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1 Editorial Manager(tm) for British Journal of Political Science Manuscript Draft Manuscript Number: BJPOLS-D Title: When and Whom to Join: The Expansion of Ongoing Violent Interstate Conflicts Article Type: Article Section/Category: Global Corresponding Author: Dr. Reşat Bayer, Ph.D. Corresponding Author's Institution: Koç University First Author: Reşat Bayer, PhD Order of Authors: Reşat Bayer, PhD; Faten Ghosn, PhD; Kyle A Joyce, PhD Manuscript Region of Origin: TURKEY Abstract: The opportunity and willingness framework has received much attention in research on interstate war expansion. We build on this framework by examining the timing of intervention by third parties and the side of intervention (initiator or target). We also expand the opportunity and willingness framework to include additional factors not considered in extant research that influence when third parties join ongoing disputes and the side they join in those disputes. We empirically analyze the timing of joining and the side of joining in militarized interstate disputes between 1816 and 2000 using a competing risks event history model. We find many of the factors associated with this framework influence the risk of joining. The results indicate that relying on statistical models that do not distinguish between joining on the side of the initiator from the side of the target can be misleading.

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3 * Cover Letter Dear Editor(s), October 16, 2008 We would like to submit our manuscript When and Whom to Join: The Expansion of Ongoing Violent Interstate Conflicts for consideration by the British Journal of Political Science. This manuscript examines interstate conflict expansion, an under-researched topic in international conflict research but of particular importance as conflicts that expand to include more countries are those that tend to result in the largest number of fatalities. We build upon the opportunity and willingness framework, which has been frequently used in research on conflict expansion, to examine the timing of third party joining in ongoing disputes and side of joining (initiator or target) in those disputes. We use a competing risks event history model, which allows us to simultaneously analyze both the timing of joining and the side selected, to examine joining in ongoing militarized interstate disputes (MIDs) between 1816 and Our results suggest that differentiating between the two originators of a dispute provides important information on how variables, such as democracy, influence the timing of joining the initiator differently from the timing of joining the target. We have attached an anonymous version of our manuscript, and separate files for our tables and figures (all in.pdf format). Thank you for considering our paper. We look forward to hearing from you. Sincerely, Reşat Bayer Bayer Assistant Professor of International Relations Koç University rbayer@ku.edu.tr Faten Ghosn Assistant Professor Political Science University of Arizona fghosn@ .arizona.edu Kyle A. Joyce Assistant Professor of Political Science University of California, Davis kjoyce@ucdavis.edu

4 * Manuscript When and Whom to Join: The Expansion of Ongoing Violent Interstate Conflicts

5 Abstract The opportunity and willingness framework has received much attention in research on interstate war expansion. We build on this framework by examining the timing of intervention by third parties and the side of intervention (initiator or target). We also expand the opportunity and willingness framework to include additional factors not considered in extant research that influence when third parties join ongoing disputes and the side they join in those disputes. We empirically analyze the timing of joining and the side of joining in militarized interstate disputes between 1816 and 2000 using a competing risks event history model. We find many of the factors associated with this framework influence the risk of joining. The results indicate that relying on statistical models that do not distinguish between joining on the side of the initiator from the side of the target can be misleading.

6 Introduction Military conflicts that expand are far deadlier than those that do not. Of the 1,267 militarized interstate disputes (MIDs) between 1816 and 2000 in which military force was used, only 8 percent expanded to include at least one additional state (i.e., third party) after the dispute began; however, these MIDs have been among the deadliest accounting for 83 percent of the fatalities across all of the MIDs during this time period. Given the extraordinary destruction that conflict expansion augurs, understanding the decision of third parties to join ongoing MIDs is important. While extant research has pointed to several factors that influence third parties decisions to join ongoing disputes, few studies have examined how these factors influence: 1) the timing of third party participation or 2) how these factors might differ for joining the initiator of a dispute compared to joining the target. In fact, past research examining the timing of third party participation (Siverson & Starr 1991, Raknerud & Hegre 1997) ignores which originator a third party joined and studies that have examined the side of third party participation have ignored the timing of that participation (Reiter & Stam 2002). In this article we examine what factors influence both the timing of third party participation and the side on which third party participation occurs during ongoing MIDs. We build upon the opportunity and willingness framework, which has received considerable attention in the study of interstate war diffusion (Siverson & Starr 1991), to examine the timing and side of third party participation. In particular, we expand this framework, originally presented in the context of alliances and contiguity, to include several additional factors that influence a third party s opportunity or willingness to join a dispute. Understanding what influences the timing of third parties decisions to join and which side they join is important and improves our understanding of the dynamic processes of conflicts and their expansion. We build upon existing research in three ways. First, we examine on the timing of third party participation. Militarized conflict is a dynamic process and participation by 1

