Fortified region of Belfort The fortified region of Belfort " ( place fortifiée de Belfort ) " formed the first line of defense in the Séré de Rivières system of fortifications in the Belfort Gap . Located in northeastern France between Épinal and Besançon , the primary line was built in the late 19th century to deal with advances in artillery that had made older defensive systems obsolete . History . Belfort has been a fortified place since the Middle Ages , when a castle was built in 1226 . After withstanding seven sieges , the medieval fortifications were modernized between 1637 and 1648 by the Comte de la Suze . Vauban added hornwork s and a barracks to the castle , and enclosed the city in a new wall according to Vauban ' s " second system . " These fortifications successfully resisted a siege in 1815 . As armies made greater use of artillery , it became necessary to extend city defenses to keep the enemy sufficiently distant to prevent bombardment of the city from neighboring high ground . The first ring of forts was built at the direction of General François Nicolas Benoît , Baron Haxo , beginning in 1817 . The forts of La Miotte , La Justice and the Le Vallon camp were built , and the city walls were improved . Further increases in the range of artillery caused the construction of more forts to the south and east , resulting in Fort Bellevue , Fort des Barres and the Forts des Perches . These forts formed a rough ring about 1200m to 1500m from the town walls . They successfully resisted attack in 1870 - 71 . Shortly after France ' s traumatic defeat in the Franco - Prussian War of 1870 - 71 , Belfort , an important road and railway node , and the frontier cities of Épinal , Toul and Verdun were chosen to constitute the first line of defense against an enemy incursion from the annexed former French provinces of Alsace and Lorraine . The fortifications were part of an extensive network of new forts proposed and carried out by Raymond Adolphe Séré de Rivières . The fortified region of Belfort runs between Fort de Giromagny , part of the High Moselle line , and Fort Lachaux of the defensive network of Lomont . The object of the program was to transform the Belfort area into a fortified camp . The fortified camp concept concentrates artillery into permanent forts built around the city , with distances calculated to place enemy artillery out of range of the city , while allowing for mutual support between forts . At the end of the 19th century the fortified camp of Belfort included the original fortified belt built between 1793 and 1870 , and the Séré de Rivières forts built between 1874 and 1886 , with numerous smaller batteries . Inter - war . Following the First World War , French defense policy was re - examined at the order of Marshal Philippe Pétain . The 1919 study recommended the creation of a mobile army that could carry war to the territory of an enemy , supported by frontier defenses that would prevent an enemy from penetrating French territory . The second part of the recommendation was to involve the creation of fortified regions ( " regions fortifiés " ) in which an enemy could be confronted . The initially recommended fortified regions included : The regions were to provide locations in which to confront an enemy and from which to launch an offensive to carry the battle onto enemy territory . The previous fortifications of the Séré de Rivières system of the 1880s were designed to defend the frontiers of France that existed following the Franco - Prussian War , in which France had lost Alsace and Lorraine . With the recovery of these territories following World War I , most of the Séré de Rivières forts were deep in the interior of northeastern France and not useful . However , the Belfort region had remained in French hands and had therefore been fortified already . A renewed RF Belfort was again proposed in 1922 by the Territorial Defense Commission ( " commission de défense du territoire " ) , which identified potential invasion routes from Germany into France at three locations : the Belfort Gap , between the Vosges Mountains and the Rhine , and across the Lorraine plateau The committee ' s final report proposed a continuous fortification from the Swiss border north to the Lauter , and west to Longwy , with the area of the Sarre , which faced the demilitarized area of the Saarland , left unfortified . The final report recommended three fortified regions : The report did not make specific recommendations on the location or character of the defenses . The Commission on Defense of the Frontiers ( " commission de défense des frontiers " ) was established on 31 December 1935 by Prime Minister Paul Painlevé , to once again consider the question of the frontiers . The commissioned was charged with formulating specific recommendations for defenses and for the incorporation of the existing defenses of Metz and Belfort . The report recommended the creation of a line of defenses about to the east of Belfort , with heavier fortifications to the north and lighter positions to the south . A second line to the rear was to be constructed after a mobilization of the army . The forts at Giromagny , Mont Bart and Chaux were to support the fortification lines with artillery . World War II . In the 1930s , several forts were included in the eastern extension of the Maginot Line fortifications along the line of the Rhine . While not officially part of the new Maginot defenses , the older positions were integrated into the fortress unit command structure of the French army and were designated the Fortified Region of Belfort , with rough equivalence to an army corps . A more ambitious 1926 plan proposed more extensive improvements , although still short of Maginot standards . In practice , a few positions were improved and some new blockhouses were built in the spaces between the forts . The Fortified Region of Belfort was under the command of the French 8th Army , Third Army Group in 1940 , with subordinate sectors including the Defensive Sector of Montbéliard , Defensive Sector of Altkirch and the Fortified Sector of the Jura . The RF Belfort became the 44th Fortress Corps ( CAF ) from 16 March 1940 , losing the Jura sector to the 45th CAF . The forts . Forts built between 1792 and 1870 : Séré de Rivières forts built between 1874 and1886 The two Perches forts of the original belt were retained and rebuilt by Séré de Rivières , by reason of their strategic value during the final stage of the Siege of Belfort in 1870 - 71 . Following a decree by Minister of War Georges Boulanger in 1887 , the military structures were renamed with more " glorious " names , preferably relating to local military events . However , the local people did not always adopt these new names . Presently , the greater portion of the sites are no longer under military usage and are abandoned . Strategic railroad . The Chemins de fer stratégique du Territoire du Belfort served the belt of forts using a 600mm narrow - gauge railway according to the Péchot systrem , which was used for a number of similar military supply systems . Part of the system was considered permanent , while the Decauville line was easily disassembled . Construction took place from 1889 to 1893 . The trains transported materials , supplies , munitions and personnel , with traction supplied by a Péchot - Bourdon steam engine . The principal forts relying on the railway were Roppe , Bessoncourt , Chévremont , Vézelois , Meroux , Bois d ' Oye , Salbert , les Barres , the Forts des Perches and Mont - Vaudois .