Civil War Intervention and the Problem of Iraq

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Civil War Intervention and the Problem of Iraq

Stephen Biddle, Jeffrey Friedman, and Stephen Long*

August 22, 2008

Civil war is the most common form of armed conflict worldwide, and has killed hundreds of millions of people in the decades since World War II. It is also among the most pressing issues in US foreign policy today, in the form of the ongoing civil war in Iraq.

There are many important theoretical and empirical questions in the study of civil warfare, but one of the most immediately consequential concerns foreign intervention. Warfare internal to a state is bad enough, but when neighboring states intervene, a local internal tragedy can become a region-wide conflagration with far worse consequences for much greater populations. Its stakes make the causes and incidence of civil war intervention an inherently important question for scholarship.

But this is also a question at the very heart of today’s debate over US policy for Iraq. This debate now turns on the need for a continued US troop presence and the consequences of US withdrawal. And the case for a large US presence turns on the argument that withdrawal risks destabilizing Iraq in a way that could cause the Iraqi civil war to spread across the region as neighbors are drawn into the fighting, yielding a conflict that could engulf the whole of the Middle East’s energy production and plunge hundreds of millions of additional people into open warfare (e.g., National Intelligence Council 2007, Byman and Pollack 2007, Boot 2008). Opponents of the US presence, by contrast, argue that such fears are exaggerated, and that the war would either burn out within Iraq’s borders or that a US withdrawal would actually solve the underlying problem and enable a peaceful settlement without a broader war (eg, Simon 2007, Gause 2008, Korb et al 2008). The outcome of this debate will shape perhaps the most consequential foreign policy decision facing the next President, with stakes that stretch far beyond the United States to the stability of an entire region.

Yet for all its importance, this debate has been almost devoid of any systematic scholarly analysis of the actual risk that others would intervene in the Iraqi civil war should the US withdraw from an unstable Iraq. There is an extensive body of empirical experience covering over 140 civil wars since 1945, and there is a significant literature on civil war intervention in these cases on which such an analysis could draw; this is an issue on which international relations scholarship could potentially offer important insight. This existing literature is not yet directly applicable, however. Its choices of dependent variables and units of analysis, for example, are structured for related but different purposes, making its findings suggestive but not dispositive for Iraq. And perhaps most important, several of the sub-issues most important for the Iraq debate – and especially the causal roles of ethnic-sectarian linkages, regional political culture, and military strength – have not been interconnected in a systematic or comprehensive way to date.

The purpose of this paper is thus twofold. First, we seek to extend the theoretical and empirical literature on civil war intervention to encompass the joint causal roles of ethnic-sectarian linkages, regional political culture, and military materiel among neighboring powers. Second, we apply the results to the specific problem of Iraq by using the resulting model to estimate the probability that this war would spread beyond Iraq’s borders in the event of a US withdrawal.

We find that the particular circumstances of Iraq and the Gulf region today create an important risk that the war could indeed spread if the United States exits and internal violence escalates. Intervention is a commonplace feature of civil warfare generally, but Iraq’s particular combination of multiple ethnic and sectarian linkages outweighs the material weaknesses of its neighbors or the accommodationalist political culture of the region to make this an unusually intervention-prone case. Even so, intervention is by no means a certainty. But our findings suggest a nearly 40 percent probability that a ten-year civil war following a US withdrawal would eventually draw in three or more of Iraq’s neighbors – and a much higher probability that the war would spread to at least one. This scale of risk warrants serious consideration in the debate over Iraq policy.

We present these findings in five steps. First, we review the extant literature on civil war intervention, discuss the problems of ethnic-sectarian linkage, regional political culture, and military materiel, and motivate our theoretical treatment of those variables. Next, we discuss our dataset and operationalize our variables. We then present the statistical results. We conclude with a series of implications from these results for scholarship and policy.

Explanations of Civil War Intervention

Civil war has been getting increasing attention in the international relations literature, and two overlapping schools of intervention analysis have emerged from this. One school emphasizes material geo-politics, and sees the causes of intervention in formal alliance ties, variations in conflict type and stakes (identity wars are seen as more prone to intervention than ideological ones), high conflict intensity and elevated casualty levels, the potential threat that civil wars pose to the stability of their neighbors, and geographic proximity between civil war states and potential interveners (Werner 2000, Regan 1998, Regan 2002, Lemke and Regan 2004, Findley and Teo 2006, Salehyan and Gleditsch 2006, Mullenbach 2008, Kathman np). Another school emphasizes cultural and ethnic affinity, and argues that intervention is more likely when sectarian or ethnic ties link civil war parties with potential interveners or when interveners and civil war states are connected by a former colonial relationship (Heraclides 1990, Carment, James, and Rowlands 1997, Davis and Moore 1997, Davis, Jaggers, and Moore 1997, Khosla 1999, Saideman 2001, Sambanis 2001, Centinyan 2002, Woodwell 2004, Austvoll 2006, Buhaug and Gleditsch 2008).1

