The conditional impact of military intervention on internal armed conflict outcomes

Abstract

Previous studies of internal armed conflict outcomes have found evidence that rebel-biased military intervention increases the likelihood of rebel victory, but little indication that pro-government interventions improve the odds of government victory. Our argument, grounded in a theory of the utility and limitations of military force in civil wars, anticipates that armed intervention increases the probability of victory for the supported side only when that belligerent’s primary challenge is a lack of conventional war-fighting capacity. Empirical analyses of internal armed conflicts from 1945 to 2010 support these expectations. Direct interventions in support of opposition movements have substantively large, robust effects on conflict outcomes. In contrast, government-biased interventions are only effective in increasing the odds of an outcome favorable to the government when the fighting capacity of rebel forces matches or exceeds that of the state.

Keywords

Civil conflict insurgency military intervention utility of force war outcomes

Under what conditions can armed intervention increase the likelihood that a government will prevail over a domestic opposition movement? How does external support provided to the rebels affect their odds of success? In the past two decades, scholarly interest in civil conflicts has swelled, producing a wealth of qualitative and quantitative evidence on important questions relating to the onset, duration and termination of civil wars. However, we are only beginning to systematically explore the determinants of civil war outcomes. In particular, there are only a handful of studies that investigate how foreign military intervention affects who prevails in internal conflicts.

In this paper, we develop an argument about the potential effects of foreign assistance on each side’s ability to prevail in an armed conflict that is grounded in a theory about the utility and limitations of military force in civil war. Superior military capacity is not sufficient to defeat rebel threats, but it is essential. Just as in wars between states, military force can be employed to accomplish a variety of tasks—disrupting enemy supply lines, physically defending territory and populations, attritting opposition soldiers—vital to attaining a military victory over a domestic adversary. While the US Army’s most recent counterinsurgency field manual, FM 3-24, maintains that “the primary objective of any COIN operation is to foster development of effective governance by a legitimate government” (United States Department of the Army, 2006: 1–21), we argue that the principal function of military intervention in violent conflicts is to provide additional military capacity to one of the belligerents. Consequently, foreign support increases the probability of victory for the supported side only when the main obstacle to strategic success is lack of military capacity.

Our theory is consistent with the extant evidence that foreign intervention benefits rebels but does not appear to systematically advantage incumbents, and also generates an additional, novel hypothesis. Government-biased foreign intervention should increase the likelihood of government victory in civil wars only when the opposition’s war-fighting capabilities match or exceed those of the incumbent regime.

To test our hypotheses, this study takes advantage of new data available on the attributes of rebel groups and external assistance provided to governments and opposition movements in armed conflicts between 1945 and 2011 (Cunningham et al., 2013). Most empirical studies have found that government-biased intervention has no effect on the probability of government victory (Dixon, 2009; Gent, 2008). Our results, however, indicate that both rebel and government-biased intervention can have a powerful impact on the likelihood that the supported side prevails under certain conditions. In particular, we find that combat assistance provided to the opposition dramatically increases the likelihood of a rebel military victory and reduces the probability of indecisive, “low activity” outcomes, regardless of the insurgents’ capacity. In contrast, military assistance provided to an incumbent regime affects civil war outcomes only when the rebels are relatively strong. Most governments have the material capacity to neutralize rebel threats without external assistance, and deploying combat troops to support a government battling militarily weak insurgents does not increase the odds of government survival. In those relatively rare cases in which an opposition movement is able to mount a serious military challenge to the central government, however, armed intervention on behalf of the government is a critical determinant of who prevails.

Contradictory evidence on the utility of foreign combat support

There is a consensus in the literature about the importance of external support for insurgent organizations. Although only a handful of quantitative studies evaluate how external intervention on behalf of rebel movements affects who prevails in the conflict, all of the studies find evidence that rebel-biased foreign involvement increases the probability that the outcome will be favorable to the rebels (Akcinaroglu, 2012; Balch-Lindsay et al., 2008; Gent, 2008). Because nonstate actors do not have access to the revenue and armed forces of the state, many rebel groups are unlikely to survive unless they receive material support from foreign governments (Regan, 2002). External patrons can provide rebels with funding, weapons, training and personnel, significantly augmenting their ability to inflict and absorb costs (Mason et al., 1999; Salehyan et al., 2011). A RAND report on how insurgencies end calls external support for intervention “critical” and claims that the type and regularity of such support often determine the outcome of civil conflicts over governance (Connable and Libicki, 2010). Jeffrey Record (2007), a professor at the Air War College who served as an advisor to the South Vietnamese military during the Vietnam war, maintains that the assistance of outside actors is the single most essential ingredient for insurgent success.

In theory, the assistance of a third-party state also has many potential benefits for an incumbent regime. Studies have found that governments with high military capacity are better able to monitor and suppress violent opposition groups (Bapat, 2005; Brandt et al., 2008; Fearon and Laitin, 2003; Herbst, 2004). Connable and Libicki (2010) and Mason et al. (1999) provide evidence that government strength is positively correlated with a government’s ability to defeat rebel movements. A foreign intervention in support of the government could boost government capacity enough to convince potential recruits to the rebel movement that the costs and risks of joining the rebellion are too high (Downes, 2007). Foreign troops can give weak government forces increased capacity to locate and capture or kill rebel combatants by providing more personnel and increasing their reach into more remote areas of the country. Foreign assistance might also improve a regime’s capacity to provide population security and address grievances by delivering public goods and services, tasks both US military doctrine on counterinsurgency operations and scholars emphasize (Fjelde and De Soysa, 2009; Kalyvas, 2006; Kalyvas and Kocher, 2007; Taydas and Peksen, 2012).

