Current studies of British citizenship and nationality neglect the development of legal frameworks prior to the Second World War. A growing body of literature, invigorated by the 2017 Windrush scandal, charts the collapse of imperial citizenship as a dimension of British decolonisation from the 1960s onwards. In contrast, this article analyses how the British empireâs framework of national belonging became strained during the early 1920s, as Dominion leaders increasingly asserted their own sense of statehood and the League of Nations mandates system introduced new forms of imperial rule. The article considers General Jan Smutsâ decision to afford British naturalisation to 7,000 German colonists residing in the mandate of South-West Africa. League officials argued that Smutsâ scheme undermined the anti-annexationist âspiritâ of the Covenant, because mass naturalisation represented a practical declaration of South African sovereignty in the mandate. Meanwhile, British mandarins in the Home, Colonial, and Foreign Offices believed Smutsâ policy would destabilise the empireâs constitutional distinctions between territorial zones of formal and informal imperial governance. They also feared it would inspire subaltern inhabitants of other British-protected foreign spaces, especially mandatory Palestine and the Indian princely states, to similarly demand naturalisation in order to claim stronger legal rights for themselves as British subjects. Ultimately, Smuts leveraged his political stature to secure British and League consent for his plan. To maintain a façade of constitutional coherence and metropolitan control, Whitehall mandarins recast Smutsâ naturalisation scheme as an imperial anomaly. Non-European inhabitants of the British mandates, and the wider informal empire, were granted the uncodified, indeterminate status of âBritish Protected Personsâ (BPPs). Recent scholarship has recognised BPP status as a form of de facto statelessness. Inter-war policymakers in the Home and Colonial Offices drew similar parallels, this article shows.