Iraq involves both geopolitical and ethno-sectarian factors simultaneously. The civil war pits Iraqi Shia against Iraqi Sunnis in the middle of a region that has experienced long historical conflict between these groups both within and between states, and in the midst of rising regional tensions along just such sectarian lines (Nasr 2006). At the same time, many who argue that the Iraqi war will not spread do so based on a geopolitical argument that Iraq’s Sunni neighbors are too weak militarily to intervene (Takeyh et al. 2008, Posen 2007). To assess the net risk of intervention in Iraq thus requires adjudication of potentially conflicting geopolitical and ethno-sectarian influences.

In the literature, however, geopolitical and affinity variables tend to be treated in different studies using different datasets, making net assessment of these effects for Iraq difficult. The affinity literature tends to use the Minorities at Risk (MAR) data, which treat only contiguous states and offer limited coverage of geopolitical issues. The geopolitical literature, by contrast, tends to use the Correlates of War (COW) data, which do not cover ethnic or cultural affinity. A recent paper by Martin Austvoll (2006) indicates that material and cultural factors may each be significant determinants of civil war intervention, but the scope of this research is limited to twenty-seven conflicts, which are suggestive but not conclusive of broader patterns.

Much of the literature, moreover, uses very broad definitions of “intervention.” Perhaps the most common definition is Regan’s “convention-breaking military and/or economic activities in the internal affairs of a foreign country targeted at the authority structures of the government with the aim of affecting the balance of power between the government and opposition forces” (1998: 756, also used by Lemke and Regan 2004; Findley and Teo 2006; Austvoll 2006; Kathman np). This has the virtue of excluding little, but it also includes much that would fall below the threshold of concern in the Iraq debate, treating modest economic sanctions and large military deployments equally. In addition, Regan codes separate observations of intervention every time a country escalates its activities, weighting certain cases very heavily in the findings and producing a higher intervention count than many in the Iraq debate would intuit. For instance, Rwanda intervenes once in the Congo (1996-97), but Cuba intervenes 11 times in Angola’s civil war and Vietnam intervenes 23 times in Cambodia. Others distinguish multiple levels of external involvement, as in a range from “ideological encouragement” to “active combat units in country” for the MAR data (eg, Saideman 2001; Cetinyan 2002); for the Iraq debate, only the most forceful of these intervention definitions speak to primary concerns.

A final issue with the existing scholarly literature concerns its units of analysis. Most recent empirical work on intervention has focused on interstate dyad-years. The benefit of this approach is that it includes every potential intervener in a civil war, and thus avoids selecting on the dependent variable. A drawback of this approach, however, is that it gives equal statistical weight to all potential interveners, even those whose small size and remote location make them implausible candidates for military action. By examining dyads where there is essentially no chance of conflict, researchers may end up overshadowing other statistical patterns and reducing a model’s relevance for assessing the likelihood of intervention in important cases like Iraq. The utility of restricting analysis to “politically-relevant” dyads limited to proximate or great-power interactions has attracted increasing attention (eg, Bennett 2006, Quackenbush 2006, Lemke and Reed 2001), but most existing work on civil war intervention does not explicitly distinguish relevant from other dyads.

Since 2007, the scholarly literature has been joined by policy analyses on the potential causes of intervention in Iraq per se. In particular, Daniel Byman and Kenneth Pollack have argued that civil wars involving a combination of ethnic links between neighbors and conflict parties, refugee flows into neighboring states, and regional arms racing create a particular risk of intervention, and that Iraq poses just such a combination (Byman and Pollack, 2007). Others, by contrast, argue that Middle Eastern political culture promotes accommodation and compromise rather than confrontation, and that Iraq’s neighbors are too weak to sustain a major cross-border intervention in any case – hence the Iraq case should be considered a low risk for regionalization (Takeyh et al. 2008, Posen 2007).

Neither Iraq camp’s analysis, however, has yet been informed by any systematic large-n empirical investigation. Nor can this be provided by simple reference to the existing literature on intervention. The affinity-oriented literature excludes the military materiel issues so important in the Iraq debate, whereas the geopolitics-oriented literature excludes the ethnic and sectarian factors. Neither scholarly sub-literature controls for Middle Eastern political culture and its effects on intervention proclivity, nor does either sub-literature consider the effects of refugee flows or arms-race dynamics on intervention.

To account for the effects considered important in the Iraq debate thus requires a synthesis and extension of the available research on intervention. In particular, this requires a new dataset with coverage of both affinity and geopolitical variables; it requires a dependent variable operationalization that focuses on the more forceful forms of intervention pertinent to the Iraq debate; and it requires an explicit treatment of region-specific features of the Middle East, the effects of potential interveners’ military capability, the regional military balance, and the effects of change in this balance as a prospective regional arms race in the Mideast unfolds.


