This chapter follows a series of important moments in the twentieth-century history of system-vulnerability thinking: the interwar articulation of strategic bombing theory in Europe and the US, which focused on the 'vital targets' of an enemy's industrial system and the development of defence mobilization and emergency preparedness in the US during the Cold War as a means of defending the industrial system against a targeted nuclear attack. The moments also include the emergence of all-hazards planning and 'total preparedness' as paradigms for response to disruptions of vital systems; and the widespread diffusion of formal models for assessing the vulnerability of vital systems. The chapter describes the process through which a new way of defining and intervening in collective security problems emerged over the course of the twentieth century. Critical infrastructure protection is only one such response, and one whose actualization in bureaucratic arrangements, resource flows, and established regimes of security is just beginning to emerge.