The purpose of this paper is to explain how and why consensus was reached on a legally binding approach given the opposition of powerful actors. Why did the US and key emerging economies change their positions? We apply tools from the regime formation literature - classical perspectives on power, interests and knowledge, and the use of different leadership tools to shed light on the issue. Knowledge-based intellectual leadership was exercised by the UNEP Secretariat, providing new information on the seriousness and scope of the problem. Power-based leadership through unilateral action was provided by the US. When the US changed position after change in domestic leadership, political costs increased for other opponents. Finally, interest-based instrumental leadership was provided by many proponents, with UNEP and among others the EU in the lead. Still, conflicts remain on control measures and the form of financial mechanism.