To test these hypotheses, we examine 142 civil wars between 1950 and 1999, each with a minimum of 200 battle fatalities. Our baseline data are configured in dyad-years: for every year in which a conflict is ongoing, there is a separate observation for every state in the international system paired with the civil war state. We use this master set to produce results based on a politically relevant dyads design.

Our dependent variable, intervention, is a dummy coded as 1 when a third party intervenes in a conflict by sending combat troops into the civil war state. Each intervention must involve state soldiers being sent across borders by an intervener for the first time in the civil war; subsequent escalation, reinforcement, or other policy changes are not coded as additional “interventions.”2 Others have operationalized “intervention” in much less restrictive ways. Regan (2002), Lemke and Regan (2004), Austvoll (2006), and Kathman (np), for example, include economic assistance, arms transfers, intelligence cooperation, or military advising, in addition to armed border crossings; these authors also code secondary troop movements such as reinforcements or escalation as additional interventions. While valid for their authors’ purposes, these broader codings include as “interventions” an enormous range of cases that most in today’s Iraq debate would not consider the primary focus of policy concern. (In Regan 2002, for example, 912 of 1036 total cases of “intervention” involve reinforcements of ongoing military action or instances of assistance short of cross-border troop movements.) We thus adopt a very conservative coding designed to speak both to the scholarly theoretical literature, whose interests include high-end as well as more modest forms of external involvement, and the Iraq policy debate – where the issue of central concern is the fear that the Iraq war will spread to engulf the neighbors in active warfare per se.

To bridge the divide between affinity and geopolitical treatments of intervention and their respective data sources, we begin with the COW data (Singer, Bremer and Stuckey 1972, Sarkees 2000, Hensel 2001, Gibler and Sarkees 2004b) given its broader coverage, and add data from other sources as necessary to account for affinity variables and a variety of controls. Following Lemke and Regan (2004), we adopt a less restrictive civil war definition than COW’s (200 or more battle deaths, as opposed to COW’s 1000); this also requires us to add conflicts fitting this definition but missing from COW, again following Lemke and Regan (2004), using Kathman’s transformation of these data into a dyad-year unit of analysis via EUGene 3.0.3. We draw our dependent variable values from Lemke and Regan (2004), but limited to cross-border troop movements alone as noted above. We also use data on regime type from the POLITY IV dataset (Marshall and Jaggers 2007) and Freedom House (2008); fatalities from the International Peace Research Institute (Lacina and Gleditsch 2005) and Lemke-Regan (2004); economic performance from Collier and Hoeffler (2004); conflict type from Lemke-Regan (2004); and refugee flows from UNHCR (2000).

The most challenging data issues involve ethnic linkages. Fearon, Kasara, and Laitin (2007) record the ethnicity of the “top political leader” in each state since 1945, but no data set provides systematic information on rebel group ethnicity.3 We therefore compiled new data on this, following Fearon et al.’s logic of coding groups by the ethnic background of their leaders. Where rebellions comprised multiple factions we included each. We were able to document rebel ethnicity for 139 of 142 civil wars in the data set.4 We then reviewed the merged data extensively, with a particular emphasis on fatalities. As a result of this review, 29 of the 142 fatality values were changed, five double-counts or erroneous civil war state identifications were corrected, and we were able to code missing ethnicity values for more than 30 state leaders (recovering several thousand observations in the master dataset).5

A key issue in the new coding was to determine whether an ethnic group in one country is the “same” as in another. For robustness, we used two sets of coding rules, but both were quite restrictive:

  • Rebel link v1, State link v1: Rebel link v1 is a dummy variable coded as 1 if the rebel group and the potential intervener are the same group with the same name in each country but the civil war state government is not, and 0 otherwise. State link v1 is a dummy variable coded as 1 if the civil war state government and the potential intervener are the same group with the same name in each country but the rebel group is not, and 0 otherwise.

  • Rebel link v2, State link v2: In addition to the rule above, actors are coded as linked if they are members of one of 18 ethnic “clusters” (e.g., Moldovans/Romanians, Americo-Liberians/Creole, Turks/Turkmen/Uighurs).

Operationalizations for other key variables are:

  • PotIntColonizer: a dummy variable with a value of 1 if the potential intervener was formerly a colonizer of the civil war state, and 0 otherwise, including if the civil war state was never a colony (Hensel 2001b).

  • CWSPowerShare: the share of material military power in the dyad controlled by the civil war state, expressed as the civil war state’s CINC (Composite Index of National Capability) score divided by the sum of the civil war state’s CINC score and the potential intervener’s CINC score (Singer, Bremer and Stuckey 1972 v3.02).