7 third parties has an important effect on the evolution of these conflicts. 1 In particular, participation by a third party has the ability to influence the eventual outcome of a dispute by changing the probabilities of each initial participant winning. Additionally, the timing of third party participation is important because it can influence the length of a dispute. For example, entry of the USA into World Wars I and II was decisive in influencing their evolution. In both wars, the decision of the USA to participate affected the length and final outcome of the war. In World War I, the USA joined after Russia had exited and Germany had improved its military position. In World War II, the USA joined after France exited and both the UK and the USSR were suffering on the battlefield. Thus, the timing of participation is, in part, informed by the current state of the conflict (i.e., who is winning and who is losing on the battlefield). Throughout a militarized dispute, each potential third party participant not currently participating reevaluates their decision and decides anew whether or not to join. Second, in addition to focusing on the timing of third party participation, we also examine how the factors purported to explain third party participation might differ for joining the side of the initiator of a dispute compared to the side of the target. We concentrate on the side on which a third party joins for two reasons. First, the decision to join an ongoing conflict and the side on which it would join are not independent decisions. For example, after Germany invaded Poland in 1939, it is unlikely that the UK first decided whether to join the war and then decided whether it would align with Germany or Poland. The decision of the UK to join was not independent from its decision on which side to join. Second, initiators of disputes often target weaker states (Gochman & Maoz 1984, Gartner & Siverson 1996) and so the distribution of military capability is likely to favor the initiator. This advantage is likely to influence both the when and the side third parties decide to join. In particular, as the discrepancy in military capability between the two initial participants increases the duration of the dispute is likely to be shorter and therefore third parties have a smaller 1 We focus on military interventions by third parties and do not consider other forms of intervention such as arbitration or mediation (Corbetta & Dixon 2005). 2

8 window within which to decide join the dispute. If third parties expect the initiator to win and to do so quickly, third parties may be more inclined to join the dispute sooner on the side of the initiator than the side of the target. Third, our research design improves upon previous research on conflict expansion by including all third parties who could potentially join an ongoing dispute, as opposed to most existing research, which only includes those third parties that actually participated in their sample. In our empirical analysis we include all third parties who are at risk of joining the dispute during each day of the dispute. Since a third party is continually updating their decision to participate or remain neutral as a dispute persists, a decision not to join at one point in time does not preclude joining in the future. Thus, if we only included third parties that actually participated we would lose important cases of non-participation. Since we are interested in both the timing of third party participation and whether a third party joins the initiator, target, or remains neutral, in our empirical analysis we use a competing risks event history model. This article proceeds as follows. First, we discuss the opportunity and willingness framework. Second, we discuss our extensions to the opportunity and willingness framework and our hypotheses for how each factor influences the time until a third party joins the initiator or the target. Third, we present our research design and discuss the key improvements we make to existing research designs that examine conflict expansion. Fourth, we present the results of our analysis, which demonstrate the importance of distinguishing between joining the initiator and joining the target and that conflating these choices produces misleading results. Finally, we discuss specific avenues for future research by examining the theoretical implications of our empirical analysis. 3

9 Opportunity and Willingness Framework The opportunity and willingness framework, developed by (Starr 1978) and (Most & Starr 1989) who built on the research of Boulding (1962) as well as Sprout & Sprout (1965, 1969), has been extensively used in conflict studies, in particular in the area of conflict expansion. Siverson and Starr conceptualize opportunity as the degree of interaction, or interaction opportunity meaning that some activity must be physically, technologically, or intellectually possible (1990, 48-49). In essence, opportunity has been developed to mean the possibilities that are available to any entity within any environment (Siverson & Starr 1991, 24). Thus, the ability of actors to interact with one another make militarized conflict possible. In their research on war diffusion, Siverson and Starr measure opportunity using the presence of a contiguous border. In addition, capability, major power status, and distance are also viewed as creating opportunities for conflict. In fact, some research designs in international relations use politically relevant dyads, which are pairs of countries where at least one country in the dyad is a major power and/or is contiguous to the other country in the dyad. While opportunity concerns the possibility of militarized conflict, willingness deals with the motivations and goals of policy makers, and the decision making processes that lead them to choose the war alternative rather than no war (Starr 1978, 365); in our case to choose to join an ongoing dispute instead of not joining. Siverson and Starr elaborate the concept to include a decision maker s calculus of advantage and disadvantage, cost and benefit, considered on both conscious and unconscious levels. It is through willingness that decision makers recognize opportunities and then translate those opportunities into alternatives that are weighed in some manner (1990, 49). In their research on war diffusion, Siverson & Starr (1990, 50) measure willingness using formal military alliances. Some scholars have used additional measures of willingness including regime type, international organizations, as well as economic factors (Kinsella & Russett 2002). This framework has been primarily used to analyze third party joining in general, without 4

