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.