The empirical case for the effectiveness of government-biased intervention is more equivocal. In one of the few studies specifically designed to test the effects of foreign intervention on who prevails in civil wars, Gent (2008) concludes that government-biased foreign intervention is uncorrelated with government victory. Balch-Lindsay et al. (2008) find that civil wars end in government military victory more quickly when external military support is provided to the incumbent government. However, their pro-government intervention variable is only marginally significant and the impact of intervention decreases over time until the direction of the effect reverses. Results from Akcinaroglu (2012) suggest that government-biased intervention reduces the probability of rebel victory but has no effect on the odds of government victory. Other research provides evidence that foreign intervention on the side of a government can increase the intensity of violence and prolong intrastate conflicts (Balch-Lindsay et al., 2008; Cunningham, 2010; Lyall and Wilson, 2009; Regan, 2002; Wood et al., 2012).

The conditional effects of foreign intervention

An existing theoretical argument by Stephen Gent provides one explanation for the apparent contradiction between the effectiveness of foreign military intervention on behalf of opposition movements and incumbent governments. Gent (2008) maintains that rebel-biased military interventions appear to be more effective than government-biased interventions because third-party interveners tend to intervene when opposition forces are strongest and therefore most likely to prevail ex ante. States are more likely to intervene in support of the government when it is facing a militarily capable rebel group that presents a genuine threat to the survival of the regime. At the same time, states that support the opposition movement will be more likely to intervene when the rebels are strong and have a real chance of defeating the government with external assistance. The result is that we observe foreign military interventions in those civil war cases in which rebel victory is most likely, which creates the appearance that only rebel-biased intervention improves the probability that the supported side will win.

We propose an alternative explanation that is consistent with the extant evidence that foreign intervention benefits rebels but not incumbents, and also generates an additional, novel hypothesis. Our theoretical expectations are derived from several important characteristics of civil wars that are already well established in the literature.

First, most, but not all, civil wars are characterized by an asymmetric distribution of military capacity that favors the incumbent regime. In their typology of civil wars, Kalyvas and Balcells (2010) code 53% of civil wars between 1944 and 2004 as asymmetric wars in which the military capacity of the rebels is substantially lower than that of the state. In the Non-State Actors in Armed Conflict Dataset (NSA), opposition forces are coded as having warfighting capacity equal to or greater than that of the government in only 14% of conflicts. In 37% of conflicts, the rebels are coded as “much weaker” than the state (Cunningham et al., 2013).

Second, opposition movements that are weaker than the incumbent regime tend to adopt unconventional military strategies and tactics such as terrorism and guerilla warfare to avoid exposing themselves directly to the risk of annihilation by superior government forces (Crenshaw, 1981; Kalyvas and Balcells, 2010). In contrast, although the government can adopt indirect, coercive military strategies in its attempts to weaken the rebel movement and discourage the population from supporting the opposition, they must adopt direct defense strategies to hold territory, defend vital infrastructure and protect personnel (Arreguín-Toft, 2005). As Butler and Gates (2009: 337) note: “government always presents a non-elusive target”.

Indirect warfighting strategies act primarily through imposing costs on an opponent over time in the hope that the opponent will decide that the costs are too high and choose to make concessions to end the conflict. This can be an effective strategy for opposition movements. The advantage of the low-tech, dispersed, clandestine organizational structure of guerilla forces and terrorists is the ability to impose costs while evading detection and elimination by superior armed forces (Beckett, 2001; Buhaug et al., 2009; Cunningham et al., 2009; Hultquist, 2013). However, in most cases, rebels cannot win a decisive victory over the government unless they gain enough military strength to transform themselves into a conventional force capable of directly challenging state security forces and forcibly seizing control of territory and the infrastructure of state (Butler and Gates, 2009). When actors must use direct offensive or defensive military strategies because they are attempting to seize, destroy or defend territory and physical assets, destructive capacity trumps motivation and the physically stronger actor generally prevails (Sullivan, 2012).

Building on these observations, we argue that the apparent disparity between the effectiveness of foreign intervention on behalf of incumbents and opposition movements is not simply the result of a selection effect. Superior troop strength, technology, equipment and operational capabilities are critical assets for government forces. In the absence of foreign intervention to assist either side, rebel military victory should be rare because nonstate actors rarely have the war-fighting capacity of incumbent regimes. Government should prevail in most domestic armed conflicts, although rebel groups may survive to engage in low to moderate levels of violence against the regime over prolonged periods of time.

For rebel movements, which are typically at a distinct disadvantage when confronting a state apparatus with much greater resources, external assistance can decisively alter the military balance between opposition and government forces. Third-party intervention can increase an opposition movement’s destructive capacity and war-fighting effectiveness, providing better trained and organized troops and access to heavy weapons and equipment that can otherwise be difficult for nonstate actors to acquire. Because incumbents that want to remain in power must adopt a conventional military strategy to defend territory and seats of power, incumbents make good targets for the conventional military forces of an intervening state. A regime cannot defend the capital, maintain control over the institutions of state and hold territory in the face of an invading army—or an indigenous opposition movement augmented by the military capabilities of a third-party state—by hiding out in remote terrain, employing hit-and-run tactics, and evading head-to-head combat. Nor can opposition movements seize control of the state by employing only guerilla tactics. Incumbent governments are difficult for relatively weak rebel movements to remove with force alone, but overthrowing an incumbent regime becomes significantly more feasible with assistance from the armed forces of a third-party state. Military intervention on behalf of an opposition movement should significantly increase the rebels’ initially low probability of military victory over the incumbent regime.

