In April 2023, the EU institutions finally adopted an innovative international climate policy instrument: the Carbon Border Adjustment Mechanism (CBAM). As the long and winding road to a CBAM has included significant EU-internal and EU-external opposition, a “shifting Multi-Level Reinforcement” (MLR) perspective is helpful for understanding this development. When France assumed initial leadership from 2007, skepticism to the WTO, generous amounts of free allowances, combined with a low carbon price, provided a strong oppositional force. Then, influential elements in the European Parliament called for a carbon border tax, which was subsequently included as a central ingredient in the “Fit for 55” package launched by the Commission, In the decisionmaking process, leadership involving all the previous forces resulted in an institutional reinforcement dynamic strong enough to counter EU-internal as well as EU-external challenges and ‘turbulence’. This case shows the relevance of an updated MLR perspective for understanding also other EU processes, taking into account the growing role of both policy packages and trilogues as decisionmaking tools, indicating a possible strengthened role for the Commission also in the decisionmaking phase.