Preventing large-scale atrocities has emerged as an important policy goal of the post–Cold War period. However, a debate exists about the effects of creating an international institution to prevent atrocities. Advocates of intervention argue that a credible threat to intervene should deter perpetrators and stop atrocities when deterrence fails. Critics argue that third-party intervention, by strengthening weak minority groups and lowering the cost of war, encourages rebellions and so makes war and atrocities more likely. We develop a model of intervention to analyze this debate. The model shows that the negative effects of intervention highlighted by critics can be mitigated if the third party is relatively neutral and if alternative costs are imposed on decision makers. We conclude that with appropriate institutional design, the net impact of stronger third-party commitments to end atrocities will be to lower the expected level of atrocities.