In fragile and conflict-affected states (FCS), governments must rebuild three core governance functions: provision of security, service delivery and political participation. We unpack the connection between service delivery and legitimacy, using a staged model of legitimation, in which progress on the governance functions forms the basis for value-based legitimacy; behavioural legitimacy may, but does not necessarily, follow. With data from Iraq, we explore the role of water services in laying the groundwork for legitimacy. The analysis underscores the complex, non-linear relationship between service delivery and increases in trust and legitimacy, and the process's sensitivity to starting points. Nascent governments can build legitimacy by improving service delivery; however, gains are contingent and often fragile.