10 considering how opportunity and willingness influence when and whom third parties join during ongoing conflicts. The only exception is Siverson & Starr (1991), who used this framework to examine the timing of joining into ongoing interstate wars. They hypothesized that the greater a nation s opportunities and willingness, the less time it will take that nation to become involved in the war (1991, 76). Furthermore, they expected that among the contiguity variables, those with direct contiguity would join sooner than those with cross water borders and colonial borders. Similarly, among the alliance variables they expected those states with defense pacts would join earliest followed by those with neutrality pacts and ententes as they view these types of alliance commitments as forming a rough ordinal index of willingness (1991, 50). While their expectations for contiguity were supported, those for alliances were not. They found that states who had defense pacts with the disputants did not join earliest, which was unexpected because defense pacts are seen as the strongest form of alliance commitment. We believe this illustrates the necessity to take into consideration side selection in addition to timing since it is likely that some of the factors that would motivate states to join on the side of the initiator would not necessarily influence joining the target. However, their research design does not differentiate the side on which a state decides to join. In addition, their design only included states that actually joined a war and not those that could have joined as well, which we argue below is a problem that affects their statistical results and therefore their conclusions. Finally, we believe that event history models are more appropriate for analyzing the timing decisions of third parties than ordinary least squares regression. Extending the Opportunity and Willingness Framework When a conflict begins there is relatively limited interaction between the two sides (Bremer 1993). As a result, at the beginning of a conflict, states contemplating joining the conflict have relatively little information. Kinsella & Russett (2002, 1049) argue that at the begin- 5

11 ning of a conflict states must rely more on contextual factors like geographic proximity and state capabilities when making decisions. The contextual factors that Kinsella and Russett refer to are the opportunity variables. As a war persists there is more interaction between the originators, and thus more information becomes available to potential joiners. Over time, other factors become more important because the factors associated with opportunity tend to change little and so other information becomes salient in influencing joining decisions. This is similar to the argument of Kinsella & Russett (2002, 1049) who maintain that [a]s more information is exchanged through signaling, the impact of contextual factors on state behavior is likely to recede. Then, influences identified with liberal theory democratic governance, economic interdependence, and membership in international organizations, all of which are indicative of dyadic interactions that communicate private information should become relatively more important than they were at earlier stages. The additional factors referred to by Kinsella and Russett are the willingness variables. In short, since there is limited interaction and information at the beginning of conflicts, we expect the factors associated with opportunity to have a greater impact on the decision to join earlier in a dispute. As a war evolves and more information is revealed we expect the willingness factors to hold greater sway. The above argument does not take into consideration who initiated the conflict and who was targeted. We believe the identity of the originators is important because it leads to the emergence of information that third parties may find pertinent when deciding whether or not to join and on which side. While there are cases when the initiator took the first military action because future and alternative prospects look bleak, it has been shown elsewhere that initiators are more likely to challenge weak targets (Gartner & Siverson 1996). Thus, potential joiners might assume that the initiator has private information on how the process of war will unfold in its favor. In addition, by striking first, the initiator can increase the chance that third parties will honor their commitments (Fearon 1995). While the international system has limited means for formally enforcing security commitments, if 6

12 potential joiners believe initiators target weak states, then those third parties that have the opportunity and/or willingness to participate are likely to do so quickly on the side of the initiator, perhaps because they expect the initiator to win quickly or because their expected costs are lower due to the expected shortened duration of the war. Therefore, the initial information that third parties have, on average, is that the initiator has an advantage over the target. The initiator s advantage results in third parties concluding that joining on the side of the target may be a more costly endeavor. As a result, the costs of not joining on the side of target need to be particularly high. Given that willingness is tied to a potential joiner s calculation of cost and benefit, this situation is likely to have a different influence on joining on the side of the initiator and joining the side of the target. If potential joiners believe the initiator has an advantage at the outset of a dispute, the willingness variables should increase joining on the side of the initiator more than on the side of the target. It is also necessary to consider how willingness factors influence the timing of joining. As discussed above, willingness refers to the motivations and goals of decision makers that encourage them to join a conflict. Since the initial information suggests that the initiator has an advantage, there is little reason to delay joining on the side of the initiator and thus, in general, states are more likely to join the initiator more quickly than the target. Therefore, the willingness factors will decrease the time until joining on the side of the initiator more than joining on the side of the target. It is also important to consider what would decrease the time until joining the target. If the costs of not joining the target are higher than joining the initiator, then for these states that would join the target, the time until joining should decrease. For example, if the security of the potential joiner is affected by not joining the target, this will encourage the third party to join. Thus, while the initial information may indicate that the initiator has an advantage, the strength of the initiator has implications on the security of the potential joiner. The opportunity variables are also influential in a potential joiners decision to join the target. If the initiator is particularly threatening, a potential joiner might decide that 7

