority group (Fox, James, & Li, 2009).

Characteristics of the civil conflict itself can also be expected to affect intervention decisions, though it is difficult to determine exactly how at this point. While it seems logical that third parties would be less likely to intervene in intense civil conflicts with high casualty rates, empirical evidence to date suggests that the opposite may be the case (Aydin, 2010; Regan, 2002). The probability of military intervention declines, however, if other third parties have intervened recently and failed to achieve their objectives (Aydin, 2010). Regional considerations can also impact intervention decisions. Third-party interveners often come from the same region as the afflicted state, and one of their main motivations seems to be to prevent the regional diffusion of civil conflict (Kathman, 2010; Kathman, 2011).

Political characteristics of the intervening state may also make a difference, although the extent to which they matter is disputed (Mullenbach & Matthews, 2008). Some evidence suggests that democratic states are more likely to deploy soldiers to defend co-ethnics within civil wars than autocracies, while autocratic states seem prone to support rebels when such non-state actors control lootable resources (Koga, 2011; Wolak, 2014; see also Findley & Marineau, 2015; Ross, 2004). Media coverage may also help to drive third-party interventions (Bell, Frank, & Macharia, 2013), a condition that clearly is more relevant for democratic than non-democratic countries.

Findings on the impact that external interventions have on civil war are also mixed. The most reliable finding to date is that biased interventions on the side of rebels increase the probability of rebel victory (Balch-Lindsay, Enterline, & Joyce, 2008; Lyall, 2010; Sullivan & Karreth, 2015; Thyne, 2009). Biased interventions in favor of the government do not seem to have a similar impact, perhaps because interventions in support of governments may only be necessary when the government’s situation has grown tenuous (Gent, 2008). Perhaps the only instance in which pro-government interventions improve the government’s chance of victory is when government forces do not have a clear capability advantage over rebels, and thus national armed forces benefit from the additional firepower provided by external troops (Sullivan & Karreth, 2015). Biased interventions on either side may increase the time required to reach some sort of settlement to the conflict (Brandt, Mason, Gurses, Petrovsky, & Radin, 2008). The tactics deployed by the government and insurgents in response to military intervention also affect duration, though empirical research on such micro-level phenomena is still developing (Linebarger & Enterline, 2016). Interventions also alter the probability that conflict will diffuse to neighboring countries. Pro-government interventions seem to reduce diffusion, while pro-rebel interventions increase diffusion (Peksen & Lounsberry, 2012). The latter may increase the capabilities and transnational activities of insurgents, increasing the odds that conflict will spill across borders.

Directions for Future Research

While we have clearly generated considerable empirical knowledge on the causes and consequences of foreign military intervention and the use of military force abroad, notable gaps in our understanding remain. There are two complementary paths future research can take to enhance our grasp of intervention dynamics and outcomes. First, we need to begin to think and more explicitly theorize about scope conditions. Second, we can pry open the black box of the state further to begin to develop an understanding of the non-state actors directly or indirectly targeted by military intervention. Opening the black box wider will also allow us to grasp some of the broad characteristics and proclivities that tend to be present among the leaders who ultimately make the decision to place soldiers in harm’s way.

With regard to scope conditions, it seems wise to give more attention to periodicity and regional variations. Many empirical results to date have period effects, particularly between the Cold War and post–Cold War eras. The field would benefit from careful theorizing about the reasons for such temporal variation and the different types of variables period effects are likely to influence.5 Regarding regional variation, it seems that most every decade scholars call for increased attention to regional patterns and the development of region specific theory to understand interstate military force (Henderson, 2009; Lake & Morgan, 1997; Pearson & Baumann, 1989; Pickering & Thompson, 1998). Empirical studies often control for regional variation and find significant differences, but a sustained effort to theorize about why patterns and effects of foreign military intervention diverge across regions has been lacking. It may be, for example, that state capacity and border fixity vary roughly along regional lines, and both indicators may systematically influence the use of armed force across borders. Norms may also vary across regions. Although norms are difficult to measure, their observable implications can be delineated and tested against alternative explanations. As Mitchell (2017) notes, the time seems ripe for the thoughtful development of theories and tests bound by time and region. Such efforts hold great potential for advancing and refining our understanding.

