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.

The defense of the area surrounding Strasbourg benefited from the width of the Rhine and its numerous oxbows and dead arms that complicate movement on the French side of the river. As with the sectors further 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.

...
Wikipedia