Rather than being constrained by the least ambitious governments, the EU has until recently set the pace of national environmental policy. The contribution examines why the EU was often able to set the pace despite the institutionalist expectation that national diversity and quasi-unanimity should produce joint-decision traps. It also explains why this is no longer the case. Adopting a long-term, diachronic perspective, I argue that the EU’s unexpected dynamism and its recent decline are related to distinct institutional exits from the joint-decision trap that have opened and closed in different periods. During the emergence of the policy-area, diversity was manageable through bargaining, and joint gains were abundant. Between the late 1980s and the Eastern enlargement, as environmental policy was increasingly adopted under the community method, supranational intervention became an additional exit mechanism, with an activist European Parliament and strong agenda-setting by the Commission. Since mid-2000 the green dynamism is in decline, as supranational interventions wane and bargaining solutions become insufficient to cope with increased diversity after enlargement, exacerbated by the economic crisis. However, non-political law-making in the regulatory mode is emerging as a novel exit from the joint-decision trap.