There are more significant limitations on the utility of foreign combat assistance provided to an incumbent regime. While weak rebels typically employ irregular warfare strategies, the armed forces of states powerful enough to project force beyond their borders are usually structured, trained and equipped to fight conventional campaigns against enemies with similar force structures (Butler and Gates, 2009; Byman, 2008; Lyall and Wilson, 2009). As a result, the destructive capacity of powerful intervening states can be especially ineffectual against weak guerilla forces (Arreguín-Toft, 2005; Krepinevich, 1986; Lyall and Wilson, 2009; Record, 2005, 2007). Rebels adopting a guerrilla warfare strategy can avoid head-to-head military contests with superior forces, relying instead on hit-and-run attacks, strikes on soft targets and remote detonation of explosive devices or long-range sniping to inflict casualties (Biddle and Friedman, 2008). Because they do not need to conduct pitched battles, they can disperse their forces and seek concealment in territory that is difficult to access—dense forests, mountainous terrain and remote, undeveloped areas of the country—or among the civilian population (Beckett, 2001; Buhaug et al., 2009). This makes identification of combatants problematic for counterinsurgent forces and especially difficult for the armed forces of intervening states that typically lack even the local knowledge and contacts within the population that government forces may have (Krepinevich, 1986; Lyall and Wilson, 2009).

In addition to these structural constraints, foreign interveners often have an inherent legitimacy deficit that can inflame nationalist sentiment and give disparate rebel groups a unifying cause (Edelstein, 2008; Byman, 2006; Galula, 1964; Lyall and Wilson, 2009; Nagl, 2002). Even if foreign intervention does not directly damage the perceived legitimacy of the host government, intervention is not likely to improve the popularity of a regime that is perceived as corrupt, repressive or unable to provide good governance. If we assume that third parties do not want to pay the costs of military intervention to prop up a regime unless the regime is likely to fail without assistance, interventions into conflicts between incumbents and militarily weak opposition movements are likely to be interventions on behalf of regimes with grave domestic political issues. When, despite its military superiority, a government is unable to prevail over a rebel movement, the key barriers are likely to be political—corruption, strong popular discontent, divisions within the regime—rather than military, and additional war-fighting capacity is unlikely to improve the regime’s position.

The civil war in the Republic of Congo in the late 1990s illustrates these dynamics. In this conflict, two relatively small rebel groups, the Ninjas and Cocoyes, mostly conducted hit-and-run attacks against government forces and intervening troops from Angola. According to observers, both groups were “weak, unorganized, and fragmented” (Englebert and Ron, 2004). Yet, even with the support of the (comparatively) powerful Angolan forces, the Congolese government was not able to completely defeat and destroy these militias. While a peace deal was signed in 2003, low-level violent conflict lingered; armed fighters remained at large in some regions and the government was not able to establish sufficient security to begin reconstruction work (Democracy Coalition Project, 2007). Despite the fact that the militias were ill-equipped, disorganized and often at war with each other, they presented a challenge to the relatively strong Angolan forces because they were difficult to identify and target and were comparatively well integrated with the local population, who, at least in part, viewed them as defenders of their region and communities (Englebert and Ron, 2004: 68).

Despite the limitations of external intervention on behalf of beleaguered governments, there are times when the benefits of receiving foreign combat assistance outweigh the disadvantages of government-biased intervention. Opposition movements with greater military capabilities are less likely to adopt guerrilla warfare strategies and less able to survive by blending into the population or hiding out in rough terrain (Balcells, 2011; Kalyvas and Balcells, 2010). For strong rebel groups, the optimal strategy is to confront the government in battle and use their strength to their advantage to press their demands (Butler and Gates, 2009; Byman, 2008). Once rebel command and control infrastructure, troop numbers and military equipment and weaponry rival that of the state, it becomes harder for the group to elude contact with government forces. While rebel military victory becomes more likely if a rebel group has access to heavy weaponry and can challenge the armed forces of the state in direct battle, this military strength also makes it more difficult for the group to hide out among the population or in inaccessible terrain. Consequently, strong rebels make better targets for the conventional military capabilities of intervening states.

We therefore predict that pro-government foreign intervention is only likely to be effective—that is, to improve the government’s odds of overcoming the rebel threat—when the insurgents have substantial military capacity. Governments that are unable to prevail over militarily strong rebels have a military problem that foreign intervention could help to ameliorate, even if a lack of military capabilities is not the government’s only vulnerability.

Empirical analysis

Our empirical analysis explores the conditions under which foreign military intervention in support of either belligerent in a violent intrastate conflict changes the expected outcome of the conflict. In this analysis, we consider all violent civil conflicts between 1945 and 2010 as identified in version 3.3 of the NSA data. This dataset supplements the Uppsala Conflict Data Program/Peace Research Institute Oslo (UCDP/PRIO) Armed Conflict Data (ACD) with information about the characteristics of rebel groups and the external support provided to both rebel groups and governments (Cunningham et al., 2009, 2013). The cases in the ACD dataset are episodes of domestic political violence between a government and an insurgent group that result in more than 25 conflict-related battle deaths per year (Gleditsch et al., 2002; Themnér and Wallensteen, 2012).

Unit of analysis

While much of the civil war literature has focused on the impact of state-level characteristics, more recent studies have moved toward modeling civil conflicts as dyadic struggles between a government and a violent opposition movement. Building on Cunningham et al. (2009) and Harbom et al. (2008), our unit of analysis is a coherent dyadic episode of domestic armed conflict. We capture the effects of third-party involvement in these conflicts by including right-hand side variables that measure external support provided to each side.¹ Because we are interested in the outcome of each violent challenge to an incumbent regime, when multiple conflicts are ongoing within one country at the same time we retain each government-rebel dyad as a separate observation as long as each rebel group acts independently of other groups and pursues a distinct objective vis-à-vis the government. In order to create a set of comparable cases likely to be driven by similar dynamics, we exclude extrastate armed conflicts, in which a government is fighting a nonstate actor located entirely outside its own home territory (e.g. colonial wars). We identify 425 conflict dyads in the NSA dataset; missing data on some covariates reduces this number to 363 cases in a model with all of the control variables.

