Studies of conflict management by international organizations have demonstrated correlations between institutional characteristics and outcomes, but questions remain as to whether these correlations have causal properties. To examine how institutional characteristics condition the nature of international organization interventions, I examine mediation and ceasefire monitoring by the Arab League and the United Nations during the first phase of the Syrian civil war (2011-2012). Using micro-evidence sourced from unique interview material, day-to-day fatality statistics, and international organization documentation, I detail causal pathways from organizational characteristics, via intervention strategies, to intervention outcomes. I find that both international organizations relied on comparable intervention strategies. While mediating, they counseled on the costs of conflict, provided coordination points, and managed the bargaining context so as to sideline spoilers and generate leverage. While monitoring, they verified violent events, engaged in reassurance patrols, and brokered local truces. The execution of these strategies was conditioned on organizational capabilities and member state preferences in ways that help explain both variation in short-term conflict abatement and the long-term failure of both international organizations. In contrast to the Arab League, the United Nations intervention, supported by more expansive resources and expertise, temporarily shifted conflict parties away from a violent equilibrium. Both organizations ultimately failed as disunity among international organization member state principals cut interventions short and reduced the credibility of international organization mediators.