Military Intervention in Interstate and Civil Wars: A Unified Interpretation
Summary and Keywords
Military intervention into interstate and civil wars is both common and important. It lengthens wars, makes them more severe, and shapes how they are fought. Even the mere possibility of intervention can alter the course of a war as belligerent powers alter their strategies to either encourage or dissuade potential interveners. These effects of military intervention are found in both civil and interstate wars. Yet, is state intervention into interstate and civil wars essentially one phenomenon or are they distinct phenomena? By looking at which states are likely to intervene, why and when they intervene, and which wars are most likely to experience intervention, it becomes clear the similarities between state military intervention into civil and interstate wars are more significant than are the differences. In other words, despite some important differences, they are subsets of the same phenomenon. In both types of wars, allies, geographically proximate states, and great powers are more likely to intervene. Also, information revealed by events within both types of wars prompts intervention and explains its timing. Last, wars in which international organizations become involved, both civil and interstate, are more likely to experience intervention. There are, however, important differences notably in the areas of cross-border ethnic ties, the presence of great powers in the war, the use of non-state proxies, and wars caused by commitment problems.
Why Military Intervention Matters
Interstate and civil wars are often influenced by the actions of states that were originally not parties to the conflict. This influence can take a variety of forms ranging from mediation, through economic and military aid, all the way to active military intervention.1 Such outside involvement in wars is quite common. For example, fully 60% of civil wars experience some sort of outside involvement and three-fifths of them attract involvement from multiple states (Findley & Teo, 2006). Even restricting the measure to military intervention—the deployment of significant numbers of state-led troops into combat—it is striking how often outside states become involved in others’ wars.2 Roughly one-third of civil wars experience military intervention and nearly half of interstate wars do as well, many by multiple states (Sarkees & Wayman, 2010).
Not only is military intervention common, but it also has important implications for international security. Military intervention on average makes both civil and interstate wars longer and more severe (Balch-Lindsay & Enterline, 2000; Balch-Lindsay, Enterline, & Joyce, 2008; Cunningham, 2010; Elbadwi & Sambanis, 2000; Gent, 2008; Kalyvas & Balcells, 2010; Regan, 2000, 2002; Regan & Aydin, 2006; Shirkey, 2012a), alters the likely outcome of wars (Altfeld & Bueno de Mesquita, 1979; Kalyvas & Balcells, 2010), and also affects the strategies used by the belligerent parties to prosecute the war (Gleditsch & Beardsley, 2004; Shirkey, 2009, 2012b). In fact, the mere possibility of intervention will shape belligerent strategies and help determine whether or not a war is even fought (Crawford, 2003, 2014; Fearon, 1994; Gartner & Siverson, 1996; Jervis, 1994; Leeds, 2005; Leng, 1993; Shirkey, 2009; Smith, 1995, 1996; Werner, 2000).
These effects of military intervention imply that theories of war that do not consider the possibility of intervention will be incomplete. This is turn suggests that the best theories of war will be ones that can explain war initiation, duration, and termination as part of a cohesive whole as such theories could answer questions about why wars happen, how they are fought, what their effects are, and how they end. This desire for unified theories of war raises questions about if civil and interstate wars can be explained by the same theories in general and specifically if military intervention in civil wars is similar to or distinct from military intervention in interstate wars. Indeed, it is probably necessary to think about the commonalities of various aspects of interstate and civil wars prior to any attempt to construct a grand, unified theory of war. Thus, it is worth exploring the similarities and differences when it comes to military intervention in interstate and civil wars.
The easiest way to do this is to think through who intervenes, why and when they intervene, and which wars are most likely to prompt intervention for both civil and interstate wars. On the whole, the answers to these questions about military intervention are quite similar for interstate and civil wars though important differences do exist. This suggests that the same or at least similar theories can be used to explain civil and interstate wars, though of course intervention is only one aspect of the war puzzle. Each of the above questions about military intervention will be covered in turn.
Who Intervenes in Wars?
