This article applies a public policy instrumentation approach to two instruments of EU security policy - civilian crisis management and enlargement conditionality. Both have come to be portrayed, by policy-makers and observers alike, as deliberate and efficient responses to specific policy challenges. Our analysis of the processes behind their adoption challenges such claims. In a complex institution like the EU, and in a sensitive sector like security, the development of new policy instruments requires negotiation within dense institutional settings. The resulting instruments do not necessarily match the initial intentions of their creators. The focus on unanticipated - albeit in retrospect not necessarily undesired - consequences in the development of the instruments of EU security policy also contributes to the broader research agenda on policy instruments, which problematises the selection of policy instruments yet nevertheless tends to perceive them as part of a deliberate strategy of policy change.