When and how do party politics matter in junior allies’ decisions to engage in multinational military operations? Developing a new role theory model of party politics and multinational military operations, we put forward a two-level argument. First, we argue that the rationale for military action is defined in a contest between political parties with expectations of what constitutes the proper purpose (constitutive roles) and functions (functional roles) of the state. Second, we hold that material and ontological insecurities reduce political space for contestation and debate, but that junior allies tend to focus on role demands for ‘good states’ and ‘good allies’ rather than the nature and aim of the military operation. To unpack our argument, we analyse the debate among political parties in Romania and Denmark leading up to the invasion of Iraq in 2003. Concluding our analysis, we outline the implications for the changing security order and current debates in NATO member states on how to respond to the Russian invasion of Ukraine.