Dependent variable

The vast majority of the quantitative literature on civil war outcomes has used a dependent variable with two categories, distinguishing between wars that end in negotiated settlements and wars that end in a military victory for one side or the other.² A few studies disaggregate the latter category to distinguish between rebel and government military victory.³ Following Cunningham et al. (2009) we use a four-category dependent variable coded from the Outcome and VicSide variables in the dyadic version of the UCDP Conflict Termination dataset v. 2010-1 (Kreutz, 2010). The most common outcome is for a conflict dyad to fade into a period of “low activity” in which fatalities from the conflict fall below 25 deaths per year for at least one year. Almost 45% of the conflict dyads in our data terminate this way. The second most common outcome, government victory, is coded when the rebel group is militarily defeated or capitulates. The government is victorious in 97 (23%) of our cases. Rebel victory occurs in just 12% of cases and 20% are terminated by some form of negotiated settlement.

We test two hypotheses with this dependent variable. The first, consistent with previous research, anticipates that foreign military intervention on behalf of the opposition will increase the probability of a conflict outcome favorable to the rebels by significantly augmenting rebel military capacity. Combat assistance is expected to increase the probability that the rebels will prevail or attain concessions in a negotiated settlement, and decrease the probability the rebels will be decisively defeated or degraded to the point that they can no longer conduct significant attacks. The second hypothesis, a novel conjecture derived from our theoretical argument, anticipates that foreign military intervention in support of the incumbent government will increase the likelihood of conflict outcomes favorable to the government only when the rebels are relatively strong. We expect pro-government intervention to increase the odds the government can militarily defeat the rebel threat and decrease the likelihood of rebel military victory when insurgent military capacity approaches that of the state. Intervention in support of the government should not improve the government’s position when rebels are relatively weak.⁴

Hypothesis 1: Foreign military intervention on behalf of the opposition will increase the probability of both rebel military victory and negotiated settlements and decrease the probability of either a low activity outcome or a government military victory.

Hypothesis 2: Foreign military intervention on behalf of the government will increase the probability of government military victory and decrease the probability of rebel military victory only when opposition military strength approaches that of the state.

Independent variables

In order to test our hypotheses, we need measures of foreign military intervention on each side of a conflict and rebel military capacity relative to the government. A government-biased military intervention involves the use of armed force by another state in an attempt to preserve the governing authority of an incumbent regime. We code the pro-government intervention variable from the NSA data as a dummy variable set to 1 if another state sent military troops to support the incumbent government, and 0 otherwise. We construct another dichotomous variable indicating that a state committed troops to assist the rebels.

Unfortunately, the NSA dataset does not include more detailed information on the scale of each intervention. Aggregating small-scale military interventions and more robust missions into a single dichotomous indicator could bias our analysis against finding a statistically significant relationship between intervention and conflict outcomes. However, it appears unlikely that minor interventions are captured in this measure, as the NSA dataset codes only 11% of the conflicts as involving third-party intervention with military force to assist the incumbent regime. Rebel-biased military intervention is even less common than government-biased intervention, occurring in only 5% of our cases. To test the robustness of our results, we also estimate our model with dichotomous variables coded from the Uppsala Conflict Data Program’s External Support Project dataset version 1-2011 (Högbladh et al., 2011). In this dataset, variables indicate whether a state “sent combat troops to fight alongside” the government or the opposition (Croicu et al., 2011). These data are only available from 1975 to 2009, restricting our sample size for the secondary analyses.

We expect rebel military capacity relative to the government to be a key determinant of the ability of insurgent forces to mount a successful challenge to the government, and an important factor influencing the likelihood of external intervention (Gent, 2008; Hultquist, 2013; Salehyan et al., 2011). A variable from the NSA dataset codes overall rebel strength relative to the government in five categories ranging from much weaker to much stronger. Relative strength is a summary measure intended to capture the rebels’ fighting capacity and ability to mobilize supporters and procure weapons. The disadvantage of this measure for our analysis is that it conflates fighting capacity and mobilization capacity, while our theory is focused on the effect of rebel material strength and war-fighting capabilities. A separate variable in the NSA data measures rebel fighting capacity relative to the government, but the three coding categories are more ambiguous. The majority of rebel forces are coded as having “low” fighting capacity relative to the government, 30% as having “moderate”, and only 3% as having “high” fighting capacity. In order to identify those opposition movements most likely to have the personnel, weaponry and equipment to present a serious military challenge to the government, we code a dichotomous measure for militarily strong rebels for all rebels with “high” fighting capacity, and the proportion of rebels coded as having “moderate” fighting capacity plus at least equal strength relative to the government according to the relative strength variable. By this measure, rebel forces are at least as strong as government forces in 14% of conflicts.

Our dichotomous indicator of rebel strength relative to the government does not allow us to distinguish the effects of strong rebels from the effects of weak governments. Our measure may indicate rebel strength because the opposition has exceptional war-fighting capacity, but it could also code the opposition as relatively strong because the government is unusually weak. In an attempt to parse out these effects, we include the natural log of GDP per capita for the conflict country as a control (Gleditsch, 2002). In order to test whether the effects of foreign intervention vary with the strength of insurgent forces (Hypothesis 2), we create an interaction term by multiplying the dichotomous variable indicating rebel fighting capacity by our government intervention dummy variable.⁵

Control variables

Based on previous studies, we identify a handful of variables that may be correlated with both the likelihood of foreign intervention and civil war outcomes. We create a variable indicating that the rebels are demanding secession, rather than the replacement of an incumbent regime, because scholars have found evidence that both civil war outcomes and the probability of external intervention are influenced by rebel demands (Balch-Lindsay and Enterline, 2000; DeRouen and Sobek, 2004; Mason et al., 1999; Regan, 2000; Saideman, 2002). Similarly, previous research has indicated that conflict outcomes are affected by whether the challenge to government authority is initiated by a faction of the military or mobilized from the civilian population (Cunningham et al., 2009). Coup threats should be easier to avert than more broad-based insurgencies. At the same time, because coups typically occur without warning and either fail or succeed in a short window of time, we expect foreign interveners to have less opportunity to intervene on behalf of the government. Both secession and coup indicators are coded from the NSA data.

