Alps. As an Alpine pass, Mont Cenis featured in several historical incidents. One example is the descent of Constantine I to Italy, to fight against Maxentius. It was the site of a military victory by the French Army of the Alps, led by General-in-Chief Alex Dumas over Piedmontese forces in April 1794, a victory that enabled the French Army of Italy to invade and conquer the Italian peninsula. It was the principal route for crossing the Alps between France and Italy until the 19th century. It was also used as the main passage by which Charlemagne crossed with his army to invade Lombardy in 773, and later by Napoleon I. Mont Cenis was one of the most used Alpine passes from the Middle Ages to the nineteenth century. The pass was part of the border between the two countries from the annexation of Savoy to the Second French Empire in 1861 until the 1947 Treaty of Paris, but is now located completely in France. The treaty allowed Savoy to retrieve its historical and political boundaries. It has historically been part of Route nationale 6. A road over the pass was built between 1802 and 1805 by Napoleon to improve military connections. By 1810, it was the most travelled road between France and Italy, as Strasbourg was closed to silk trade traffic from Vienna, leading to Lyon becoming a major trade centre instead. The Mont Cenis Pass Railway was opened alongside the road in 1868, but was dismantled in 1871, on the opening of the Fréjus Rail Tunnel. It was the first ever railway based on the Fell mountain railway system and was worked by English engine-drivers. The Fréjus Rail Tunnel acquired the alternative, and geographically incorrect, name of Mont Cenis Tunnel because the traffic which formerly used the Mont Cenis Pass was transferred to it. When the Kingdom of Sardinia-Piedmont ceded Savoy to France, in 1860, the Mont Cenis became a frontier pass, and consequently a part of Savoy was left on the Italian side. It was therefore highly fortified as a protection against an invasion of the Val di Susa route towards Turin. In 1874-1880 the Italian Regio Esercito built three stone forts: Fort Cassa, Fort Varisello and Fort Roncia, supported by several batteries and fortifications, such as those at top of Mont Malamot. Two further armored batteries, La Court and Paradiso, were added in the early 20th century,