13 joining the target is advantageous because at the end of a dispute, the initiator may be more powerful than when the dispute began, and is now more threatening to the potential joiner. The factors that decrease the time until joining on the side of the target should be in cases where not joining is likely to be particularly costly to the third party. Below we discuss several variables within the opportunity and willingness framework that influence a third party s decision to join an ongoing conflict. Although some of these factors have been considered in extant research, these factors have not been examined in relation to how they influence the timing of third party participation or the side of that participation. For each factor we discuss its influence on the risk (i.e., timing) of third party participation and, more importantly, how the risk of joining differs for the decision to join the initiator from the decision to join the target. Since we are going to estimate an event history model to conduct our empirical analysis, we present our hypotheses in terms of how each factor affects the risk of joining. If a factor increases (decreases) the risk of joining it decreases (increases) the time until joining is expected to occur. In general, we expect the opportunity variables to decrease the time until joining more than the willingness variables and the willingness variables to have a stronger effect on joining the initiator compared to the target. Opportunity We focus on capability and geographic proximity as the variables that influence a third party s opportunity to join. These factors have been put forward as the most physically apparent determinants of opportunity (Clark & Regan 2003, 101). Both are expected to influence the timing of third party joining and side on which that joining is expected to occur. Capability A potential joiner is likely to consider whether its capability contribution is sufficient to influence the dispute s eventual outcome and the time it will take the originator it joins to reach that outcome, preferably a victory. The realist tradition is particularly concerned about capability because of its connection to the balance of power. According 8

14 to balance of power theory, states are concerned with the acquisition of power by other states because it threatens their own security and survival and, as a result, states need to be concerned about their relative power. Since military conflict provides states with an opportunity to gain additional power which, in turn, could lead to a change in the distribution of power in the international or regional system, a third party is likely to consider the threat posed to its external security by an ongoing militarized conflict when making its joining decision (Haldi 2003). Stoll (1995, 136) asserts that findings across a variety of studies are consistent with the premise that states are motivated by basic realpolitik considerations when they decide to join or abstain from a war. Regardless of the distribution of capabilities among the originators, participation by a third party increases the probability of winning the dispute for the originator it joins, thus changing the dynamics of the dispute. For example, if there is an approximate balance of capabilities between the originators, then participation by even a relatively weak third party can shift the balance to one originator, potentially resulting in a victory by the joined side and a decrease in the time it takes to achieve that victory. When an originator has a large capability, this can sometimes dissuade a state from joining, because of the small increase in the probability of winning a potential joiner could make. For example, in the Russo- Ottoman War of 1878, Russia restrained several small Balkan states from joining on its side. Of particular importance in international relations theory are instances when a third party joins the weaker side in an imbalanced conflict. By joining on the weaker side, the third party is engaged in balancing against the powerful side and in doing so the third party s participation improves the weaker side s chances of victory. In this case, the motives for the third party are at least partially seen as preventing the stronger originator from becoming even more dominant. Although participation by relatively weak third parties can affect the dynamics of an ongoing conflict, the more powerful a third party is relative to the originators, the greater its ability to influence the outcome of the ongoing conflict. As a third party s capability 9

15 increases its opportunity for joining also increases. We expect that third parties with higher levels of material capability, relative to the initial participants, will be more likely to join an ongoing dispute (Altfeld & Bueno de Mesquita 1979) and will do so sooner than less capable third parties. If a potential joiner with enough capability to significantly alter the chances of victory for one side delays participation other third parties could join or the side it would join could become significantly weaker as the dispute persists. In either case a third party s ability to influence the outcome of the dispute decreases. Delaying entry is particularly relevant for third parties who would join on the side of the target, as initiators are more likely to challenge weak targets (Gartner & Siverson 1996), potentially reducing the length of the conflict and the time frame in which a third party must make its decision. Since initiators are more likely to challenge weak targets their victory might be seen as particularly threatening to the distribution of power globally or regionally. Walt (1987, 25) argues that states that are viewed as aggressive are likely to provoke others to balance against them. 2 By joining the target, a third party can help ensure the survival of the target and prevent the initiator from increasing its power, or at least, acquiring enough power to alter the status quo, all of which are vital concerns to states according to the realist tradition. While relatively more capable third parties may also join the initiator we expect there to be less urgency compared to joining the target. In sum, while we expect that relatively more capable third parties have a higher risk of joining an ongoing dispute than relatively less capable third parties, we expect the risk of joining to be higher for the decision to join the target than the initiator. Geographic Proximity In addition to increasing the opportunity to join, a third party s geographic proximity to the ongoing conflict is likely to influence its assessment of how threatening the conflict is to its external security. Third parties in close proximity to the conflict, especially contiguous ones, are more likely to be sensitive to local events than those farther away, and local conflict is naturally more threatening to these proximate states. A third party s geographic proximity to the originators increases the chances that a conflict will 2 Empirical support for whether states balance or bandwagon against external threats has not been conclusive (see Walt (1987), Schweller (1994), Werner & Lemke (1997), Reiter & Stam (2002)). 10

