Abstract: Europe's relative political fragmentation is often suggested to have contributed to its unique institutional and economic development. But why was Europe so fragmented? Here we compile data on geography and the locations of borders between sovereign entities in Europe and its surroundings from 1500 to modern days. We find that borders tend to be located in mountainous terrain, by rivers, and in areas suitable for rainfed - but not irrigation-based - agriculture. We conclude that Europe's political fragmentation was at least partly due to its geography, in turn hinting at a novel causal link from geography to institutions. We also examine how the link from geography to borders has changed over time, finding that borders have become more correlated with elevation, and also propose a simple model explaining why.