The question of who intervenes in others’ wars has been the most thoroughly explored aspect of military intervention. For interstate wars the answer almost always is other states. Intervention by non-state actors is more common in civil wars, though often as proxies for outside states (Salehyan, 2010). Still, even in civil wars, states are both regular and powerful, direct interveners. But which states? Geographic proximity, alliances, and great power status have all been found to be good predictors of which states will intervene in both civil and interstate wars. Additionally, being the former colonial ruler of a given state and having cross-border ethnic ties have been found to be good predictors of state intervention in civil wars. Though these last two factors indicate that there are important differences in who intervenes in interstate and civil wars, the effects of these factors are weaker when compared to the factors held in common. Thus, they do not undermine the overarching pattern of similarity between civil and interstate wars.
First, geographically proximate states—contiguous states and those in the same region as the war—are more likely to intervene in both civil and interstate wars (Findley & Teo, 2006; Hammerström & Heldt, 2002; Houweling & Siccama, 1988; Raknerud & Hegre, 1997; Richardson, 1960; Siverson & Starr, 1991; Shirkey, 2009, 2012b; Vasquez, 2009; Wallensteen & Sollenberg, 1998; Ward & Gleditsch, 2002). The scale of this effect is substantial. Depending on the study, geographically proximate states are from three to six times as likely to intervene in ongoing wars. This strong relationship between proximity and military intervention holds for several reasons. First, the military logistics of intervention are far easier for proximate states (Boulding, 1962). Supply lines are shorter and for contiguous states, one’s troops need only march across the border to reach the conflict. Indeed, outside of a few great powers most states cannot deploy military assets to distant continents unaided. On the rare occasion when non-great powers do intervene in wars outside of their region, they usually do so in conjunction with an allied great power (e.g., Canada’s involvement in Afghanistan as a U.S. ally after 9/11), with a great power’s assistance if not active participation (e.g., Cuban intervention in the Ogaden War with Soviet assistance),3 or as part of a United Nations action (e.g., Colombia’s involvement in the Korean War).4
Second, proximate states are more likely to have a strong interest in the outcome of the war than are more distant states. While distant states might still have preferences about the outcome of the conflict, these preferences are unlikely to be strongly held. Furthermore, the limited nature of state resources means that it would be unwise to devote scarce resources to such distant, and therefore, less important conflicts. For most states, especially non-great powers, their security interests are confined to their own region (Buzan & Waever, 2003) meaning they will likewise confine their military efforts to their own region. Also, proximity may create specific incentives for intervention that more distant wars would not. For example, a nearby war could provide opportunities to annex territory or depending on who won the war threaten the state’s safety. Wars might also create regional instability or disrupt regional economies (Gleditsch, 2007; Murdoch & Sandler, 2002) providing an incentive to become involved in the war. Additionally, proximity can create direct incentives to intervene in the form of refugee flows (Kathman, 2010; Salehyan & Gleditsch, 2006). Contiguous states in particular may be subjected to large flows of refugees fleeing the conflict. These refugees could destabilize that state’s economy or politics. This in turn could cause some states to intervene in the conflict in hopes of stanching the flow of refugees, though it is not clear that such strategies are effective or wise (Salehyan & Gleditsch, 2006).
Last, proximate states are more likely to be attacked by belligerent parties, thereby forcing them to intervene (Shirkey, 2009). This may happen because such states possess raw materials crucial to the continuation of the war (e.g., Norway in World War II)5 or are along a belligerent’s best line of advance for attacking an enemy (e.g., Belgium in World War I). Such gross violations of neutrality appear to be limited to world wars and are the least of the reasons that proximate states are more likely to intervene. Thus, proximity creates a number of strong incentives and opportunities for military intervention in both civil and interstate wars.
Allies of states in ongoing wars are also more likely to intervene militarily than are other states (Findley & Teo, 2006; Raknerud & Hegre, 1997; Shirkey, 2009, 2012b; Siverson & King, 1979; Siverson & Starr, 1991). This is especially true if the war in question invokes the specific legal obligation spelled out in the alliance treaty (Leeds, Long, & Mitchell, 2000). Vasquez & Rundlett (2016) even argue that prewar alliance formation is a necessary condition of multiparty wars and hence for military intervention. This is likely a bit of an overstatement as it is possible to think of wars where military intervention has occurred—such as the Crimean War—in the absence of antebellum alliance formation. The magnitude of the effect of alliances is quite large—roughly comparable in size to the effect of geographic proximity—and again varies somewhat from study to study.
