This study examines the anti-whaling strategy of the Sea Shepherd Conservation Society (SSCS). Despite being relatively small and resource poor, this confrontational marine conservation organization has been successful in frustrating Japanese whaling in the Southern Ocean through the use of what it describes as ‘aggressive non-violent direct action.’ Adopting an inductive approach, the study uses participant observation and process tracing in order to uncover those mechanisms which make the SSCS strategy effective. In understanding this strategy, which is unlike any other described in the transnational environmental activism literature, the study seeks to add to our understanding of the role and power of non-state actors in international affairs. A close examination reveals that the organization is engaging in a strategy which can be described as ‘direct enforcement’ (DE) – whereby it seeks to enforce existing marine conservation laws. The SSCS supports its claims as an enforcement organization through the use of legal language, symbols and imagery. It also selects targets which can be accused of violating the law, and gathers evidence to support these accusations. Once it has identified such a target, the SSCS interferes with the operations and attempts to prevent illegal and environmentally harmful activities, and to directly increase the target’s costs of operation. This study explores some of the mechanisms upon which DE relies. Aggressive intervention exposes activists to potential retaliation from targets and states. Several mechanisms reduce potential retaliation. First, activists are protected by the principle of unclean hands: targets do not wish to draw attention to their own wrongdoings by indicting activists. Second, activists surround themselves and their actions in a complex web of international laws, which tends to deter state prosecution, because states generally wish to avoid complications or potentially embarrassing international incidents. By actively enforcing laws where states lack capacity and/or political will to do so, DE enhances the compliance pull of international laws. Through the use of DE, activists also exert powerful legal leverage against states. Eschewing traditional activist approaches such as the ‘mobilization of shame,’ DE not only criticizes states for their failure to live up to their international obligations and commitments, but supports these claims with confrontational actions which cannot be ignored.