Fortified Sector of Mulhouse The Fortified Sector of Mulhouse ( " Secteur Fortifié de Mulhouse " ) was the French military organization that in 1940 controlled the section of the French frontier with Germany in the vicinity of Mulhouse . The sector ' s principal defense was the Rhine itself , which could be crossed only by boat or by seizing a bridge crossing . The sector ' s fortifications chiefly took the form of casemates and blockhouses . The sector did not see significant action during the Battle of France , since the German crossings of the Rhine took place father north , near Colmar . The fortifications in the sector were built as part of the overall plan for the Maginot Line , but did not include the major fortified positions that characterized the Maginot Line of northeastern France . The sector did not see major fighting in World War II . It was bordered to the north by the Fortified Sector of Colmar and on the south by the Fortified Sector of Altkirch . Concept and organization . The fortifications of the Mulhouse sector were laid out in two major lines , one right on the Rhine , and another to the rear on the eastern edge of the Hardt forest . No fortifications were established south of Hombourg , owing to the provisions of the 1815 Congress of Vienna treaty , which forbade permanent fortifications within of Basel . Two positions were placed between the front and rear lines , and were considered a second line . The fortifications cosisted entirely of casemates and blockhouses , constructed by the " Commission d ' Organisation des Régions Fortifiées ( CORF ) " , CORF was responsible for the major fortifications of the Maginot Line , but in this area no major positions or " ouvrages " were built . The Hardt forest was regarded as a significant obstacle between the third line and Mulhouse . The riverbank fortifications were of a basic nature , with protection only up to 155mm caliber , machine gun armament and no electrical system . The second and third lines were more robust in construction and equipment , with electric generators and anti - tank weapons . The heaviest concentration of fortifications were in the area of Neuf - Brisach , itself a preserved fortress from the Vauban era . Neuf - Brisach guarded a strategic bridge across the Rhine . Command . The Mulhouse sector was under the overall command of the French 8th Army , which was in turn under the French 3rd Army Group . The SF Mulhouse was commanded by General Didio , with a command post at Rixheim . The fortifications were manned by the 10th Fortress Infantry Regiment . The interval troops , the army formations that were to provide the mobile defense for the sector , to support and be supported by the fixed defenses , were under the command of the 13th Corps ( " 13e Corps d ' Armee " ) , General Misserey , commander . Artillery support for the sector was provided by the 159th Position Artillery Regiment ( " Régiment d ' Artillerie de Position ( RAP ) " ) . From 16 March 1940 the SF Mulhouse was designated the 105th Fortress Infantry Division , or " Mulhouse Division . " At the midpoint of the Battle of France on 1 June 1940 , the fortress troops of the 105th DIF amounted to one fortress infantry regiment ( 10th RIF ) in three battalions , comprising 135 officers and 4 , 500 men . Description . The sector includes , in order from north to south , the following significant casemates and infantry shelters ( " abris " ) in each sub - sector : Sub - sector of Schliebach . 10th Fortress Infantry Regiment ( " 10e Régiment d ' Infanterie de Forteresse ( RIF ) " ) , Lt . Colonel Thiervoz < / div > Battle of France . The German offensive ( Operation " Kleiner Bär " ) across the Rhine was concentrated in the area near and to the north of Colmar , with little fighting in the Altkirch sector . The offensive , launched on 15 June , was rendered unimportant when the German XIX Corps under General Heinz Guderian reached the Swiss border on 17 June , behind the main French line . The riverside positions were evacuated on 16 June , with a general withdrawal of the 105th DIF into the Vosges on 18 June , regrouping in the Markstein area . An attempt was made to defend Kruth , but were dislodged with many captured by the Germans . Remnants fell back to the Col de Stiftkopf on the 22nd , then the Rouge - Gazon farm where they were finally captured on 26 June . Units . The 10th Fortress Infantry Regiment was the principal operational unit in the SF Mulhouse , and its operations in June mirrored those of its parent unit , the 105th DIF . Present situation . Most of the Rhine defenses were simply abandoned after World War II . Most of the first line of blockhouses on the banks of the river were destroyed when the river was widened as part of navigational improvements in the 1970s .