Opening the black box further has the potential to improve our understanding as well. Many of our measures for foreign interventions and their targets remain relatively crude. They are frequently binary, indicating whether an intervention occurred or not and whether or not it was supportive of the target government. Interventions can, however, target a wide variety of non-state actors and still be categorized as being either pro- or anti-government. To date, we do not have reliable data on such targets, and thus we do not have information on how interventions affect various non-state actors and their relationships both with one another and with the government. Such details on the ground are important, as they represent arguably the most crucial missing link between our relatively dichotomous theory and data on intervention and a wide range of intervention outcomes. Foreign military intervention may cause non-state groups to splinter or coalesce or help to transform relations among such groups (i.e., Gleditsch & Beardsley, 2004; Olson Lounsbery, 2016). Developing theory to understand these changes and data to evaluate such theory will represent a major advance in our knowledge and should help to develop more reliable information on foreign military intervention’s utility.6 We also need more refined categorizations of military intervention itself. There are, of course, many types of overt military intervention—from benevolent missions to rescue nationals or deliver humanitarian intervention to large-scale invasions hostile to the target government. Few studies reliably capture variation in scale or intention across interventions. It seems wise to begin to differentiate interventions, while at the same time delving more deeply into their impact on the myriad actors that can be important within the target state.

Opening up the black box even further to examine leaders’ perceptions and decision-making processes can also be fruitful. Empirical research on the use of interstate force rarely examines individual level variables to further understanding, perhaps because such proximate causes are thought to be too uncertain and inconsistent to be included among the structural conditions used to explain military force. Returning to Thompson’s analogy of firewood, individual-level variables are often conceptualized as idiosyncratic “matches” that set the kerosene soaked (by structural factors) firewood ablaze. Individual-level variables do not have to be considered either distinctive or unsystematic, however. Scholars have made good progress in determining systematic variations at the individual level, and these variations can be usefully added to the stack of structural “firewood” the field has developed to bring understanding to interstate force (Prins, 2010). Adding this layer of complexity to our models is important, because leader perceptions of structural conditions may be more important for determining foreign policy decisions than the actual structural conditions themselves (Keller, 2005). The work of Keller and Foster (2012, 2016; Foster & Keller, 2014) offers an example of the utility of building the cognitive characteristics of leaders into our theoretical and empirical frameworks. They show that leaders who believe that they can have a decisive influence on events are more prone to use military force abroad for domestic reasons than counterparts who assume that external phenomenon shape outcomes. In the psychological literature, the former are said to have an internal “locus of control” while the latter have an external “locus of control.” Keller and Foster also demonstrate that leaders who do not view the world as complex, or who have low cognitive complexity, are more prone to react to stimuli with military force than those who have high levels of cognitive complexity. Cognitively complex leaders are more likely to consider all conceivable policy options and to carefully weigh various risks. They may ultimately choose to use military force, but they will not jump to this conclusion with minimal provocation and without a thorough review of the evidence and alternatives. Saunders (2011) offers an additional example of the utility of analyzing decision-makers. She demonstrates that leaders’ perceptions about the source of threats—whether they emanate from the domestic institutions of target states or merely poor policy choices by the target—determine the type of interventions that have been undertaken by U.S. leaders.

In sum, empirical theory and evidence have provided considerable insight into the causes and consequences of foreign military intervention. The field seems to be adding even greater precision to these findings through a critical examination of scope conditions, while also adding more fine-grained detail to theory and data. The prospects for the continued advancement of our understanding seem bright.

Acknowledgment

We appreciate the suggestions that Amanda Guidero and two anonymous reviewers provided on this essay. Any remaining errors are, of course, the authors’ alone.

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Notes:

(1.) Foundational works on this approach include the following: Blainey, 1973; Bueno de Mesquita & Lalman, 1992; Fearon, 1995; Goemans, 2000; Morgan, 1994; Powell, 2004; Schelling, 1966.

(2.) Critical junior parties are defined as parties whose departure would cause the coalition to lose its parliamentary majority (Beasley & Kaarbo, 2014).

(3.) Note that these recent findings contradict earlier claims that diversionary targets will tend to be relatively weak targets to ensure success and a potential rally effect for the leader (Clark, Fordham, & Nordstrom, 2011; Fordham, 1998; Foster, 2006; Morgan & Bickers, 1992).

(4.) A category of states that some have called anocracies and Gleditsch et al. (2007) term unstable semi-democracies.

(5.) State-level variation across time might also be examined. Pickering and Peceny (2006) show how U.S., British, and French interventionary activity has changed over time, and variation in activity level and activity type can be profitably traced for other countries from Russia to Nigeria. Comparative state-level studies could determine how often such variation results from broader, system-level or narrower, state-specific influences.

(6.) Note that such data would reverse the direction of the data available in the Uppsala Armed Conflicts Database. The Uppsala data begin with domestic groups involved in active conflict and attempt to determine if they received external military support, even if such support is only alleged (see Salehyan, Gleditsch, & Cunningham, 2011). The data proposed here begin with overt foreign military intervention, which is clearly traceable, and seek to determine its impact on non-state actors in target states.