Previous studies find that decisive military victories have become less common and there are more negotiated endings to civil conflicts since the end of the Cold War because the international community has become more willing and able to mediate conflicts (Fortna, 2009; Kreutz, 2010; Toft, 2010). We include a dummy variable for those conflicts that end after 1989 as a control for the possibility that state intervention behavior has changed and domestic armed conflicts terminate in systematically different ways in the post-Cold War era.

Finally, a number of scholars suggest that the duration of a conflict is negatively correlated with government success because rebels are most vulnerable early in a conflict, but gain capacity over time (e.g. Brandt et al., 2008; DeRouen and Sobek, 2004; Mason et al., 1999). Previous research has also shown that the duration of a conflict is correlated with foreign intervention because long conflicts are more likely to draw in additional states and because intervention on either side of the conflict may prolong the fighting (Akcinaroglu and Radziszewski, 2005; Cunningham, 2010; DeRouen and Sobek, 2004; Elbadawi and Sambanis, 2000; Regan, 2002). We control for duration dependence by including the natural log of the duration of a conflict in years, again taken from the NSA data.

Main findings

To assess our hypotheses, we estimate multinomial logit models of the simplified form:

\Pr (Y_{i} = j) = \frac{{\exp (X}_{i} β_{j})}{\sum_{j = 1}^{J} {\exp (X}_{i} β_{j})}

Here, j indexes each of the four outcomes—government victory, rebel victory, settlement, and low activity; i indexes our unit of analysis, a coherent dyadic episode of domestic armed conflict. We omit low activity (the most common outcome) as the baseline category and report coefficient estimates for each of the remaining outcomes: government victory, settlement and rebel victory. The coefficients then summarize the relationship between the X matrix and the odds of each of these outcomes relative to a low activity outcome.

Table 1 displays estimation results for three multinomial logit models. Model 1 has only one independent variable: the dummy variable indicating that a third-party state intervened to provide combat support to the government. The variable is not a statistically significant predictor of any of the outcome categories. Model 2 adds a variable indicating whether rebel fighting capacity was at least equal to that of the government and the interaction between rebel fighting capacity and government-biased intervention. Model 3 adds our indicator of military intervention on behalf of the opposition and the control variables discussed above in an attempt to isolate the effects of intervention and rebel strength from potential confounding factors.

Table 1.

Determinants of internal armed conflict outcomes (multinomial logit estimates)

	Model 1	Model 2	Model 3
Rebel Victory compared with Low Activity
Pro-government Intervention	0.195 (0.541)	−0.325 (1.067)	−2.127 (1.288)
Strong Rebels		3.849 (0.511)	2.897 (0.615)
Intervention × Strong Rebels		−0.591 (1.445)	0.873 (1.667)
Pro-rebel Intervention			2.946 (0.948)
Coup Threat			2.512 (0.794)
Secessionist Conflict			−3.136 (1.101)
GDP per capita (log)			−0.300 (0.263)
Duration (log of years)			0.173 (0.250)
Post-Cold War			0.263 (0.457)
Constant	1.294 (0.163)	−2.240 (0.248)	0.070 (1.969)
Negotiated Settlement compared with Low Activity
Pro-government Intervention	0.551 (0.420)	−0.060 (0.545)	−0.281 (0.678)
Strong Rebels		0.713 (0.622)	0.750 (0.697)
Intervention × Strong Rebels		1.341 (1.154)	1.896 (1.269)
Pro-rebel Intervention			1.602 (0.819)
Coup Threat			−12.83 (543.1)
Secessionist Conflict			−0.161 (0.314)
GDP per capita (log)			0.037 (0.161)
Duration (log of years)			0.234 (0.144)
Post-Cold War			1.759 (0.363)
Constant	0.861(0.139)	−0.896 (0.143)	−2.560 (1.258)
Government Victory compared with Low Activity
Pro-government Intervention	0.835 (0.384)	0.606 (0.432)	0.620 (0.586)
Strong Rebels		0.080 (0.720)	−1.107 (0.883)
Intervention × Strong Rebels		1.003 (1.178)	1.835 (1.344)
Pro-rebel Intervention			0.349 (0.876)
Coup Threat			2.721 (0.676)
Secessionist Conflict			−0.265 (0.336)
GDP per capita (log)			0.169 (0.154)
Duration (log of years)			−0.524 (0.184)
Post-Cold War			−0.754 (0.334)
Constant	0.770 (0.134)	−0.773 (0.137)	−1.497 (1.189)
N (Internal armed conflict dyads)	425	425	363
LR χ ²	5.14	108.73	258.20
Pr > χ ²	0.1617	0.0000	0.000
AIC	1093.669	1002.085	741.945

Standard errors in parentheses. The unit of analysis is the armed conflict dyad.

LR, Likelihood ratio; Pr, probability; AIC, Akaike information criterion.

Owing to the structure of the multinomial logit model, our nonlinear outcome variable, and the added complication of an interaction term, the substantive effects and statistical significance of the factors in our model are not easily discernible from the coefficients and standard errors reported in the table. We therefore proceed directly to evaluating the statistical significance of each of our independent variables and discussing substantive effects in the form of first differences. For model 3, Table 2 presents changes in the predicted probability of each conflict outcome as we simulate a change in one element of the model while holding all other factors constant at their median value. Each of the dichotomous predictor variables is varied from 0 to 1, and the two continuous variables are adjusted from the 25th to the 75th percentile using Clarify (Tomz et al., 2003). We calculate the effect of military intervention when the rebels are relatively weak by holding the rebel fighting capacity and rebel strength × intervention interaction terms at zero and varying only the military intervention variable in our simulations. To estimate the effect of government-biased intervention when the opposition is strong, we hold the rebel fighting capacity variable constant at 1 and simultaneously change both our pro-government intervention variable and the interaction term from 0 to 1. The table displays the mean predicted change in the outcome of interest based on 1000 simulations. We also calculate the 95% confidence interval around each prediction based on these simulations and judge the statistical significance of each of our variables using this standard. For reference, predicted probabilities for each outcome when all variables are set to their median values are listed in the rows labeled “Baseline”. In the interest of readability, we exclude from the table control variables that do not have a significant effect on a particular outcome.