The correlation between sharing an alliance with a belligerent party and military intervention results from the fact that alliances are proxies for shared interests (Snyder, 2007). Alliances are formed to protect states from threats and also to exploit potential opportunities. So if an ally is attacked, this may well be the manifestation of the feared threat. Even if it is not, the attack may reduce the ally’s ability to assist when the threat does manifest or reduce the ability to exploit any opportunities. The quicker an ally can triumph and return to being a potential aid in any future conflict, the better. Thus, states will prefer to see their allies win their wars, and therefore, will often join their allies’ wars in order to make victory more likely.
Interestingly, the relationship between alliances and military intervention holds only early in wars. As time passes, the likelihood that an alliance will prompt a state to intervene drops sharply (Joyce, Ghosn, & Bayer, 2014; Melin & Koch, 2010; Shirkey, 2009; Stevenson, 2011). Presumably this is because if antebellum alliance considerations are insufficient to prompt intervention immediately, they never will be sufficient. Certainly, any reputational benefit gained from honoring an alliance is lost if the alliance is not honored quickly, meaning concerns about being seen as a reliable ally are unlikely to prompt intervention after a war’s early stages. More importantly, if a state decided the shared interests represented by the alliance were insufficient to warrant military intervention at the war’s outset, it is unlikely these considerations would ever be sufficient, barring significant changes in the expected intervener’s calculations about the likely outcome of the war. More will be said of this below.
Surprisingly, the converse—that states would be more likely to intervene in wars so as to oppose long-standing enemies—may not be true. The evidence on whether or not rivals of states engaged in wars are more likely to intervene is decidedly mixed with some studies finding rivals are more likely to intervene (Colaresi, Rasler, & Thompson, 2007; Lee & Thompson, 2015) while others find they are not (Shirkey, 2009). Further study is warranted on the role of rivalry in relation to military intervention.
Thus, alliances are good predictors of military intervention in the early stages of civil and interstate wars. This is because they are proxies for shared interests and states often are willing to make sacrifices to help their allies win wars. However, this relationship only holds for the early stages of a war because if the relationship is insufficient to prompt immediate intervention, it likely will never be sufficient, barring a change in the expected outcome of the war occurring.
Great powers are also more likely to intervene militarily in ongoing civil and interstate wars (Altfeld & Bueno de Mesquita, 1979; Biddle, Friedman, & Long, 2012; Findley & Teo, 2006; Pearson, Baumann, & Pickering, 1994; Richardson, 1960; Shirkey, 2009, 2012b; Siverson & Starr, 1991; Wright, 1965).6 This is true for several reasons. One, great powers have the logistical capacity to project military power far from home, meaning unlike most states they can intervene outside of their own region. Also, great powers have important interests in many regions around the globe. This means that they have strong preferences concerning the outcomes of many wars, even those that are not geographically proximate. As discussed above, this distinguishes great powers from most states whose security interests are primarily restricted to their own region. Finally, the vast military assets of great powers mean they have the ability to influence the outcome of most wars whether near or distant. This is not to say great powers always triumph in the wars they become involved in—they certainly do not (Mack, 1975)—but rather they have the potential ability to alter the probable outcome of many wars (Altfeld & Bueno de Mesquita, 1979). Smaller states’ militaries often cannot materially influence the likely outcome of a war, thereby reducing their incentive to intervene. Thus, great powers have greater abilities and incentives to intervene in ongoing interstate and civil wars in comparison to non-great powers.
Areas of Difference: Former Colonial Powers, Ethnic Ties, and Non-State Actors
Three areas of significant difference exist between civil wars and interstate wars in regard to which states are likely to intervene. These areas are the roles of former colonial powers, cross-border ethnic ties, and non-state actors. Former colonial powers are more likely to intervene in civil wars (Biddle et al., 2012; Findley & Teo, 2006) but not in interstate wars (Siverson & Starr, 1991; Wright, 1965). Likewise, cross-border ethnic ties have been found to be a good predictor of which states will intervene in civil wars (Balch-Lindsay & Enterline, 2000; Biddle et al., 2012; Buhaug & Gleditsch, 2008; Cetinyan, 2002; Findley & Teo, 2006; Jenne, 2006; Kathman, 2010; Saideman, 2002; Salehyan, 2010; Trumbore, 2003; Woodwell, 2004). No such correlation has been found for interstate wars. Finally, non-state actors are more likely to intervene in civil wars than in interstate wars. This is likely because civil wars happen most often in weaker states (Fearon & Laitin, 2003) and also because civil wars weaken states (Taylor & Botea, 2008; Theis, 2005). This places non-state actors on a more level playing field with the belligerent powers, meaning their involvement is more likely to significantly impact the war than would normally be the case in interstate wars. Also, states often intervene militarily in the civil wars of others indirectly rather than directly by using rebel groups as proxy actors (Salehyan, 2010). Such proxy interventions are usually sponsored by neighboring states, states with ethnic ties to rebel groups, great powers, and by states that are enduring rivals of the target state (Salehyan, 2008, 2010; Shirkey, 2012b). Again, this points to the fact that in civil wars non-state actors are able to marshal military forces that can contend with those of the state, whereas this is rarely the case in interstate wars.
