Matthew Krain (2005). International Intervention and the Severity of Genocides and Politicides. International Studies Quarterly. 49, 3: 363-387.

ABSTRACT
This study examines the effectiveness of overt military intervention in slowing or stopping the killing during ongoing instances of genocide or politicide. Six alternative hypotheses regarding the potential effects of intervention on genocide/politicide severity are tested in a cross-national longitudinal analysis of all ongoing genocides or politicides from 1955 to 1997. The results suggest that interventions that directly challenge the perpetrator or aid the target of the brutal policy are the only effective type of military responses, increasing the probability that the magnitude of the slaughter can be slowed or stopped. Impartial interventions seem to be ineffective at reducing severity, and interventions to challenge the perpetrator do not make matters worse for the targets of genocide or politicide. The findings are consistent with recent arguments that attempts to prevent or alleviate mass killings should focus on opposing, restraining or disarming perpetrators and/or removing them from power.