This article examines the United Kingdom’s use of drones in an act of self-defence in a counter-terrorism operation. The government justified the targeted killing of a UK citizen in Syria – a country with which it was not at war at the time – with reference to existing laws and norms. In doing so, it contested a number of established concepts to justify its conduct as lawful activity. The article argues that modern weapons such as drones, which are used to address novel threats, lead to legal justification that in turn has the potential to create new laws. In this way, the intersection of norms, warfare and new technologies becomes a productive site of political contestation. The contribution of the article is twofold: empirically, it contributes to debates on targeted killing and discussions surrounding the meaning and interpretation of ‘imminence’ in the context of preventive self-defence in the United Kingdom; and theoretically, it adds to the constructivist literature by examining norm contestation and resulting normativity in this area. The article concludes that the legal justification in this particular instance has important implications for other emerging technologies that require discussions about how states justify their actions to conform with existing legal and normative frameworks.