Despite these important differences, on the whole there are more similarities than differences between civil and interstate wars in regard to who is likely to intervene. Importantly, civil and interstate wars share the strongest indicators of who will intervene: geographically proximate states, allies, and great powers, whereas the differences between them are in factors that are weaker predictors of intervention such as colonial ties. As will be seen below this pattern of overarching similarity holds as well for why and when states intervene in civil and interstate wars.
Why and When Does Military Intervention Occur?
The previous section showed that there are significant similarities between civil and interstate wars in which states are likely to intervene. Of course, knowing which states are most likely to intervene, while useful, is insufficient. Knowing when they are most likely to intervene and why they do so is also crucial. One thing that should be immediately obvious is that the relatively static variables discussed above such as geography, alliances, and great power status will be unable to tell us much about timing (Christia, 2012). While such variables are helpful in understanding which states join wars, given that they are invariant over the short and even medium run, they cannot explain when something happens. Rather something must vary in order to explain the timing of intervention as many states that intervene militarily in both civil and interstate wars wait months or years to join the conflict. If time invariant factors could explain intervention in and of themselves, such states would intervene immediately after the outbreak of the war.
In fact, some states do intervene immediately. Of the 102 states in the Correlates of War Interstate War dataset that joined ongoing wars, 29 did so in the first 30 days of the war (Sarkees & Wayman, 2010). These early interventions tend to reflect antebellum alliances (Shirkey, 2009). This leaves the majority of states as intervening later in wars. This is true for both interstate and civil wars (Sarkees & Wayman, 2010). If these later interventions cannot be explained by the time invariant variables discussed above—recall for instance that the effect of antebellum alliances on intervention rapidly declines after the first few months of a war—what is going on?
The key to the puzzle lies in the fact that wars—both interstate and civil—are bargaining processes and that wars are mechanisms for revealing information (Fearon, 1995; Powell, 2004; Slantchev, 2003; Wagner, 2000). Specifically shifts in power and changes in the belligerent parties’ intentions, as revealed by events within wars, cause states to reconsider their initial decisions about whether or not to intervene (Shirkey, 2009, 2012b, 2016). This helps to explain why states would intervene months or years after a war began even though they opted not to join at the outset. This revealed information could be linked to a variety of motives for intervention including perceived opportunities to revise the status quo, the desire to join one side for the intervener’s own protection, a need to balance, and a desire to affect the outcome of the wars they enter (Altfeld & Bueno de Mesquita, 1979; Gent, 2008; Gleditsch, Salehyan, & Schultz, 2008). This logic of revealed information triggering intervention can apply as well to non-state actors that might intervene in an ongoing civil war. This is important as such interventions have become common (Cunningham, 2010; Salehyan, 2010).
To more fully understand why revealed information plays a critical role in triggering military intervention, the nature of war itself must be considered. All wars, whether interstate or civil, are a continuation of bargaining in which information about belligerent parties’ relative power, reservation points, and goals are revealed and costs imposed causing the range of acceptable bargains for all parties to shift (Fearon, 1995; Goemans, 2000; Powell, 2004; Slantchev, 2003; Smith, 1998; Wagner, 2000). As belligerent powers learn, they continuously weigh the value of proposed offers against the expected value of continued bargaining. This leads belligerents to alter both their alignments with their fellow belligerents and their demands until an agreement acceptable to all sides is reached (Christia, 2012; Cunningham, 2010). The lack of such an agreement, tacit or otherwise, is what causes war (Goemans, 2000).