Table 2.

First differences calculated from multinomial logit estimates in Table 1, model 3

		Hypothesized change
Government victory
Baseline probability^a	0.30
Pro-government Intervention vs Weak Rebels	n.s.	0 (H2)
Strong Rebels	–0.24
Pro-government Intervention vs Strong Rebels	+0.34	+ (H2)
Pro-rebel Intervention	n.s.	− (H1)
Coup Threat^b	+0.44
Duration (25th to 75th percentile)	–0.20
Post-Cold War	–0.18
Rebel victory
Baseline probability^a	0.08
Pro-government Intervention vs Weak Rebels	n.s.
Strong Rebels	+0.52
Pro-government Intervention vs Strong Rebels	–0.45	– (H2)
Pro-rebel Intervention	+0.41	+ (H1)
Secessionist Conflict	–0.07
Negotiated settlement
Baseline probability^a	0.07
Pro-government Intervention vs Weak Rebels	n.s.
Strong Rebels	n.s.
Pro-government Intervention vs Strong Rebels	n.s.
Pro-rebel Intervention	n.s.	+ (H1)
Coup Threat	–0.07
Duration (25th to 75th percentile)	+0.04
Post-Cold War	+0.26
Low activity outcome
Baseline probability^a	0.55
Pro-government Intervention vs Weak Rebels	n.s.
Strong Rebels	–0.28
Pro-government Intervention vs Strong Rebels	n.s.
Pro-rebel Intervention	–0.33	– (H1)
Coup Threat	–0.45

Predicted probability of outcome when all variables set to median values.

Substantive effects of control variables are only shown when the effect is significant at p < 0.05.

Statistically significant results are in bold type.

Rebel-biased military interventions

Hypothesis 1 predicts that foreign military intervention on behalf of the opposition will increase the probability of a conflict outcome favorable to the rebels by significantly augmenting rebel military capacity. Combat assistance provided by a state sponsor was expected to increase the probability that the rebels would prevail and decrease the probability the rebels would be decisively defeated or degraded to the point that they could no longer conduct significant attacks. The analysis indicates that rebel-biased intervention significantly increases the odds that the opposition will achieve a decisive victory and avoid a low activity outcome, but does not affect the likelihood of either a negotiated settlement or government victory. The probability of rebel victory is six times higher, increasing from just 8% in the median case to 49% when the opposition receives combat support from another state. At the same time, the probability of a low activity outcomes falls by 33 percentage points when a third-party intervenes with military force to assist the rebels. Interventions on behalf of the rebels do not reduce the probability of government victory or improve rebels’ prospects for achieving concessions from the incumbent regime in a negotiated settlement. Instead, combat support provided to insurgents appears to shift outcome odds away from the indecisive “low activity” outcome toward decisive rebel victories over the government. These findings are consistent with our conjecture that simply subsisting as a clandestine organization conducting low-level attacks becomes less desirable and more difficult as insurgents gain military capacity. By boosting the military strength of rebel forces, rebel-biased foreign intervention raises the likelihood that the rebels will prevail, but it does not decrease the odds of rebel defeat (i.e. government military victory). Instead, foreign support makes it less likely that rebels can evade open military conflict, withdraw and regroup under reduced conflict activity.

Government-biased military interventions

Our theoretical argument emphasizes the conditional role of pro-government intervention. The data support the hypothesis we derive from this argument. Results from both model 2 (without control variables) and model 3 indicate that foreign regime maintenance interventions significantly increase the odds of government victory and decrease the odds of rebel victory, but only when the rebels are relatively strong. Based on model 3, we find that when the government is facing rebel forces with fighting capacity at least as great as government forces, a foreign intervention on behalf of the government increases the likelihood of a decisive government victory by 34 percentage points (from 6 to 40%) and reduces the likelihood of rebel victory by 45 percentage points (from 60 to 15%). In contrast, when the rebels are classified as weaker than the government, foreign intervention on the government’s side does not significantly affect the odds of any of the conflict outcomes. This helps to explain why few studies have found evidence that incumbent regimes are more likely to prevail when a third-party intervenes on their behalf. In the majority of conflicts, government forces have a clear material advantage over rebel forces and pro-government intervention does not increase the odds of an outcome favorable to the regime. Only in those cases in which the opposition’s war-fighting capacity approximates that of the government does combat assistance provided to the government prove critical to regime survival.

We find no relationship between pro-government interventions and the probability of low activity and negotiated settlement outcomes—regardless of the strength of rebel forces. This is not particularly surprising as we had no basis for predicting how government-biased interventions would affect these outcomes. Both negotiated settlements and low activity outcomes are better for an incumbent than rebel victory, and less favorable than a decisive defeat of the opposition, but the categories are too broad to reveal whether foreign assistance improved the government’s position at the negotiating table or moved rebels that would have otherwise prevailed into a hibernation mode.

Control variables

In congruence with results from previous analyses (Cunningham et al., 2009; Gent, 2008; Regan, 1996), rebel strength relative to the government is a strong predictor of rebel victory in our model, regardless of whether the incumbent government receives external support. Interestingly, the effects of external assistance are similar to the effects of endogenous rebel fighting capacity in our model, and the effects appear to be additive rather than conditional or interactive. Both rebel strength and pro-rebel foreign assistance increase the odds of rebel military victory and decrease the probability of a low activity outcome dramatically. When rebels with military capacity that meets or exceeds that of the state receive combat assistance from another state, the predicted probability of rebel victory exceeds 90%—10 times greater than the likelihood of rebel victory in the median case—and the likelihood of a low activity outcome drops to just 3%. The correspondence between the effects of endogenous military capacity and external assistance to the rebels lends additional support to our contention that foreign intervention affects conflict outcomes through its impact on the balance of military capabilities in the conflict. We do find, however, that unlike military intervention on behalf of the rebels, the endogenous military capacity of rebel forces has a significant, negative effect on the likelihood of government victory. While the probability of government victory is 31% when the regime has a material advantage over the rebels, the probability of government victory falls to just 6% when the regime confronts a rebel movement with military capabilities equal to or greater than its own.

