This Paper investigates expert contributions to public debates where joint military response mechanisms are invoked. Security threats triggering collective security systems provide an interesting case for foreign and security policy decision-making. In times of collective security crisis, policy responses are frequently developed under conditions of extensive confidentiality. Given this limited government publicity, non-governmental foreign and security policy experts may play a crucial role by triggering debate and contextualizing policy-options for a domestic audience. Thus, they may reconstruct the publicity that is an essential precondition for public opinion formation and, ultimately, democratic legitimacy. The central objective of the Paper is to better understand the role played by experts in this process. The publicity-generating role of experts is analysed through two cases of collective defence: the terrorist attacks in New York/Washington (2001) and Paris (2015), when the United States and France invoked solidarity clauses, Art. 5 NATO Treaty and Art. 42(7) EU Treaty respectively. The paper studies the debate on policy options in two of such allied countries, the UK and Germany. We map and analyse government statements and expert interventions in major news outlets on the basis of framing analysis. This approach allows us to map the diversity of frames under which interventions are being discussed. We further seek to obtain an understanding of the types and variety of experts involved in collective defence debates. The longitudinal analysis of two cases allows us to consider differences in the role of expert interventions in the public sphere both between countries and over time. Previous research suggests that experts have a substantial ‘framing power’ in news reporting. This Paper seeks to investigate whether the public intervention of experts do in fact succeed in diversifying and fomenting a critical public debate, or whether they instead the official frames promoted by public authorities prevail.