Non-belligerent states view ongoing interstate and civil wars in much the same manner (Balch-Lindsay & Enterline, 2000) meaning that if an agreement, tacit or otherwise, between a belligerent and a non-belligerent unravels, intervention will occur (Cunningham, 2010).7 The revelation of information may lead directly to such an unraveling. This is because the revealed information may indicate a need to balance, an opportunity to pick up spoils, an opening to alter the war’s outcome, or a chance to obtain a seat at the conference table. Such information is most likely to be revealed by battles and significant changes in the composition of governments and rebel groups, though other events within interstate and civil wars could reveal information as well. Therefore, military intervention can be thought of as occurring in the following manner. At the beginning of an interstate or civil war states decide whether to become belligerents or not. As states become privy to new information revealed by the war, they alter their beliefs about the benefits of belligerency relative to peace and potentially become prompted to intervene.
Of course, revealed information is not a sufficient cause of intervention. Only revealed information that increases the appeal of belligerency would increase the probability of intervention. Information consistent with non-belligerents’ original estimates or which makes belligerency less attractive would not increase the likelihood of intervention. Furthermore, not all states would respond to revealed information equally. The types of states discussed above as more likely to intervene—great powers, proximate states, and allies of belligerent powers—would be more likely to be responsive to revealed information compared to the general population of states.
Strategic Delay and Domestic Politics
Other explanations for intervention midwar are possible, but less convincing or in fact indicate that a decision to intervene has not been made. Mobilization, bureaucratic delays, and the political mechanisms of actually declaring war could delay intervention. Such factors, however, are unlikely to delay intervention for more than a month or two. Also, many states faced with such obstacles declare war before the delays are resolved even though it may take many months before they can make substantive contributions to the war effort. The United States’ entry into World War I is an example of a state intervening well before it could make a significant contribution on the battlefield.
Alternatively, states may be waiting to complete rearmament. It is not clear, however, why hostile belligerent states would allow intervention to occur at the time that suits their potential foes. In such cases preemption should occur (Murray, 1984). This sort of preemption, however, almost never occurs (Reiter, 1995) This suggests states rarely wait to complete rearmament prior to intervening, in turn limiting the number of cases in which states would have an incentive to preempt. Thus, while such strategic delay is logical, it rarely occurs.
Last, rifts in domestic politics—either among elites or between elites and the broader public—could delay intervention. Such divisions are entirely plausible and likely commonplace, but their very existence also means that the state has not in fact decided to intervene. In other words, domestic politics divisions are not an example of strategic delay, but an obstacle to decision-making. For intervention to occur something must happen to allow for these divisions to go away or for one side to triumph over the other in debates about war and peace. Thus, domestic politics divisions cannot alone explain the timing of intervention as some additional factor must cause those divisions to vanish or allow one side’s views to win out. Interestingly, there is some evidence that events within the war can play a role in shifting power between pro- and contra-intervention camps and can also influence the views within those camps (Shirkey, 2009). This suggests revealed information may explain intervention even in some cases where domestic politics divisions are relevant. Importantly, all three of these alternative explanations—mechanistic delays, strategic delay, and domestic politics—should apply to military intervention in interstate and civil wars equally.
This means that events within wars, in particular those events that are surprising or unexpected, are the most compelling explanation why states alter their calculations about the benefits of belligerency and decide to intervene militarily. The information revealed by these events provides both the motive for and explains the timing of intervention. Of course, the specific motives of interveners may be quite diverse and include at a minimum balancing as well as bandwagoning for both territorial and nonterritorial spoils (Savic & Shirkey, 2017). There is both large-n and case study evidence that revealed information does in fact trigger military intervention and explains its timing in both civil and interstate wars (Shirkey, 2009, 2012b, 2016). Thus, for both civil and interstate wars, states either intervene quite quickly based on the situation antebellum or intervene later in response to information revealed by events within the war that suggest their antebellum expectations about the course and likely settlement of the war were incorrect.
Which Wars Are More Likely to Experience Intervention?
