Between 2009 and 2015, both Islamist and anti-Islamist protest groups were active in Britain and Norway. However, while these opposing groups regularly clashed violently in the UK, such interactions never occurred in Norway. This paper seeks to explain why seemingly similar group dyads produced different outcomes in different cases. In doing so, we trace relevant causal mechanisms derived from social movement theory in a comparative case study design. The paper can also be read as a response to Busher and Macklin’s call for improving conceptual clarity in research on “cumulative extremism.” Part of our response is introducing an alternative concept: Reciprocal intergroup radicalisation (RIR). Our analysis further shows that in Britain, RIR was fuelled by the presence of militant activists on both sides, a competent leadership, a central enemy image of the adversary, and a perception of unjust repression by security authorities. Conversely, the absence of these factors contained RIR between the Norwegian groups. In conclusion, our paper cautions against exaggerating the threat from RIR as multiple conditions must combine for RIR to occur. Even in Britain, by many considered a hotbed for RIR, the combined presence of these conditions was short lived.