Once Savoy became part of France, the Fréjus Tunnel became a possible invasion route from Italy to France, avoiding the difficulties of the Mont Cenis pass. Accordingly, an extraordinary amount of fortification was placed around Modane. The Fort du Replaton and the Fort du Sapey were built in the late 19th century on the heights across the valley of the Arc. In the 1930s the Maginot Line fortifications Ouvrage Saint-Gobain, Ouvrage Saint-Antoine and additional fortifications at Le Sappey were built.