  • PotIntAlliance: a dummy variable with a value of 1 if the potential intervener and the civil war state have a formal alliance consisting of a “defense pact, neutrality or non-aggression treaty, or entente agreement,” and 0 otherwise (Gibler and Sarkees, 2004 v3.03).

  • Intensity: the natural logarithm of the average number of battle-related fatalities per month of the conflict. We based our data on Regan (2002), and checked them against figures from Clodfelter (2002), Correlates of War (Sarkees 2002), the International Peace Research Institute (PRIO) Battle Deaths data (Lacina and Gleditsch 2005), and other sources cited in the PRIO documentation. When Regan’s values differed from multiple crosschecks by more than a factor of two, we replaced them with the figure given in PRIO.6

  • Mideast: a dummy variable with a value of 1 if the civil war state is Iran, Turkey, Egypt, Syria, Lebanon, Jordan, Israel, Saudi Arabia, the Yemen Arab Republic, Yemen, the Yemen People’s Republic, Kuwait, Bahrain, Qatar, the UAE, or Oman.7

  • Refugees: the natural logarithm of the average number per year of refugees from the civil war state residing in the potential intervener, using data from UNHCR (2000).8

Control variables include:

  • Coldwar: The Cold War superpower competition created an intervention incentive for great powers that may not have been present since then. The Coldwar variable controls for this potential influence, and is coded as a dummy variable with a value of 1 if the dyad-year is before 1989, and 0 otherwise.

  • African: Sub-Saharan Africa is widely considered an unusually intervention-prone region.9 The African variable controls for this potential influence, and is coded as a dummy variable with a value of 1 if the civil war state is in Sub-Saharan Africa (COW country codes 402 through 591) and 0 otherwise.

  • Resources: States whose economies turn on the export of oil or other primary commodities are often considered attractive predation targets and subject to unique internal political dynamics that might make them unusually intervention-prone. The Resources variable controls for this potential influence, and is coded as the ratio of the civil war state’s primary commodity exports to total GDP, coded in 5-year increments following Collier and Hoeffler (2004).

  • Democratic state, Democratic intervener, Joint Democracy: Democracies are widely expected to display distinctive security behavior that might reduce their proclivity to intervene in civil warfare, or to be subject to intervention by others when beset by civil warfare. To control for this potential influence, Democratic state is coded as a dummy variable with a value of 1 if the civil war state scores 6 or greater in the POLITY IV data set (Marshall and Jaggers, 2007); Democratic intervener is coded as a dummy variable with a value of 1 if the potential intervening state scores 6 or greater in POLITY IV; Joint Democracy is coded as a dummy variable with a value of 1 if the potential intervening and civil war states both score 6 or greater in POLITY IV. POLITY contains a large number of missing values, and where that was the case, we considered a state a “democracy” if it was listed by Freedom House as “Free” (Freedom House 2008), following the procedure in Regan (2002).


Table 1 presents statistical findings for a probit analysis with robust standard errors on our politically relevant dyad year data for our binary intervention/no intervention dependent variable.10 Table 2 presents a comparison of the magnitude of substantive effects for each statistically significant variable from Model 1, showing changes in estimated net intervention probabilities as key variables’ values are altered around their means. The results suggest several key findings.

First, as the affinity literature suggests, ethnic linkages can be important for civil war intervention. But not all linkages are created equal. In particular, Rebel link v1 is positive, statistically significant at the .01 level, and substantively important: a potential intervener with an ethnic link to rebels is more than five times as likely to intervene as one without a link (Table 2). State link v1, however, is statistically insignificant with an opposite sign. The results thus imply that links with rebels in the civil war state are much more conducive to intervention than links with governments. Analyses that do not disaggregate thus risk underestimating the importance of ethnic affinity for intervention, and the scale of the difference in empirical performance suggests important theoretical gaps in the affinity literature: the logic of affinity clearly operates very differently for rebels and governments.

Colonial history similarly matters for intervention. As the affinity literature expects, former colonial powers are more likely to intervene in their former colonies’ civil wars: the PotIntColonizer variable is significant at the .01 level and almost as important substantively as ethno-sectarian linkages to rebel groups (the probability of intervention rises by more than a factor of four for former colonizers: see Table 2).

Table 1: Probit Model of Intervention by a Politically Relevant Potential Intervener

Model I:

Full Model

Model II:

and Controls

Model III:

and Controls

Rebel link v1





State link v1



























Democratic state







Democratic intervener





















Ethnic Conflict







































Wald 2 (15)=107.55

Prob >2=0.0000

M&Z’s R2=0.249

McF’sAdj R2=0.132


Wald 2 (11)=99.31

Prob >2=0.0000

M&Z’s R2=0.202

McF’sAdj R2=0.123


Wald 2 (12)=94.97

Prob >2=0.0000

McK&Z’s R2=0.226

McF’s Adj R2=0.116

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