Following the First World War, French defense policy was re-examined by Marshal
Philippe Pétain. The 1919 study recommended the creation of a mobile army that could carry war to the territory of an opponent, supported by frontier defenses that would prevent an opponent from penetrating French territory. The second part of the recommendation was to involve the creation of fortified regions () in which an opponent could be confronted; the recommended fortified regions included • The Rhine frontier, with two fortified regions, one to the south of
Mulhouse and a northern region defending the Alsace plain to the frontier on the
Lauter. • An area to the west of the Rhine to prevent an incursion across the land frontier with Germany onto the Alsace plain. • An area in Lorraine to defend the coal-mining regions in the Sarre basin The regions were to provide locations in which to confront an opponent and from which to launch an offensive to carry the battle to the opponent. The fortifications of the Séré de Rivières system of the 1880s were designed to defend the new eastern frontier following the
Franco-Prussian War. With the recovery of these territories following World War I, most of the Séré de Rivières forts were deep in the interior of north-eastern France and not useful. The Belfort region had remained in French hands and had been fortified already. A new (RFB) was again proposed in 1922 by the Territorial Defense Commission (), which identified three potential invasion routes from Germany, 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
Fortified Region of Metz-Thionville-Longwy • The
Fortified Region of the Lauter • The Fortified Region of Belfort The report did not make specific recommendations on the location or character of the defenses. The Commission on Defense of the 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 making 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 , 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 was under the command of the
Eighth Army,
Army Group 3 in 1940, with subordinate sectors including the
Defensive Sector of Montbéliard,
Defensive Sector of Altkirch and the
Fortified Sector of the Jura. The RFB became the 44th Fortress Corps (CAF) from 16 March 1940, losing the Jura sector to the 45th CAF. ==Forts==