Beyond who joins wars, when they join, and why they join, understanding which wars are most prone to experiencing military intervention is vital. Yet, it is one of the less-developed aspects of the military intervention literature. In part, this is because researchers who have tried to categorize wars by type have largely focused on the causes of truly catastrophic wars such as world wars (see, for example, Copeland, 2000; Gilpin, 1981; Kugler, 1990; Levy, 1982; Midlarsky, 1990; Organski & Kugler, 1980; Thompson, 1988, 1990; and Vasquez, 2009). While studying the causes of world wars certainly makes a great deal of sense, they are likely not the only wars that are especially prone to military intervention. It seems probable that there are other, more limited wars that are more prone to military intervention than the average conflict is. Several possibilities exist as to why some wars would be more likely than others to draw in outside states. One, the type of states involved in the war, in particular the presence of great powers, could alter the odds of intervention. Second, the nature of the cause of the conflict could alter the likelihood of intervention. Specifically, are commitment problems a central cause of the war? And last, regional and global institutions could increase the chances of intervention by acting as collective security institutions or by endorsing military intervention in a given war. Each of possibilities—and whether or not the implications are similar or divergent for interstate and civil wars—will be covered in turn.
Wars Involving Great Powers
Why great powers themselves are more likely to intervene in ongoing civil and interstate wars has already been covered. However, this is not the only way in which great powers influence the likelihood of military intervention. Their presence in both interstate and civil wars impacts the odds that other states will join those wars though the effect is different for civil and interstate wars.
The presence and especially prior intervention of great powers in interstate wars likely alters the scope of the issues over which the war is being fought and the overall stakes of the war for regional systems and for the international system as a whole (Kim, 1991; Yamamoto & Bremer, 1980). Evidence of this effect is clearest and strongest for the prior intervention of great powers in comparison to their presence in an interstate war from the outset (Shirkey, 2009).
This effect of great powers increasing the odds of further intervention does not appear to translate to civil wars. There is some evidence that the presence of a great power in a civil war actually reduces the odds that other states will intervene. Shirkey (2016) finds that the presence of a great power in a civil war reduces the odds that another state will intervene for civil wars in the Correlates of War dataset but not for civil wars in the PRIO dataset where the presence of a great power in a civil war has no effect. The finding in the Correlates of War dataset that great powers can dissuade further intervention into civil wars is likely the result of the fact that great powers are usually strong enough to discourage other states from intervening in civil wars that occur within the great power itself and perhaps also from intervening in the civil wars of states that are located within the great power’s sphere of influence. The null effect for the PRIO dataset is likely the result of PRIO covering only the post-1945 period, a time when the United States has often intervened in civil wars in conjunction with its many allies. This “democratic posse” effect is limited to the post-1945 era (McDonald, 2015) and may obscure great powers’ ability to dissuade intervention in their own civil wars and those of their allies. Still, the exact nature of the relationship between military intervention in civil wars and the presence of great powers is underdeveloped and warrants further study.
Wars Fought Over Commitment Problems
Next, interstate wars caused by commitment problems probably are more likely to experience military intervention than are wars with other causes. Commitment problems occur when one party to an agreement has sufficiently strong incentives to renege that it cannot credibly pledge to honor the agreement (Fearon, 1995; Powell, 2006). Commitment problems often arise due to expectations of future shifts in relative power or where the agreement itself significantly alters relative power (Fearon, 1995; Powell, 2006; Weisiger, 2013). These expected power shifts create incentives for the disadvantaged party to wage a preventive war in the hopes of averting the expected power shift (Copeland, 2000; Reiter, 2009; Weisiger, 2013).8
That there could be a relationship between wars fought over commitment problems and wars that experience military intervention is suggested by the fact that both interstate wars that experience military intervention and interstate wars caused by commitment problems are deadlier and longer on average (Copeland, 2000; Powell, 2006, 2012; Reiter, 2009; Shirkey, 2012a; Slantchev, 2004; Weisiger, 2013). This correlation hints at the possibility that the higher severity of such wars is rooted in a common cause. Of course, the fact that both types of wars are correlated with higher severity is not enough in itself to establish that wars caused by commitment problems are more likely to experience military intervention. That said, there are strong theoretical reasons to believe wars caused by commitment problems are more likely to prompt military intervention than wars are with other causes.