16 expand because it provides third parties, that are willing, with an opportunity to participate and preserve their external security (Most, Schrodt, Siverson & Starr 1990, Siverson & Starr 1991, Raknerud & Hegre 1997). This finding was illustrated in both of the Balkan Wars where many nearby states rushed to gain land first from the Ottoman Empire and then from Bulgaria. Additionally, transportation costs make participation in long distance conflicts more difficult due to the loss of strength gradient, and thus, proximate potential joiners have a higher likelihood of influencing a conflict than non-proximate third parties (Boulding 1962). In fact, it is hard to imagine Greece and Ethiopia making the trip to fight on the Korean Peninsula during the Korean War if it had not been for the transportation assistance that they received from the USA. In the case of a contiguous third party, its participation can open up an additional front that the side it aligns against will now need to defend, in turn, increasing the probability of victory for the side joined. Proximity could also present a third party with an opportunity to acquire additional territory by joining the conflict. Ancient Indian and Chinese as well as medieval European doctrines address a particular element of geographic proximity, the idea of geopolitical counterpoise (Doyle 1997, ). Under the geopolitical counterpoise premise, if state A is non-contiguous to state B but both states are contiguous to state C, then A and B should view each other as potential friends. The logic of geopolitical counterpoise had led Turkish policy makers during the late Cold War period to develop scenarios where it was envisaged that in a war between Turkey and Greece, Syria would not be far behind in joining against Turkey (Elekdağ 1996). We expect a third party s geographic proximity to the ongoing dispute to increase the risk of participation. We also expect third parties that are contiguous to the initiator to be more likely to join the target and third parties that are contiguous to the target should be more likely to join the initiator. Furthermore, we expect the risk to be higher for joining on the side of the target. This expectation follows from our earlier arguments about the initiator targeting weaker states and opportunity factors in general decreasing the time until joining. 11

17 By delaying, a third party is at risk of joining when the target is at a disadvantage and joining on the losing side might reduce its external security because of its now antagonized, contiguous, and perhaps stronger neighbor. Willingness Past research has primarily focused two factors thought to influence a third party s willingness to join: regime similarity and alliance commitments. We consider several additional factors: civilization similarity, joint membership in intergovernmental organizations, rivalry, territory, and joining by a previous major power that influence both the timing and side of third party joining in ongoing disputes. Regime Type States with similar domestic political institutions are likely to be more familiar with each other s decision making process, decreasing the uncertainty between such states and making it easier for them to build mutual ties. Past research shows that institutional similarity influences whether states engage in violent conflict with each other. Findings on the existence of both a democratic peace and possibly a dictatorial peace (Russett & Oneal 2001, Peceny, Beer & Sanchez-Terry 2002) are relevant with respect to which side a third party will join. Previous studies demonstrate that third parties are more likely to join states with similar domestic political regimes (Bremer 1992, Werner & Lemke 1997, Raknerud & Hegre 1997, Huth 1998, Reiter & Stam 2002). For example, Werner & Lemke (1997) find that institutional similarities are important in a state s decision to join and that there is little merit to the argument that opposites attract. In 1938, the president of the USA, Franklin Roosevelt, assured the British PM, Neville Chamberlain, that Great Britain would have the industrial resources of the American nation behind him in the event of war with the dictatorships (Thompson 2005, 673). The primary focus of past research has been on explaining the behavior of democracies. Democracies do not shy away from joining (Bremer 1992) and tend to join ongoing conflicts more frequently than autocracies (Raknerud & Hegre 1997). The two world wars and the Gulf 12