Our model indicates that governments become less likely to prevail the longer a conflict persists, a result that is in line with one of the most robust findings in the literature to date (Brandt et al., 2008; Cunningham et al., 2009; DeRouen and Sobek, 2004; Mason et al., 1999). Our results also suggest that a negotiated agreement becomes slightly more likely as a conflict drags on (Balch-Lindsay et al., 2008; Mason and Fett, 1996). Consistent with findings from studies by Fortna (2009) and Toft (2010), conflicts after the Cold War are significantly more likely to result in negotiated settlements and less likely to end in a military victory for the government. As one would expect, we find that attempted coups are far more likely to result in victory for the incumbent regime than a negotiated agreement or a low-activity outcome. Most coups fail and military leaders that initiate failed attempts to overthrow the regime seldom have the opportunity to bargain for concessions from the regime or transform their opposition to the incumbent into a low-level campaign of violent attacks. Our model also indicates that rebels are slightly less likely to win a military victory in territorial conflicts, but we do not find evidence that separatist conflicts are any more or less amenable to negotiated settlements, government military victory or low activity outcomes (Cunningham and Lemke, 2013; Walter, 2002).

Model quality

The proportional reduction in error using model 3 is 28% compared with a null model that assigns the modal outcome, low activity, to all conflicts. Multicollinearity among the predictors does not present a problem in our fully specified model. Many countries experience more than one dyadic episode of domestic armed conflict, so we also ensure that our results are not biased by unit heterogeneity. Estimating our models with clustered standard errors or varying intercepts, each by country, returned substantially equivalent results. In addition, estimating varying-intercept models indicated that the variance at the country level is negligible. To examine the validity of the assumption of independence of irrelevant alternatives (IIA) underlying the multinomial logit model, we estimate the probability of each outcome against the other remaining outcomes separately. Our inferences remain the same and a Hausman test indicates no violation of the IIA assumption. Finally, because the essence of our argument is conditional and thus requires the addition of an interaction effect to the model, we test whether the interaction term improves the model and find that it does. Adding the interaction term improves the ability of the model to correctly predict conflict outcomes and a likelihood-ratio test indicates that the model with the interaction term is a better fit to the data with reasonable certainty (p < 0.1).

Robustness tests

One of the most significant limitations of our analysis is that our measures of foreign intervention and military strength do not vary across time within conflict dyads. This makes it more difficult to establish that military intervention and relative military capabilities have independent causal effects on conflict outcomes. Our final analysis explores how disaggregating conflicts into yearly units of analysis affects our results. Table A1 in the Appendix presents results from a multinomial logit analysis of conflict dyad-year outcomes. This analysis serves as a particularly strong test because, in addition to switching to annual observations, we adopt alternative measures of foreign military intervention that vary over time and, consequently, must limit the dataset to observations after 1975.

In the NSA dataset, a new conflict dyad spell is created when any of the variables change during the course of the conflict (Cunningham et al., 2013). Moving to annual observations should allow us to capture changes in relative rebel strength over time, although, in the dataset, rebels rarely move from one category to another (e.g. from weaker to at parity or stronger) during the life of a conflict dyad. There is more temporal variation in indicators of external involvement in the UCDP External Support Project dataset version 1-2011 (Högbladh et al., 2011), which adopts the conflict dyad-year as its unit of analysis. We therefore use the alternative measures of foreign military intervention provided by the External Support Project, despite the fact this forces us to limit our analysis to observations between 1975 and 2009. In this analysis, two dichotomous variables indicate whether there is an ongoing commitment of combat troops to fight alongside each of the belligerents. If a military intervention is terminated before the conflict ends, the intervention variable would be coded with zeroes for all years after the troops are withdrawn. Like our previous models, our dependent variable is coded from the UCDP Conflict Termination dataset; however, in this analysis, it has five categories: rebel victory, government victory, negotiated settlement, low activity, and continue. Continue is the most common conflict-year outcome; internal armed conflicts are sustained into the next year in 82% of our annual observations.

As might be expected, the results from estimation of this model differ somewhat from the results we obtain from our conflict level model because this model only estimates the effects of ongoing troop deployments. Nevertheless, the results continue to be consistent with our theoretical argument. Just as in our conflict level models, combat assistance provided to a rebel movement increases the probability of rebel victory in the median conflict year. As before, the effect of rebel-biased intervention appears to be in addition to the significant positive effect of indigenous rebel fighting capacity in a given year. Owing to the inclusion of a “continue” category, we also now have evidence that rebel-biased invention significantly decreases the duration of internal armed conflicts.

The effect of government-biased intervention again varies with the strength of rebel troops. The likelihood of rebel victory in the median conflict-year declines when foreign troops are fighting alongside the regime. Unlike in our previous models, the effect is statistically significant regardless of rebel capacity, but the decline is greater when the rebels are strong militarily. While our primary models suggest no correlation between pro-government intervention against a weak rebel threat and the overall likelihood of rebel victory, it is not surprising to find that an active deployment of foreign troops effectively eliminates the possibility of a decisive rebel military victory, regardless of the strength of rebel forces. On the other hand, if the rebels are weak, the probability of a decisive government victory in the median conflict year is slightly lower when foreign troops supporting the government are present in the country. When rebel fighting capacity is high, an ongoing pro-government intervention has a positive but statistically insignificant effect on the probability of a decisive win for the government. These results are likely to reflect some endogeneity induced by the change in unit of analysis. Among those conflicts which experience a foreign military intervention, foreign troops will still be actively deployed in the year a conflict ends primarily in those cases in which government victory is less assured. The negative correlation between an ongoing pro-government military operation against weak insurgents and government military victory is consistent with our argument; intervening states have difficulty eliminating weak insurgents entirely and governments that are unable to prevail over militarily weak rebel movement without assistance are frequently plagued by political problems that foreign combat assistance does not ameliorate.