This is so for two reasons. First, wars caused by commitment problems are more likely to produce greater war aims such as state death, regime change, significant territorial annexation, and absolutist war-fighting strategies. These goals arise as a way to eliminate the commitment problem by attempting to halt or minimize shifts in the future distribution of power (Reiter, 2009; Weisiger, 2013). Such extensive aims, however, are more likely to substantially alter the existing power structure of a subregion, region, or even the world as a byproduct of the war (Weisiger, 2013). Belligerent states benefiting from these power shifts may in turn be unable to credibly commit to not use this new advantage to harm third parties. This threat to third parties may prompt them to intervene (Shirkey, 2012b; Wolford, 2014a).9
Second, wars fought over commitment problems are likely to be more severe. This is because eliminating a commitment problem often involves the destruction of a state, regime change, the destruction of much of the good that is at stake, or a large degradation of a state’s power (Leventoglu & Slantchev, 2007; Powell, 2006, 2012; Reiter, 2009; Stanley, 2009; Weisiger, 2013). In other words, the severity is the result of the expansive war aims discussed above. The greater severity and larger war aims mean these wars are more likely to result in significant territorial realignments creating opportunities for interveners to pick up territorial spoils. This increased severity also means belligerent states should be willing to make greater efforts to win such wars, and therefore, be desirous of attracting allies to their side in order to help with these strenuous efforts. Taken together, the desire to attract allies and the availability of spoils mean that belligerents should be more willing and able to distribute territorial spoils to attract allies, thereby increasing the odds of outside intervention. Germany’s willingness during World War II to reward bandwagoners such as Hungary with territory is an example of such behavior (Savic & Shirkey, 2017). Belligerents should also be more willing to offer nonterritorial spoils, such as aid or promises of future alignment, in order to induce states to intervene. Of course, the high costs and risks associated with such wars may also deter potential interveners (Haldi, 2003), but for many states the opportunities offered by intervention are likely to outweigh these greater costs and risks.
There is some evidence that interstate wars caused by commitment problems are in fact more likely to attract military intervention. A number of studies have established that significant prewar shifts in the relative balance of power are a cause of and good proxy for commitment problems (Powell, 2006, 2012; Weisiger, 2013). These prewar relative power shifts have further been shown to be correlated with interstate wars that experience military intervention (Shirkey, 2015).
Whether this relationship between wars caused by commitment problems and military intervention also holds for civil wars has not been studied. In fact, it seems unlikely to be the case. This is because the commitment problems in question would be internal to the state experiencing the civil war and thus less likely to threaten regional or global orders. True, the commitment problems would still prompt more extreme war aims, but this severity might not increase the desire to attract outside aid, as rebel groups in civil wars are already highly likely to be desirous of such outside aid due to their relative weakness and governments are unlikely to distribute their own territory as spoils to attract support. Thus, the relationship between interstate wars caused by commitment problems and military intervention seems unlikely to translate to civil wars.
The Role of International Organizations
Last, international organizations may play a role in determining the rate of military intervention. To the extent that the United Nations or various regional organizations act as collective security organizations, they would encourage states to intervene in ongoing wars against perceived aggressors. In other words, collective security organizations tend to widen wars if they failed to deter war in the first place (Betts, 1992; Morgenthau, 1973). Indeed, it is hard to explain the presence of states such as Colombia and Ethiopia in the Korean War or the wide anti-Iraqi coalition in the Gulf War (1990–1991) without the discussing the role the UN played in those wars. Likewise, UN peacekeeping and peacemaking missions in civil wars may expand the number of states that become involved in those conflicts. Quantitative studies have found some support for both notions (Shirkey, 2009, 2016; Uzonyi, 2011) though more systematic study is needed.
Presumably for both civil and interstate wars, the involvement of international organizations may lower the diplomatic costs of intervention and even potentially create costs for nonintervention (Finnemore, 2003). Also, since interventions conducted under the auspices of international organizations are often done by broad-based multilateral coalitions, states with weaker logistical and power projection capacities may be able to intervene by piggy-backing off of the logistical capabilities of great power allies that are also intervening. Of course, this could occur without the involvement of international organizations and may be a factor in the relationship discussed above between the presence of great powers in interstate wars and further military intervention.
