The central contention of this article holds that scholars do not adequately assess and explain the influence of transboundary security issues on government behaviour. Their assessment is not adequate because they do not fully conceptualize the relationship between internal and external security concerns. Their explanations are not adequate because existing theories cannot fully explain how and why states respond to transboundary security issues. To rectify these concerns, stimulate and structure further research, and encourage scholarly dialogue, we build an analytical framework for (a) understanding what we describe as the ‘nexus’ of internal and external security matters, and (b) explaining why that nexus may change state behaviour on transboundary security issues. The resulting framework encourages a strong focus on the nature of transboundary problems before studying their implications for changes in perceptions, policies, politics and polity.