Civil War Intervention and the Problem of Iraq

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*** p-value 0.01 or less; ** p-value 0.05 or less; * p-value 0.10 or less

A second major finding is that affinity alone is an inadequate explanation of intervention: geopolitical variables matter, too. Material military power is significant at the .05 level, and its effects are substantively important (though less so than for ethnic linkages with rebel groups or colonial relationships): the materially weakest interveners in the dataset have a predicted probability of intervention 71 percent lower than the strongest. Combat intensity is significant at the .01 level and is extremely important: intervention is nearly 16 times more likely when the intensity level is at its lowest in the estimation sample than when it is at its highest.12 But not all geopolitical variables noted in the literature are consequential: conflict type does not pass even the .1 threshold for statistical significance, while alliance relationships are significant only at the .1 level.

The results also show that to consider either affinity or geopolitics in isolation is to produce an unnecessarily narrow and potentially biased picture. To illustrate the point, Model II in Table 1 considers only affinity variables plus controls; Model III considers only geopolitics plus controls. Neither partial model performs as well as Model I; statistical performance for the partial models varies by measure but can exceed a 30 percent falloff, as in the Efron’s r2 for Model III.13 And omitted variable bias, while generally modest, can sometimes affect coefficients in problematic ways: in Model III, for example, the estimated effect of military materiel is biased upward by about 40 percent relative to the more complete analysis in Model I, and the effect of democracy in the potential intervener is diminished by more than 50 percent.

Table 2: Model 1 Effects on the Predicted Probability of Intervention



Predicted p


Predicted p


Rebel link v1
























Democratic intervener






























Predicted probabilities calculated holding continuous variables at their means and dummy variables at their modes. Means, minimums, maximums and modes are for the estimation sample. Only variables with a significance level of 0.05 or better are presented.

The results suggest several other findings of note. Democracy’s effects on international politics have been a major theme in recent scholarship, and Model 1 suggests that regime type is indeed important for intervention: Democratic intervener is significant at the .01 level and substantively important. Yet Model 1 also suggests that, like ethnic linkage, democracy’s effects on intervention are differentiated. Democratic outsiders are significantly less likely to intervene in civil wars, but democracies undergoing civil wars are no more or less likely to suffer intervention than others, and democratic outsiders are not significantly more likely to intervene when the civil war state is a fellow democracy than when it is not.

The Refugees variable is significant at the .01 level, and its effects are substantively important: the potential intervener containing the highest average of refugees per civil war year in the sample is over eight times more likely to intervene in a given year than a potential intervener with the lowest average of refugees per civil war year in the sample. The nature of the civil war state’s economy also matters: the Resources variable is significant at the .01 level, and even more important than ethno-sectarian linkage in its effects: the case with the highest ratio of primary commodity exports to GDP is more than twelve times more likely to experience intervention than the case with the lowest ratio. States with primary commodity-centered economies are thus substantially more likely to be targeted for intervention in the event of civil war than are states with other economic foundations. The Cold War appears to have involved distinctive dynamics: Cold War civil wars were more prone to intervention than subsequent conflicts, and the effect is both statistically significant at the .01 level and fairly important substantively (post-Cold-War civil war year dyad observations are about 71 percent less likely to receive intervention: see Table 2).

Regional distinctions can be important. African civil wars, for example, are two times more susceptible to intervention than elsewhere. But here, too, the effect is not universal – and in particular, Middle Eastern civil wars are not meaningfully less likely than others to see intervention. Thus there is no evidence in these data to support the claim that Middle Eastern states are unusually free of intervention risk by virtue of a distinctive political culture.

Conclusions and Implications

Our findings thus suggest that ethno-sectarian affinity and geopolitical dynamics can be significant contributors to the risk of outside intervention in civil warfare. In particular, links between the civil war rebel group and the governments of neighboring states significantly increase the risk that neighbors will intervene by sending troops across the border, as do material power advantages for potential interveners and high levels of combat intensity in the civil war. In addition, a number of factors noted in the policy debate but largely absent from the scholarly literature on civil war intervention also have important effects: refugee flows and primary commodity dependence both demonstrate empirical performance with causal importance comparable to or greater than that of many variables which have heretofore received greater attention in the civil war literature. The results improve our understanding of intervention in particular and civil war in general, and suggest the importance both of combining geopolitical and affinity approaches to the study of internal warfare, and of extending analysis to new explanatory variables outside either tradition.

These findings also have important implications for US foreign policy and the debate over Iraq. In particular, they imply an important danger that the Iraq war could spread if the US withdraws and internal violence in Iraq escalates. Many proponents of prompt withdrawal have argued that the risk of intervention is exaggerated, and this assessment is often supported by arguments that Iraq’s neighbors are too weak, or that Middle Eastern states resolve differences by appeasement rather than invasion (e.g. Takeyh et al. 2008, Simon 2007, Posen 2007, Gause 2008, Korb et al 2008). The findings above, however, imply that the unique features of Iraq and its neighborhood could have the opposite effect – increasing, not decreasing, the risk of intervention in this war relative to others. Iraq presents an unusually interconnected ethno-sectarian conflict in a neighborhood with a large number of potential interveners who share the Sunni majority populations and/or regimes of Iraq’s Sunni insurgency. The neighbors are relatively weak now, but so is Iraq, and the region’s ongoing arms race stands to increase those neighbors’ material capacity to intervene over time. Iraq is also a major oil exporter, a major source of destabilizing refugee flows into neighboring countries, and a state with a civil war of very high average intensity. These factors are all strongly linked with an elevated risk of intervention in the data as a whole. And the findings above show no reason to expect that anything unique to the Middle East region per se should imply any unusual freedom from danger: whereas Africa, for example, is an especially intervention-prone region, the Mideast is not significantly different from the rest of the world in this regard.

