Fortified Sector of the Lower Rhine The Fortified Sector of the Lower Rhine ( " Secteur Fortifié du Bas - Rhin " ) was the French military organization that in 1940 controlled the section of the French frontier with Germany in the vicinity of Strasbourg . The sector ' s principal defence was the Rhine itself , which could be crossed only by boat or by seizing a bridge crossing . While it was constructed by CORF , the organization responsible for the construction of the Maginot Line , the SF Lower Rhine was not a part of the core Line fortifications . The sector ' s fortifications chiefly took the form of individual casemates and blockhouses . Additional support was provided by the fortress ring around Strasbourg , whose fortifications were still active in 1940 . The SF Lower Rhine was flanked to the north by the Fortified Sector of Haguenau and to the south by the Fortified Sector of Colmar . The Rhinau section of the SF Lower Rhine was attacked by German forces in June 1940 as a diversion from the main German invasion operation in the SF Colmar . Follow - up incursions from the north and at Strasbourg left much of the SF Lower Rhine in German hands by the armistice of 25 June . Concept and organization . The defense of the area surrounding Strasbourg benefited from the width of the Rhine and its numerous oxbow s and dead arms that complicate movement on the French side of the river . As with the sectors father south , three lines of fortifications were built : a line of casemates right on the riverbank , a line of infantry shelters a few hundred metres to the rear , and a line of heavier casemates between two or three kilometres back from the riverbank . No interconnected " ouvrages " of the type found in sections of the main Maginot Line just to the west were built in the Lower Rhine sector . Strasbourg , directly on the riverside , could not be defended in depth , and so made do with casemates on the riverbank . Strasbourg was effectively considered an open city , as it could otherwise be easily destroyed by German artillery on the other side of the Rhine . However , some of the German " festungen " from the late 19th century surrounding the city were reactivated from 1935 , with blockhouses at Fort Pétain , Fort Ducrot and Fort Foch . Artillery positions were placed at Fort Ullrich , Fort Ducrot and Fort Ney - Rapp . 65mm naval guns were added at six additional positions . No substantial garrison was stationed at any of the old forts , only crews sufficient to serve the arms . 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 . Command . The Lower Rhine sector was under the overall command of the French 5th Army , headquartered at Wangenbourg , under the command of General Bourret , which was in turn part of Army Group 2 under General André - Gaston Prételat . The SF Lower Rhine was commanded by General Pichon , then General Renondeau from 5 January 1940 , then General Vallée from 28 May . The command was located at Fort Ducrot in Mundolsheim . Artillery support for the sector was provided by the 155rd Position Artillery Regiment ( " Régiment d ' Artillerie de Position ( RAP ) " ) , which controlled both fixed and mobile artillery , commanded by Lieutenant Colonel Agabriel . Fortress infantry units included the 34th and 172nd Fortress Infantry Regiments under Lieutenant Colonel s Brocard and Le Mouël , respectively . A variety of other units were attached to the sector , including three battalions of the 237th Infantry Regiment of the Fortified Sector under Lieutenant Colonel Vigneron . The Strasbourg ( " Place de Strasbourg " ) area was garrisoned by the 226th Infantry Regiment of the Fortified Sector under Lieutenant Colonel Marteau , and the 205th Regional Regiment . The interval troops , the army formations that were to provide the mobile defence for the sector , to support and be supported by the fixed defences , were under the command of the 17th Corps ( " 17e Corps d ' Armee " ) , General Noël , commander . The 6th Corps in turn comprised the 62nd Infantry Division under General Sarrebourse de la Guillonère . The 62nd ID was a Class B reserve formation , not suitable for heavy or sustained combat . The sector became the 103rd Fortress Infantry Division from 5 March 1940 , also known as the Strasbourg Division . At this time the Herrlisheim sub - sector passed to the control of the neighboring Fortified Sector of Haguenau . At the midpoint of the Battle of France on 1 June 1940 , the fortress troops of the 103rd DIF amounted to two fortress infantry regiments in four battalions plus one regular infantry regiment with three battalions , comprising 485 officers and 15 , 500 men . Description . The sector includes the following major fortified positions . The casemates were generally about apart , with gaps in areas of marshy ground or old sections of the Rhine . Sub - sector of Strasbourg . 172nd Fortress Infantry Regiment ( " 172 Régiment d ' Infanterie de Forteresse ( RIF ) " ) , Lt . Colonel Le Mouel , command post at Fort Pétain , then Schiltigheim . Sub - sector of Erstein . 34th Fortress Infantry Regiment ( " 34 Régiment d ' Infanterie de Forteresse ( RIF ) " ) , Lt . Colonel Brocart . History . Battle of France . The French Rhine defences 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 metres . German forces , under General Dollmann , amounted to seven divisions of the Seventh Army , supported by about three hundred artillery pieces , chiefly concentrated opposite the SF Colmar , just to the south . 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 , all in the SF Colmar . Supporting operations were carried out by the 554th ID at Neuf - Brisach ( SF Colmar ) and the 557th ID at Rhinau in the Lower Rhine sector . 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 . Most positions suffered a fate similar to Rhinau Nord , which was quickly overcome by the attackers with one French death . The German bridgehead was extended on 16 June , with a general withdrawal by French forces in the SF Colmar , leaving the Lower Rhine guarding the rear of the SF Haguenau and SF Vosges . The Germans continued to systematically assault and capture positions moving north from Rhinau . Meanwhile , the German 165th ID , having penetrated the main Maginot Line at the weakly defended Sarre sector , moved south into the SF Lower Rhine . The German 555th ID crossed the Rhine on 21 June and occupied Strasbourg . From then until the armistice took effect on 25 June , there was little additional movement . Units . The 172nd RIF was stationed along the riverside between Drusenheim and Rhinau . The regiment traded fire with German positions across the Rhine through the winter . The regiment ' s interval units received orders to fall back to the Bruche valley in the Vosges on 13 June . The riverside units retreated 18 June . The regiment surrendered 23 – 24 June in the area between Schirmeck and Rothau . 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 status . Most of the Rhine defences 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 .