Conclusion

At the beginning of 2014, the International Crisis Group documented almost two dozen violent conflicts in which governments and internal opposition movements battled for control in countries around the world.⁶ In many of these conflicts, external actors had intervened to provide armed assistance to one side or the other. How is foreign military intervention likely to affect the trajectory of these conflicts? In this article we provide evidence that the utility of third-party military intervention on behalf of incumbent regimes is dependent on the military capabilities of the opposition. Government-biased interventions often target militarily weak insurgents that can disperse their forces and seek concealment in territory that is difficult to access—qualities that limit the effectiveness of an intervener’s conventional war-fighting capabilities. Moreover, if we assume that states do not want to pay the costs of military intervention to prop up a regime that can survive without assistance, interventions into struggles against militarily weak rebels are especially likely to be interventions on behalf of regimes with grave political liabilities. In contrast, governments that are unable to prevail over militarily strong rebels face a military challenge that foreign intervention could help to ameliorate, even if a lack of military capabilities is not the government’s only vulnerability. When an opposition movement is able to mount a serious military challenge to the central government, armed intervention on behalf of the government is critical. Although government-biased intervention has no effect on conflict outcomes when the opposition is weak, we find that a regime confronting rebel forces that are at least as strong as government forces is 6–10 times more likely to prevail with foreign combat support than without.

We also uncover interesting dynamics on the opposition side. Combat assistance provided to rebel groups dramatically increases the probability of a decisive military victory for the opposition, and conflicts in which the rebels receive foreign support are much less likely to end in a low-activity outcome or to continue into another year. By boosting the military strength of rebel forces, rebel-biased foreign intervention raises the likelihood the rebels will prevail, but it does not decrease the odds of rebel defeat (i.e. government military victory). Instead, foreign support makes it less likely that rebels can evade open military conflict, withdraw and regroup under reduced conflict activity.

Our analysis helps to explain why previous studies have found consistent evidence that rebel-biased intervention increases the likelihood of an outcome favorable to the rebels, but very little indication that pro-government interventions increase the odds of government victory. Because governments that want to remain in power must adopt a conventional military strategy to defend territory and seats of power, they make good targets for the conventional military forces of an intervening state. While it is generally not possible for relatively weak rebel movements to forcibly remove incumbent regimes, assistance from the armed forces of a third-party state makes the task considerably more feasible. In contrast, most governments have the material capacity to neutralize rebel threats without external assistance, and those governments that have superior military capabilities but are still unable to eliminate a militarily weak rebel movement do not benefit from foreign intervention on their behalf.

To better understand the effects of military intervention, future research should investigate the types of strategies and tactics that foreign interveners employ and how these strategies relate to the behavior of domestic governments in internal conflicts. A limitation of our study is that data constraints have compelled us to use broad, aggregate measures of both conflict outcomes and third-party involvement to test arguments that imply effects on conflict dynamics. Our ongoing research aims to investigate the efficacy of different intervention strategies, consider alternative outcome measures and track conditions within conflicts over time.

Footnotes

Appendix

Table A1.

Determinants of conflict-year outcomes (multinomial logit estimates)

	Rebel Victory vs Continue		Government Victory vs Continue
Pro-government Intervention	−2.970	(1.196)	−1.654	(0.856)
Strong Rebels	1.717	(0.455)	−2.488	(1.150)
Intervention × Strong Rebels	−0.911	(1.546)	3.113	(1.762)
Pro-rebel Intervention	3.186	(0.762)	−0.047	(1.067)
Coup Threat	3.742	(0.786)	4.314	(0.704)
Secessionist Conflict	−3.372	(1.128)	−0.317	(0.436)
GDP (natural log)	−0.527	(0.284)	0.186	(0.192)
Duration (log of years)	−0.107	(0.224)	−1.174	(0.248)
Post-Cold War	0.265	(0.428)	0.164	(0.384)
Constant	0.294	(2.103)	−3.583	(1.496)
	Negotiated Settlement vs Continue		Low Activity vs Continue
Pro-government Intervention	−0.281	(0.592)	−0.209	(0.338)
Strong Rebels	0.614	(0.411)	−0.450	(0.455)
Intervention × Strong Rebels	0.029	(1.037)	−0.326	(1.188)
Pro-rebel Intervention	0.680	(0.548)	−0.520	(0.632)
Coup Threat	−16.167	(4979.7)	1.203	(0.901)
Secessionist Conflict	−0.044	(0.300)	−0.016	(0.217)
GDP (natural log)	−0.154	(0.155)	−0.103	(0.109)
Duration (log of years)	−0.275	(0.133)	−0.435	(0.100)
Post-Cold War	2.076	(0.419)	0.542	(0.213)
Constant	−2.805	(1.230)	−1.047	(0.839)
N (conflict-years)	1299
LR χ²	314.293
Pr > χ²	0.000
AIC	1661.820

Standard errors in parentheses. The unit of analysis is dyadic conflict-year. The outcome category “continued conflict” is new to this model compared with the models presented in Table 1 owing to the yearly structure of the data.

Acknowledgements

The authors would like to thank Navin Bapat, Mark Crescenzi, David Cunningham, Stephen Gent and participants at annual meetings of the Peace Science Society (International) and the International Studies Association for helpful comments at various stages of this project. We are also grateful to Glenn Palmer and the reviewers at CMPS whose careful readings greatly improved the final project. Bryce Loidolt provided research assistance. Replication files for the analyses performed in the article are located at plsullivan.web.unc.edu and .

Funding

Partial funding for this research was provided by a grant from the Office of Naval Research, US Department of the Navy, no. N00014-09-1-0557. Any opinions, findings or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the Office of Naval Research.

Notes

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