18 War are examples of democracies joining a coalition against a small number of autocracies. The extent to which autocratic third parties behave similar to democratic third parties in their propensity to join has been less explored. While it is debatable whether autocracies have the same set of attachments to each other as democracies, Raknerud & Hegre (1997) find that autocracies join fellow autocracies but to a lesser extent than democracies join with other democracies. We expect regime similarity to differ for third party decisions to join the initiator than the target. Mousseau (1997) finds that while democracies are more likely to join democratic initiators, there is no evidence that democracies are more likely to join democratic targets (see also Reiter & Stam (2002)). In part these findings indicate that leaders of democratic states want to avoid situations of risk that could lead to national (and potentially electoral) defeat. As initiators are more likely to target opponents that are weaker and as democracies are more likely to initiate conflicts that they expect to win (Bueno de Mesquita, Morrow, Siverson & Smith 1999, Reiter & Stam 2002), democracies might resist joining a democratic target but may not have the same reservations about joining a democratic initiator. Third parties are likely to perceive the chance of joining the losing side as being low in the case of joining a democratic initiator and high in the case of a democratic target. Therefore, we expect the time until democracies join democratic initiators to be shorter than the time until joining democratic targets. In terms of autocratic third parties, we expect the risk of joining to be similar for joining both an autocratic initiator and an autocratic target because autocratic leaders are less sensitive to the electoral costs associated with joining with one the originators and ending up on the losing side. Alliance The importance of alliances as a primary reason for conflict expansion has been widely examined (Altfeld & Bueno de Mesquita 1979, Siverson & King 1979, Levy 1982, Bueno de Mesquita 1989, Siverson & Starr 1991, Smith 1996). Siverson & Starr (1991), find that alliances increase the willingness of third parties to participate (see also Sabrosky (1980), Most et al. (1990)). 13

19 While alliances might increase the willingness of third parties to participate, whether third parties will honor their alliance commitments is the central question. Potential joiners are likely to consider the costs of maintaining their alliance commitments during an ongoing conflict. Third parties that fail to do so face potential reputation costs, the potential loss of an ally and a potential victory by a non-ally. Recent work suggests that states tend to honor their alliance commitments (Leeds 2003) but Gartzke & Gleditsch (2004) find that democracies are less likely to come to their allies assistance than non-democracies. As mentioned above, we believe that democracies are less likely to join democratic targets because of the risk of being on the losing side. In contrast, the incentives for democratic leaders to join democratic initiators is much higher and thus we expect democracies to join on the side of democratic initiators. Previous research has distinguished between three types of alliances; defense pacts, ententes, and non-aggression pacts. Defense pacts are likely to have the largest effect on third party decisions since they require members to militarily come to each other s aid in the event of a conflict. Non-aggression pacts, which require its members to remain neutral or at least not use force against other members in the event of a dispute, have been found to slow the expansion process, though with conflicting levels of support (Siverson & Starr 1991, Kim 1991, Raknerud & Hegre 1997, Kadera 1998). Ententes should have a smaller effect than defense pacts as they only require members to consult with each other if a dispute should occur. We expect defense pacts and ententes to increase the risk of third party participation, while non-aggression pacts should decrease the risk. While alliances have been put forward as a crucial element of the expansion process, existing research has not considered how the type of alliance influences third party participation differently for joining the initiator compared to the target. A potential joiner that has a defense pact with the initiator is likely to have a variety of common views on foreign policy. Delaying participating or not participating at all, will lead to reputation costs, which are likely to have implications beyond the current conflict. Moreover, by not acting quickly, the 14

20 potential joiner likely increases the problems for the initiator with which it shares a defense pact. While reputation costs also exist for potential joiners that have defensive pacts with the target, here the greater costs involved with participating on the target will also deter some countries from joining. We expect ententes to have a similar effect to defense pacts. However, the fact that ententes are likely to have less clear provisions than defense pacts on rules of engagement leads us to expect that the risk of joining the initiator will be lower than states with defense pacts. Finally, a third party that shares a non-aggression pact with the initiator is at a higher risk of joining on the side of the target. Similarly, a third party with a non-aggression pact with the target has a higher risk of joining the initiator However, there are relatively few non-aggression pacts, and the need to write such agreements in the first place leads us expect little influence from such agreements. Civilization Civilization dissimilarity has been posited to encourage the onset of interstate conflict. Huntington (1996) suggests that states that share civilization membership are likely to be friendlier with each other than states from different civilizations (Henderson 1997, Henderson & Tucker 2001, Russett & Oneal 2001). We expect states that share civilization ties to have similar norms, values and ideals. Consequently, shared civilization membership between an originator and a potential joiner is likely to influence its decision to participate. A third party that observes violent conflict erupt between a state with which it shares civilization ties and another state is likely to view the conflict as an attack not only on the friendly state but on the culture of the potential joiner itself. For example, the common Orthodox Christian civilization partly explains why the Ottoman Empire faced a hostile Czarist Russia in its dealings in the Balkans, particularly over Greece. In this way, if states within a civilization view conflict from an us versus them or an attack on the self by the other perspective, the presence of civilization ties increases the risk of joining. We expect civilization similarity to increase the risk of third party participation in general. We also expect that shared civilization membership between a third party and an originator will increase the risk that the third party will join on the side with which it shares civilization 15

