Geopolitical and societal changes are increasingly narrated using the concept of complexity. In a range of policy contexts, from climate to finance and peacekeeping to defence and security, complexity is often prominent. This article explores the contemporary expressions of complexity in defence and security scholarship and policy. To understand the expressions of complexity, we view it as a ‘folk theory’ – what others have called a category of practice. We illustrate how this folk theory has gained prominence in NATO policy regarding the organizing of future war and warfare as one example of a policy context in which expressions of complexity are produced and promoted. Specifically, we examine NATO’s future warfighting concept – multi-domain operations – and explore how the concept offers a way and a new language to make sense of an uncertain future and make it governable from the present. We discuss some consequences of a ‘folk theory’ becoming the explanation rather than one of many explanations of the future defence and security landscape. As a representative of folk theory, complexity offers a strong performative master narrative for a variety of orientations, yet one whose practical relevance for the organization of security can be questioned.