This history of colonial legacies in UN peacekeeping operations from 1945–1971 reveals how United Nations peacekeeping staff reconfigured the functions of global governance and sites of diplomatic power in the post-war world. Despite peacekeeping operations being criticised for their colonial underpinnings, our understanding of the ways in which colonial actors and ideas influenced peacekeeping practices on the ground has been limited and imprecise. In this multi-archival history, Margot Tudor investigates the UN's formative armed missions and uncovers the officials that orchestrated a reinvention of colonial-era hierarchies for Global South populations on the front lines of post-colonial statehood. She demonstrates how these officials exploited their field-based access to perpetuate racial prejudices, plot political interference, and foster protracted inter-communal divisions in post-colonial conflict contexts. Bringing together histories of humanitarianism, decolonisation, and the Cold War, Blue Helmet Bureaucrats sheds new light on the mechanisms through which sovereignty was negotiated and re-negotiated after 1945.