This does not imply a certainty of intervention in a post-withdrawal Iraq. The findings above are based on necessarily imperfect data, and statistical analyses never explain the totality of the variance in the data; a degree of caution is always in order in drawing policy implications from empirical analysis. It is also possible that US withdrawal could reduce rather than increase internal violence within Iraq (though this is unlikely: Biddle, O’Hanlon, and Pollack 2008). Nor does the empirical analysis above suggest anything like a guarantee of disaster in the event that Iraqi internal violence does escalate in the wake of a withdrawal.

But the odds of intervention implied by the findings above are daunting all the same. To explore this more concretely, we extrapolated from the individual-state intervention probabilities estimated in the probit analysis above to compute the net probability of one-, two-, or more-than-two-state interventions in a post-withdrawal Iraq war using a Monte Carlo simulation methodology. That is, we computed unique time-dependent intervention probabilities per year for each of Iraq’s neighbors using the coefficients in Model I; compared these to random number draws for each neighbor in each simulated year; scored a neighbor as having intervened if the random draw was below the computed probability for any simulated year; then summed the interventions over potential interveners and over simulated time.15 Table 3 presents the mean results from more than 4,000 replications of the simulation, broken out for results observed in five, ten, and fifteen years of simulated post-withdrawal civil warfare.

Table 3: Net probability of intervention


0 states

>0 states

>1 state

>2 states

>3 states

5 yrs






10 yrs






15 yrs






The results suggest that the probability of intervention by at least one neighbor become extremely high after even five years of post-withdrawal warfare, with over a 70 percent estimated probability of at least one intervention, and over a 30 percent probability of more than one. Within ten years of withdrawal, the probabilities rise above 90 percent for at least one intervention and approach 70 percent for more than one. And within 15 years these probabilities rise to 98 and 88 percent, respectively. The odds of three or more of Iraq’s neighbors intervening in the war approach 40 percent within 10 years, and exceed 60 percent if the war continues for 15 years.

Table 4: Country-specific results (probability of intervention for each state)







Saudi Arabia

Syria (Sunni)

Syria (Shiite)


5 years










10 years










15 years










The simulation also enables an examination of the identity of prospective interveners and the relative magnitude of risk across states in the region. These findings are presented in Table 4, which reports the estimated probability of intervention for each of Iraq’s neighbors assuming five, ten, and fifteen years of post-withdrawal civil warfare.16 Given Monte Carlo simulation results based on the statistical findings in Model I, the simulation implies that the greatest threats of intervention are from Saudi Arabia, Jordan, and Turkey. Syria poses unusual coding complexities given its Sunni majority population but heterodox Alawite Shiite regime: if coded by reference to its Sunni majority, it is likelier than Saudi Arabia to intervene; if coded by reference to its Shiite leadership, it is less likely than Kuwait or Qatar to intervene.

Note that none of these individual-state intervention probabilities exceed 50 percent for 10 years of warfare; only two (Saudi Arabia and Syria coded as Sunni) exceed even a one-in-three chance of intervention in ten years. But because Iraq has many neighbors, even modest probabilities of intervention individually cumulate into serious risks over time.

Interestingly, of these neighbors Iran is among the least likely to intervene, with less than a 10 percent probability of intervention after even 15 years of Iraqi civil warfare – it is overwhelmingly Iraq’s Sunni neighbors who pose the greatest intervention risk. To some extent this is a function of the absence of contagion or reaction dynamics in Table 1’s statistical modeling; in the results here, all interventions are considered statistically independent events. In reality, the odds of counter-intervention following an initial entry by a rival are likely to be much higher than those for initial interventions (Findley and Teo 2006). In particular, an entry by a Sunni state into an Iraqi civil war would probably swing the military balance within Iraq dramatically in the Sunni rebel’s favor, and this would greatly increase Shiite Iran’s incentives to counter-intervene in order to avoid a Sunni takeover of Iraq. But considered in the broader context of the empirical record as a whole, these results suggest that Iran is unlikely to be the first state to cross the border with uniformed military formations, and may pose a smaller danger of regionalization for the conflict than do Iraq’s Sunni neighbors.

Of course, none of these values reach unity, and for the odds of multiple interventions to reach dangerous levels requires multiple years of post-US warfare in Iraq. Intervention is far from a certainty, whether for any given neighbor or across the region as a whole. And it cannot be known how long the Iraqi civil war would continue after a US withdrawal – it could last less than five years or more than fifteen.

But given the potential consequences – both strategic and humanitarian – of regional warfare in the Persian Gulf, the results in Tables 3 and 4 are grounds for concern. Certainly these findings give no basis for dismissing the danger of regional intervention in the Iraq war if the United States withdraws. This is a nontrivial risk which must be considered carefully in the design of US policy for Iraq and in any planning for troop reductions there – it cannot safely be ruled out on the basis of a belief that Iraq’s unique conditions make the war unlikely to spread. And the results also suggest some priorities for US diplomatic efforts to contain any post-withdrawal warfare within Iraq’s borders in the event that the conflict re-escalates after a US departure. In particular, the ethno-sectarian affinity between the Sunni insurgency in Iraq and its Sunni neighbors demands special attention – and greater concern than does Shiite Iran to the east, at least for the problem of containing an Iraqi civil war.

Perhaps most broadly of all, the results above suggest that important policy debates need not be conducted in isolation from empirical scholarship in international relations. The theoretical and empirical literature has much to offer if framed appropriately and extended where necessary to account for the particular issues at stake. Yet such decisions are often made in the absence of any systematic consideration of the range of evidence and experience that empirical scholarship can consider. Knowledge is important in its own right. But where the stakes in public decision are as grave as those in war and peace, opportunities to apply knowledge to inform public debate can be – and should be – exploited more often.


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