Why has Europe implemented a quite-proactive climate policy while the US has adopted a far less ambitious climate strategy? Does variation in security concerns or other factors better explain this difference in policy? Using a multimethod case study approach, the authors find that in the US, constructions of climate change as a security threat play an important role in developing public support. In Europe, leadership and opportunity discourses predominate. Other factors including centralization of governance, trust in the technocratic elite, and cultural norms contribute to the variation in policy construction.