Great Britain is regarded as a classic example of the Weberian state, and thus as a model of a developed state that might be contrasted with developing states. However, this view conceals the formative role of empire in the evolution of the British state. Rather than take the distinction between a ‘metropolis’ and a ‘periphery’ as given, this article explores the mutual constitution of state and empire. What it finds is that the political identity of the British state changed dramatically during the first half of the twentieth century as British intellectuals and policy-makers attempted to develop a new political community, primarily through the vehicle of the Commonwealth. The British state of the interwar years decentralised its decision making and embedded itself firmly in new multilateral networks. A rationalised, centralised British state emerged after the Second World War and only then within a context of multiple (principally Atlantic and European) political identities. The modern British state is as much a post-colonial invention as are states of the ‘developing’ world.