In the wake of World War I, geographers helped advise national delegations at the 1919 Paris Peace Conference whose purpose was to delimit Europe’s new boundaries. The paper examines the role played by British geographers, specifically Alan Ogilvie and the British geographical delegation, in the Treaty of Trianon (1920) which greatly reduced Hungary’s territorial extent. Attention is paid to contemporary published work on the new Europe, particularly Marion Newbigin’s Aftermath: A Geographical Study of the Peace Terms and Ogilvie’s Boundary Settlement (1922). Assessment of manuscript diaries and correspondence reveals the complex circumstances faced by geographers engaged in peace work. The work of different practitioners – in the Royal Geographical Society (RGS), over how national boundaries should be arrived at (on either ethnic or physiographic grounds) – was hindered by inadequate map provision from British geographical institutions. This led Ogilvie to propose a new geographical body for Britain at a time when the RGS was facing criticism and when the meetings of the British Association for the Advancement of Science, not the RGS, provided the forum for discussion of the new post-war Europe.