Fortified Sector of Colmar The Fortified Sector of Colmar ( " Secteur Fortifié de Colmar " ) was the French military organization that in 1940 controlled the section of the French frontier with Germany in the vicinity of Colmar . The fortifications were built as part of France ' s Maginot Line defensive strategy , but the sector lacks the large interconnected fortifications found along France ' s land border with Germany . 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 SF Colmar was flanked to the north by the Fortified Sector of the Lower Rhine to the north and the Fortified Sector of Mulhouse to the south . The Colmar sector was directly attacked by German forces on 15 June 1940 , capturing or destroying most of the fortifications in the sector in three days . Concept and organization . The Maginot Rhine defenses employed three lines of defense , with blockhouses or casemates close to the Rhine ( the first line ) , backed by infantry shelters ( the second line ) . The third line was a strong series of casemates , built on the model of interval casemates in the northeastern sections of the Line , but without lower levels . All of the Colmar fortifications were built in the mid - 1930s 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 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 Colmar sector was under the overall command of the French 8th Army , which was in turn part of Army Group 3 under General Besson . The SF Colmar was commanded by General Coradin , then General Poisot and finally General Cousse from 1 January 1940 . The command post was at Colmar . 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 . The primary interval formation was the 54th Infantry Division . Artillery support for the sector was provided by the first battalion of the 170th Position Artillery Regiment ( " Régiment d ' Artillerie de Position ( RAP ) " ) , which controlled both fixed and mobile artillery , commanded by Chef d ' Escadron Maigret . The 54th ID was a Class B reserve formation , not suitable for sustained or heavy combat . From 16 March 1940 the SF Colmar was designated the 104th Fortress Infantry Division , or " Colmar Division , " incorporating the 9th and 10th battalions of the " Chasseurs Pyrénéens " . At the midpoint of the Battle of France on 1 June 1940 , the fortress troops of the 104th DIF amounted to two fortress infantry regiments in four battalions and one regular infantry regiment in three battalions , comprising 300 officers and 10 , 300 men . Description . The sector includes , in order from west to east , the following major fortified positions , together with the most significant casemates and infantry shelters ( " abris " ) in each sub - sector . Sub - sector of Elsenheim . 42nd Fortress Infantry Regiment ( " 42e Régiment d ' Infanterie de Forteresse ( RIF ) " ) , Lt . Colonel Fonlupt . Sub - sector of Dessenheim . 28th Fortress Infantry Regiment ( " 28e Régiment d ' Infanterie de Forteresse ( RIF ) " ) , Lt . Colonel Roman History . Battle of France . The French Rhine defenses did not see significant action until the middle of June 1940 . By this time the main French army was in full retreat . The casemate lines along the Rhine were not supported by significant mobile forces or field artillery , which had been diverted to more urgent tasks . The casemates were designed to be mutually supporting , with fields of fire along the Rhine rather than across it . The first line was exposed to enemy fire from across the Rhine , a distance of a few hundred meters . German forces , under General Dollmann , amounted to seven divisions of the Seventh Army , supported by about three hundred artillery pieces . The codename " Kleiner Bär " ( " Little Bear " or " Lesser Bear " ) was given to the planned German cross - Rhine operation . The initial attack started at 0900 ( some sources say 1000 ) on 15 June with artillery bombardments . 8 . 8 cm guns concentrated on the French positions just across the river , destroying many in a few minutes . At 0920 the first assault crossed the river , composed of the German 218th Infantry Division at Schoenau , the 221st Infantry Division at Limbourg and the 239th Infantry Division at Sponeck . Supporting operations were carried out by the 554th ID at Neuf - Brisach and the 557th ID at Rhinau in the Lower Rhine sector . In most locations the French were unable to mount a serious defense , lacking artillery support and mobile forces . Riverbank positions were cleared quickly or destroyed by the Germans . The strongest resistance was posed by Casemate Schoenau Sud , which caused the 218th ID to suspend a portion of its crossing operations . Stronger resistance came from the third defensive line , supported by interval troops . Heavy fighting took place around Neuf - Brisach , where the German 554th ID ' s 623rd Infantry Regiment encountered Casemate Fort - Mortier and its supporting troops from the 28th RIF . The 623rd was pinned to the riverbank . The 557th ID ' s attack at Rhinau was intended to be a diversionary attack , but it produced significant success . The artillery bombardment and infantry attack destroyed most French positions , with only Casemate Rhinau Sud holding out through the day . Several positions in the third line were taken by the Germans on the 15th at Rhinau . In all , at the end of 15 June , German forces held a shallow bridgehead wide , extending to the third line , about back from the riverbank . The Germans used the night to land reinforcements . 16 June opened with another artillery bombardment , followed up by attacks on the third line of casemates , supported by unopposed Stuka attacks . By evening third - line casemates in the central portion of the front were being captured or were surrendering . The most significant resistance was mounted by Casemates Marckolsheim Sud and Nord , but Sud was captured by evening and Nord the next morning . Apart from some casemate crews left to cover their retreat , the survivors of the 104th and 105th DIF abandoned the remaining fortifications overnight , retreating to new positions in the Vosges mountains . Colmar was occupied on the 17th , but some casemates held out until the 18th . The French Army Group 2 , of which the 104th and 105th were a part , was encircled in the Vosges , remaining there until they surrendered at last on 25 June . Units . The 28th Fortress Infantry Regiment was positioned in the Dessenheim sub - sector . German fire on the 15 June forced many riverside casemate garrisons out of their positions , followed up by aerial bombardment the next day One the 18th the regiment retreated , the first battalion to Hohneck and the second to the Markstein and the Petit - Ballon in the Vosges . By the 21st the first battalion was pushed back to the Reinkopf , the La Bresse , finally surrendering at Wildensein . The second battalion was surrounded at Markstein , where most of the unit was captured . The 42nd Fortress Infantry Regiment held the Elsenheim sub - sector . On 17 June the regiment retreated to Kaysersberg in the Vosges , fighting an action against the Germans at the Col du Surceneux . The regiment was captured in 22 June . 1944 . As Allied forces approached the Rhine in November 1944 , the Germans destroyed many of the bankside fortifications that had escaped damage in 1940 . 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 . Many village line positions remain , but have been stripped by salvagers . Marckolsheim Sud ( 35 / 3 ) casemate has been preserved and is operated as the Memorial Museum of the Maginot Line of the Rhine ( " Musée Mémorial de la Ligne Maginot du Rhin " ) . The STG blockhouses at Aschenbach and des Vernes are preserved as well .