Not that such research was needed, since the evidence is contemporary and obvious. But here it is, anyway:
From the abstract of an influential 2002 paper, Patrick Regan:
Recent research has begun to focus on the role of outside interventions in the duration of civil conflicts. Assuming that interventions are a form of conflict management, ex ante expectations would be that they would reduce a conflict’s expected duration. Hypotheses relating the type and timing of outside interventions to the duration of civil conflicts are tested. The data incorporate 150 conflicts during the period from 1945 to 1999, 101 of which had outside interventions. Using a hazard analysis, the results suggest that third-party interventions tend to extend expected durations rather than shorten them.
From the paper:
The policy implications of these results are fairly stark. If the objective of an intervention is to shorten the length of a civil conflict, then an outside military or economic intervention is not a terribly effective strategy to do so. Regardless of how the intervention is conceived – or empirically operationalized—there seems to be no mix of strategies that lead to shorter expected durations. Even maintaining a neutral posture or organizing the intervention under the auspices of a multilateral rubric is not sufficient to form an effective means of conflict management (p. 31).