The effect of the Treaty of Trianon (1920) upon Hungary’s boundaries is well understood. Surprisingly, however, almost no attention has been paid to the work of the various national geographical delegations which helped advise politicians as the boundaries of Europe were re-cast after World War I. This paper examines the work of the British geographical delegation in Paris as it advised upon Trianon. Particular attention is paid to the work of Alan Ogilvie, and to Ogilvie’s relationships with Isaiah Bowman, effective head of the American delegation. The paper examines Ogilvie’s diaries and correspondence to provide detailed insight into how ethnic identity, linguistic difference and other criteria were used (or not) to define and map the new Hungary and the new Europe. The paper shows how questions to do with the complexity of Hungary’s ethnic diversity were known to British geographers and map makers as early as 1915. It shows, too, that no single view was held over how to map boundaries: some British geographers favoured delimitation based upon physiographic divides, such as river and drainage basins. Others favoured ethnic difference based on use of the mother tongue. For yet others, attention was paid to lines of communication and to economic market area. Because such differences were apparent in the British geographical community during World War One, notably between prominent members of the Royal Geographical Society, British geographers offered no consistent view upon Hungary’s delimitation and ‘dismemberment’ following Trianon. British geographers turned to the meetings of the British Association for the Advancement of Science in order to debate these differences and to review Hungary’s position in the new Europe after 1920.