What explains the effect of external intervention on the duration of civil war? The literature on intervention has made some progress in addressing this question, but it has been hindered by an assumption that states intervene in civil wars either to help one side win or to facilitate negotiations. Often, however, external states become involved in civil war to pursue an agenda which is separate from the goals of the internal combatants. When states intervene in this fashion, they make wars more difficult to resolve for two reasons. First, doing so introduces another actor that must approve any settlement to end the war. Second, external states generally have less incentive to negotiate than internal actors because they bear lower costs of fighting and they can anticipate gaining less benefit from negotiation than domestic insurgents. Through Cox regressions using data on the goals of all interventions in civil wars since World War II, this article shows that when states intervene with an independent agenda, they make wars substantially longer. The effect of independent interventions is much larger than that of external interventions generally, suggesting that the established finding that external interventions prolong civil war is driven by a subset of cases where states have intervened in conflicts to pursue independent goals.
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