Many studies have found that third-party intervention tends to prolong the duration of civil conflict. However, some studies have suggested that mere expectations of external intervention can prolong conflict duration even in the absence of actual intervention. Therefore, the effects of external interventions in prolonging the duration of civil conflicts remain unclear. This study examines the direct effect of third-party intervention on the duration of civil conflict by controlling for the indirect effect of expectations of external intervention. The probabilities of intervention were estimated, and the direct effect of interventions was tested by controlling for the effect of ex ante expectations. The empirical findings were as follows: (1) Third-party intervention has no direct effect of prolonging the duration of a conflict when expectations are controlled for, and (2) Third-party intervention may have an indirect effect of shortening conflict duration, which contradicts the findings of previous studies.