How do states build and lose the capacity to decarbonize? Based on a comparative-historical analysis of Brazil’s drastic reduction and later resurgence of CO2 emissions from 1985 to 2022, this article conceptualizes climate change as an emerging state-building domain with global and scientific dimensions. It uncovers a novel pathway to state performance: National actors can pursue state building outside its traditional terrain, the nation-state. Bureaucrats and scientists leverage transnational ties that provide shelter from politics and resources for bureaucratization and experimentation. Over time, this transnational embeddedness helps consolidate the infrastructural reach of decarbonization policies. New policies trigger economic elite attention and opposition, yet capacity persists as long as decarbonization is minimally recognized as a state responsibility. Democratic backsliding collapses that agreement: Opponents occupy the bureaucracy and erode the foundations of decarbonization capacity. The article concludes with a comparative framework and its contributions to political and environmental sociologies.