By focusing on military experts and so-called experts on ‘terrorism’, this chapter tries to make a contribution to an international political sociology of war and security through an analysis of the latter’s constitutive theoretical practices. More specifically it tries to highlight how theoretical practices infuse and permeate security-policies, while focusing on two overlapping ‘macro-practices’: war and counterterrorism. In the first part, we focus on military strategy as theoretical practice and its relation to warfare, showing how strategic theory is itself imbued and pervaded with a ‘practical logic’ that is however silenced in the process of theorization. In the second part we turn towards a specifically contemporaneous preoccupation of countering ‘terrorism’, showing how self-proclaimed experts on ‘terrorism’ have managed, through their theorizations of violence and conflicts, to put the concept of ‘terrorism’ front and centre in contemporary security know-how, thus contributing to the mutation and even de-differentiation of warfare and policing. Overall, this chapter shows that security-practices are infused with specific problematizations and theorizations, while the latter are themselves pervaded by a practical logic the understanding of which, we argue, is central to the more general sociology of war and security.