21 membership, regardless of whether that side is the initiator or the target. However, the risk of participation is expected to be higher in the case of joining the initiator because of its capability advantage. After all, the Russian Czar, even as the protector of the Orthodox Slavs, chose not to come to the aid of many Balkan states when it did not seem prudent. Intergovernmental Organizations The role of intergovernmental organizations (IGOs) in conflict expansion has not been examined. Our focus is not solely on institutions that have a collective security or defense mechanism (e.g., the role of the UN in the Korean War and first Gulf War). Rather, we suggest that states that share overlapping membership in several IGOs are more likely to share common interests. We expect the common interests embodied in joint IGO membership to increase the risk of third party participation in ongoing disputes. While we expect shared membership in IGOs to increase the risk of joining in general, we expect the risk to be higher for joining the initiator. Shared membership between a potential joiner and the initiator suggests that both states might be in agreement over the threat posed by the target and may have already discussed the threat posed by the target within an IGO, leaving little reason for the potential joiner to delay joining. In terms of joining on the side of the target, a potential joiner has more flexibility in terms of when it joins and so we expect the risk to be lower. Rivalry Although not addressed in the conflict expansion literature, the presence of an entrenched hostility between a potential joiner and one of the original belligerents (i.e., rivalry) is likely to influence a third party s assessment of how threatening the ongoing conflict is to its external security and therefore its willingness to join. States involved in rivalries are likely to perceive each other as aggressive (Bennett 1996, Diehl & Goertz 2000, Thompson 2001). Therefore, when the rival of a third party is involved in a conflict with another state its willingness to become involved will increase as it can inflict costs on its rival and/or prevent the rival from acquiring additional power by joining the conflict on the side opposite the rival. By joining a rival s adversary a potential joiner can improve its external security. The enemy of my enemy is my friend rationale leads us to believe that 16

22 third parties will take the chance to capitalize on the possibility of dealing a decisive blow to an adversary even if there is not a good relationship between the potential joiner and the other conflict participant. In this case, a third party does not need to feel any particular fondness for the side it joins but joins anyway because of a strong dislike for the opposing side, its rival. 3 We expect the presence of a rivalry to increase the risk of a third party joining the conflict and that the risk is higher for joining on the side opposite its rival. If the rival of the potential joiner is the initiator (target), the potential joiner is more likely to participate on the side of the target (initiator). In either case, third party participation is likely to occur quickly, particularly if delay increases the prospects of victory for its rival originator. But, following our previous argument about the initiators advantage, we expect the risk to be higher for joining the side of the initiator than the target. Territory Another factor not addressed in the conflict expansion literature is the issue in dispute between the originators. The presence of a dispute over territory increases the probability of interstate conflict (Vasquez 1996, Huth 1996). Third parties should be especially concerned about conflicts over territory, since territorial exchanges can result in a transfer of power as states obtain additional strategic locations and access to natural resources. Indeed, there is a general aversion in the international community to land transfers by force, particularly in the late twentieth century. It is not surprising that Iraq faced one of the largest coalitions ever assembled during the first Gulf War following its occupation of Kuwait given that the invasion was largely perceived to be a land grab. Thus, when a conflict involves the issue of territory, we expect that third parties are more likely to participate. Following our previous argument about initiator advantage, we expect third parties to have a higher risk of joining on the side of the initiator than the target. Previous Major Power When making their decisions about whether or not to join an ongoing conflict, third parties are likely to consider the participation of other third parties, 3 While Kim (1991) finds that third parties are more likely to join friends than to oppose foes in ongoing conflicts, he relies on similarities in alliance commitments and does not consider rivalries. 17

