Using a case study of the UN's experiences in Croatia, this essay addresses the question of why some UN peace-keeping missions succeed, while others fail. The essay develops wider criteria of success than usually employed in peace-keeping literature and analyzes the performance in Croatia based on these measures. It then takes hypotheses extracted from the international relations literature on peace-keeping and comparative politics literature on civil conflict management and tests them against this case. First,‘international’ factors, those related to the UN itself, such as its level of commitment to a mission, and the presence or absence of leadership by a major power, are considered. Second, ‘domestic’ factors are addressed, including the level of consent and cooperation of the warring parties and the existence of a military stalemate. In light of these findings it is asserted that peace-keeping can indeed be successful when certain necessary and sufficient conditions are met. In Croatia, success seems mostly dependent on the domestic factors. This supports the notion of a ‘ripeness’ point for resolution.