This paper explores the utility of the interpretive device of ‘Balkanism’ for the analysis of British foreign policy-making in the 1920s. It does so through the case study of the efforts to construct a regional security pact involving guarantee and arbitration treaties reconciling former enemies, a so-called ‘Balkan Locarno’. This was not the most important or fruitful initiative in interwar diplomacy, but it is nonetheless revealing. On the one hand, it illustrated the existence amongst British policy-makers of a network of deeply embedded negative and essentialising assumptions about the region and its peoples. On the other, it casts new light on the nature of the 1925 Locarno settlement itself: the terms in which British policy-makers construed the extension of the Locarno principles indicates that in endowing it with mythic significance, they overlooked the fact that it was concluded only because of the coincidence at a particular moment of otherwise divergent national interests.