Fortified Region of Metz The Fortified Region of Metz comprised the central and most heavily fortified portion of the Maginot Line . The region was established in 1926 as a military organization for the French fortifications along the frontier with Luxembourg and Germany to the east of Longuyon in northeastern France , forming a shield to the north of the industrialized areas of Metz and Thionville . The region was dissolved as a military organization on 18 March 1940 , its command personnel moving to the 42nd Army Corps . The region was divided into four sectors : The region was given the highest priority in funding , manning and equipment of all Maginot regions . Creation . 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 Metz region was considered a potential base for a counter - offensive should border defenses be pierced and Lorraine threatened , possibly using the former German fortifications of Metz and Strasbourg . 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 . The " Conseil Supérieure de la Guerre " ( " Supreme War Council " ) created a committee in March 1920 to study France ' s newly adjusted frontiers , with Marshal Joseph Joffre as chairman . Disagreement arose between supporters of Pétain , who favored continuous light defenses covering concentrations of troops and construction materials for field fortifications , and supporters of Marshal Joseph Joffre , who advocated fortified regions like the ring of forts around Verdun , that had performed well during the first world war . The committee dissolved without reaching any conclusions . The RF Metz 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 last two directly affecting the Metz industrial basin , home of much of France ' s coal and steel industry . The CDT recommended that the industrial area of Metz - Thionville - Longwy be protected by new fortifications . The report noted that the frontier with Belgium would be difficult to fortify . 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 CDT ' s report proposed that offensive operations into Germany should focus on an incursion to establish a line from Konz to Trier , then to Ottweiler , Homburg and Kaiserslautern . No specific recommendations on the location or character of the defenses were made . The Commission on Defense of the Frontiers ( " commission de défense des frontiers " ) was established on 31 December 1925 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 again listed potential invasion routes , listing the area to the northeast of Hunsrück in addition to the previously - identified routes . It reiterated the proposed fortified regions around Metz , Belfort and along the Lauter . The report formalized the concept of the fortified region , to be about in depth , a distance dictated by the range of heavy artillery . The RF Metz - Thionville - Longwy was planned to provide a protected area from which to counter - attack an enemy moving into France along the Sarre , to protect the industrial regions , and to anchor the right flank of the northern armies . The German fortifications of Thionville and Metz were to be incorporated into the region , while the forts of Verdun , to the rear , were to be kept in reserve without renovation The line of the fortifications was modified in 1927 to leave Longwy exposed , anchoring the right end of the region instead at Longuyon . This allowed optimal siting of fortifications on the heights behind Longwy , rather than contending with the town ' s basin and its close proximity to the border . Construction . The " Commission d ' Organisation des Régions Fortifiés " , or CORF , was established on 30 September 1927 to oversee the design , financing and construction of the Maginot Line . Its first priorities were the Metz and Lauter regions . CORF built 14 " gros ouvrages " ( large artillery positions ) and 24 " petit ouvrages " ( more lightly armed infantry works ) , as well as more than one hundred casemates and infantry shelters in the RF Metz . In 1929 an initial appropriation of 1 , 498 , 000 , 000 francs was allotted to the RF Metz , the most expensive region in the Maginot system . The highest - priority positions in the RF Metz were " ouvrages " Rochonvillers and Hackenberg , with work beginning in 1929 . A continuation of the construction campaign starting in 1930 resulted in the construction of three more large positions , six lesser artillery " ouvrages " , and seven infantry positions . Four interval casemates were built in 1930 . At the same time , a program to provide the Line with long - range artillery was gradually dropped as costs mounted . By November 1930 the RF Metz had been allocated 2 , 298 , 000 , 000 francs . From 1931 new projects were developed to extend the RF Metz to the west from Rochonvillers to Longuyon , and eastward to the Sarre . The left , or western wing was allocated an additional 400 , 000 , 000 francs . By the end of 1935 , when CORF was disestablished , the sector ' s positions were being commissioned and garrisoned . However , budget cuts were felt in the eastern wing , where the expansion of several " petit ouvrages " was deferred to 1940 , never started after war threatened . Operations . The Fortified Region of Metz was organized as the 6th Military Region in peacetime . On 2 September 1939 a general mobilization of French forces was declared . Reserves were called up and the Maginot Line was fully manned . The Phoney War continued into 1940 . The Fortified Region of Metz was disestablished on 18 March 1940 , two months before the Battle of France , with Army command exercising a more direct control over the four sectors . At the time of its dissolution , the RF Metz was commanded by General Le Maignon de Kérengat from the Fort de Saint - Julien at Metz . After the dissolution of the fortified region , the staff was transferred to the 42nd Fortress Army Corps , formerly the SF Crusnes .