Drawing on extensive archival research at the UK National Archives, this volume examines how “London” (primarily British diplomats in Spain and officials at the Foreign and Commonwealth Office) perceived the radical Basque nationalist group ETA's campaign of violence from 1968 to 1993. In the summer of 1968, reports of the killing of a Spanish Guardia Civil officer by the then little-known ETA appeared in British diplomatic despatches. Over the next five decades, ETA would claim more than 850 lives. All the while, UK diplomats and officials gathered information and analysed ETA within the constantly changing, tumultuous sociopolitical context of Spain’s transition from dictatorship to democracy. This book offers a rare external perspective on what London often referred to as the “Basque problem”, while also shedding light on the relationship between ETA and the IRA, and the joint efforts by Madrid and London to confront their shared threat. ETA and the “Basque Problem” will appeal to students and scholars of Basque nationalism, contemporary Spanish history and terrorism studies.