Thus, wars in which the stakes are raised (through the presence of great powers and commitment problems) and wars where the costs of intervention are lowered (through the involvement of international organizations and perhaps also great power allies that provide logistical support) are more likely to experience military intervention than are other wars. In general, these factors appear to be more important for interstate wars than for civil wars, though that varies depending on the factor being considered. However, wars where costs are raised—such as through the presence of a great power in a civil war—are less likely to experience intervention.
As can be seen from the above discussion, there are more commonalities between interstate wars and civil wars when it comes to the nature and causes of military intervention than differences. The effects are similar: longer, more severe wars and shifts in the likely outcomes and strategies employed. Likewise, which states are prone to intervention is largely similar. In both interstate and civil wars great powers, geographically proximate states, and states allied to one of the belligerent parties are more likely to intervene. Another strong similarity is in the motives for and timing of intervention. Events within interstate and civil wars which revealed information to non-belligerent states cause those states to reconsider whether they wish to intervene. Revealed information also explains the timing of those states’ interventions. Last, the involvement of international organizations increases the odds of military intervention in both interstate and civil wars.
There are, however, important differences. Some of the clearest differences arise in who is likely to intervene in wars. Former colonial powers are more likely to intervene in the civil wars of their former colonies, but not in the interstate wars of their former colonies. Also, while cross-border ethnic ties are a strong predictor of intervention in civil wars, the result has not been found to transfer to interstate war. Similarly, how the presence of one or more great powers in a conflict affects the chances of future military intervention differs sharply between civil and interstate wars. The presence of great powers in interstate wars increases the odds of future military intervention, while their presence in civil wars appears to either have no effect or to reduce the odds of future military intervention. Last, wars caused by commitment problems are correlated with increased military intervention in interstate wars, but the relationship is unlikely to hold for civil wars.
Overall the similarities outweigh the differences suggesting that future lessons learned about intervention in interstate wars may well apply to civil wars and vice versa. This fits with a general view that it is unwise to draw sharp lines between interstate and civil wars when constructing theories or even more broadly between international relations and comparative politics. The forces at work between and within states are often the same or at least have parallels in the other sphere. Likewise, many of the insights about military intervention are drawn from general theories of war. This suggests that future studies of military intervention should seek to set intervention within both the broader international relations literature and also seek to link explanations of intervention in interstate wars with those in civil wars. Likewise, studies of both interstate and civil war must account for intervention as both its occurrence and even just its possibility shape how those wars are fought. It is only by keeping these linkages and broader literatures in mind that we can truly understand the nature of military intervention and the other international phenomena that are influenced by military intervention.
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(2.) Military intervention requires significant actual combat, not simply sending weapons or military advisors. The Correlates of War dataset counts states as participants if they suffer 100 combat fatalities or have at least 1,000 troops engaged in combat (Sarkees & Wayman, 2010). While such cutoff points may be a bit high, especially for smaller states intervening in wars in conjunction with larger states, the numbers give a rough sense that military intervention requires sending a significant number of troops to the war and that they actually engage in combat. Of course, at times an outside state will call their own combat forces “advisors” or “volunteers” in an attempt minimize the state’s involvement in the war. The key is not the labels used but the scale of the deployment, whether the troops are engaged in combat, and whether they are under the control of an outside state.
(4.) More will be said of the United Nations’ role in military intervention below. Of course, Colombia’s intervention into the Korean War was also done on behalf of and with the assistance of its ally, the United States.
(5.) The iron ore that Germany was hoping to secure was in Sweden, but the occupation of Norway was deemed necessary to prevent the allies from interfering with the Swedish ore shipments.
(6.) Precise definitions of great powers, especially definitions that do not rely on the behavior of the states, and where the cutoff between great and middle powers lies are both notoriously elusive (Gecelovsky, 2009). Great powers are defined herein using Levy’s (1981) definition. They are states that possess substantial military and economic resources, participate frequently and significantly in major international conferences, are given enhanced de facto or de jure powers within international organizations, and are states that are recognized as great powers by their fellow states and international institutions. They are also states that can project military power far from their borders (Richardson, 1960).
(9.) Wars caused by commitment problems may also increase the odds that belligerent states will deem that the stakes of the wars are sufficiently high to make it worthwhile to violate the neutrality of neutral states (Shirkey, 2015; Wolford, 2014b). This helps explain that finding discussed above that gross violations of neutrality, such as military invasion, generally occur only in world wars.