© 2018 The Author(s). Literature on war and the environment has examined a wide range of militarized landscapes, but massive fortification systems such as the Maginot or Siegfried lines, which are symbols of a military trend in vogue during the interwar period, have largely been ignored. These military walls interwove natural and national values and constituted massive landscape interventions, aimed at reinforcing political borders, embedded in-and relying upon-geographical features. This article examines a late example of this trend: the fortification of the Pyrenees border that the Spanish dictatorship carried out during the 1940s. Particularly after the liberation of France in 1944, the Francoist regime engaged in a serious effort to build a fortification system in the Pyrenees, fearing a potential invasion; by the early 1950s, several thousands of bunkers formed what became the Pyrenees Line. Through these efforts, the Francoist army attempted to convert what Spanish fascists regarded as a spiritual wall-the political border with France-into a truly physical separation. Today, the remaining fortifications are material ruins of the Spanish isolation after the Second World War, when the Francoist regime closed in on itself, until the Cold War, when Francisco Franco became a US ally. Altogether, the case of the militarization of the Pyrenees shows how walls, fences, and other forms of fortification can be a fertile ground for environmental history to explore the mix of culture and nature as well as the political implications of the concept of natural borders.