How do the qualities of United Nations peacekeeping operations (PKOs) influence the duration of peace after civil wars? Recent work shows that UN peacekeeping extends the duration of peace. However, this work has only been able to assess whether the presence or absence of UN missions affects post-conflict peace processes. Such analyses offer little in the way of policy prescriptions for planning and structuring PKOs to effectively pursue their goals. By employing fine-grained data on the personnel composition of PKOs, and generating expectations from rationalist bargaining models of civil war, we argue that the number and type of personnel deployed to a PKO influence the UN's ability guarantee peace by addressing the information and commitment problems that so often lead to the collapse of post-conflict peace. We analyze how the composition of missions influences the duration of peace, finding that, as the number of UN military troops deployed increases, the chance of civil war recurring decreases. However, other personnel types do not have the same effect. We conclude that the effectiveness of post-conflict peacekeeping lies in the ability of PKOs to alleviate commitment problems through the deployment of military troops that are able to defend the peace.