23 especially if those other third parties are influential states in the international system and have the potential to dramatically influence the outcome of the conflict. Along these lines, we focus on how the prior participation by major powers influence the willingness of other third parties to join. Past research has shown that states with more material capability are more likely to participate in ongoing conflicts (Altfeld & Bueno de Mesquita 1979, Bremer 1980, Huth 1998). When a major power joins an ongoing conflict, it is likely to lead other third parties to reevaluate their decision to participate (Yamamoto & Bremer 1980). Thus, in deciding whether or not to participate, potential joiners will not only consider the originators of the conflict but also whether a major power has previously joined. While this argument has not received much empirical attention, Yamamoto & Bremer (1980) examine how major power joining into an ongoing conflict influences other major powers decisions to participate. Yamamoto & Bremer (1980) find that a major power s decision to join is highly dependent on the participation of other major powers (Kim 1991). However, the decision of a major power to participate influences the decisions not only of other major powers, but all other potential joiners as well. An examination of both the Korean and Vietnam Wars shows that many states joined these wars due specifically to the participation of the USA. Once a major power joins a war, states with relatively less capability might conclude that the side the major power joined has dramatically increased that sides probability of winning. In this case, less capable states might believe they can gain by participating on the same side that the major power joined. For example, in return for joining the side the major power joined in an ongoing conflict, a minor power may increase the likelihood of obtaining benefits such as, gaining support from that same major power in any future conflicts in which the minor power becomes involved, increased trade, financial aid or inclusion in an international organization such as NATO. Thus, participation by a major power should increase the probability of participation by other potential joiners on the same side. And, since participation by a major power will likely reduce the length of the dispute and thus the amount of time during which additional third parties may obtain any 18

24 gains offered by participation, the risk of joining is higher when major power third parties join. The side a major power joins leads us to expect third parties to display different behavior for joining the initiator compared to the target. In the case of joining on the initiator, previous major power participation on the side of the initiator is likely to increase the risk of joining for other third parties. In the case of joining the target, the risk of participation is also higher following major power participation on the target but the risk is smaller compared to joining the initiator because the potential costs of joining are higher. Given that major powers have a greater propensity to initiate conflict (Gochman & Maoz 1984), participation by a major power on the side of the target might make some states fear that the dispute will last longer and be more deadly, thus delaying joining until uncertainty about the outcome is reduced. Research Design We use the militarized interstate dispute (MID) data set (Ghosn, Palmer & Bremer 2004) to identify a sample of disputes, the timing of third party participation (if any) in those disputes, and the side on which that participation (if any) occurred. 4 Our sample only includes MIDs that reached a level of 4 or 5 (i.e., use of force or interstate war ) and that last for more than one day between 1816 and We use these MID levels because they indicate that military force has been used. These are the most serious disputes and we expect third party decisions to be important during these disputes. We follow the MID codings in identifying the initiator, target, and third party participants. The initiator is defined as the side that took the first codeable militarized action, while the target is defined as the recipient of that action (Ghosn, Palmer & Bremer 2004). 5 A state is coded as a third party participant if it 4 All of the data used in this article, unless otherwise specified, was complied using EUGene (Bennett & Stam 2000). 5 The label initiator may not truly reflect who initiated the dispute in the broader sense of instigator. 19

25 participated militarily in the dispute after the first recorded day of the dispute. 6 Our research design improves upon previous research in two important ways. First, we use a triadic approach to examine third party participation in ongoing MIDs (Bremer 1992, Mousseau 1997, Reiter & Stam 2002). Most extant research has only included those third parties that actually joined a MID (Werner & Lemke 1997, Siverson & Starr 1991), which is problematic because important information about third parties that could have joined the MID but chose not to is discarded. Our data set includes all third parties that could potentially join a MID. We restrict our attention to those third parties that are politically relevant. 7 We define a third party as politically relevant if 1) the third party is contiguous to one of the originators or 2) the third party or one of the originators is a major power (Reiter & Stam 2002). For each year in each MID we include both the initiator, the target, and all politically relevant third parties (e.g., a politically relevant triad). For example, consider World War I, in which Austria-Hungary was the initiator and Serbia was the target. For each year of the MID, Greece is a potential joiner until June 29, 1917, when it joins Serbia and Switzerland is a potential joiner until November 11, 1918 because it did not join either side by the end of the war. Our sample includes 91,195 politically relevant triads. Our unit of analysis is the mid-triad-day. Our second improvement is that we distinguish between joining the initiator and joining the target. As the labeling of the two originators highlights, different processes are likely to be at work for a third party s decision to join the initiator compared to the target. Prior research has focused primarily on third party participation in general (Siverson & Starr 1991), failing to capture the crucial distinction between joining the initiator and joining the target. To the extent that the decision to join the initiator is different from the decision to join the target, our covariates should have a different effect on the risk of joining the initiator 6 We also include instances where a third party joined a dispute because it was invaded. These third parties are coded as joining even though they were not willing participants. The number of such cases is relatively small and primarily occurred during the world wars. 7 We focus our attention on politically relevant third parties in order to avoid inflating our sample by including third parties for which there is only a small probability of joining. 20