Defensive Sector of the Rhône The Defensive Sector of the Rhône ( " Secteur Défensif du Rhône " ) was the French military organization that in 1940 controlled the section of the French border with Switzerland and Italy in the area of Geneva . The area was not regarded as a likely point of invasion , owing to the neutrality of Switzerland and the extremely difficult terrain along the Italian portion of the border . It was therefore lightly fortified . Its chief fortified position was the 19th - century Fort l ' Écluse , manned by a small force , which held its strategic position against a " Panzerkorps " for a week in June 1940 . Concept and organization . The SD Rhône was largely ignored until 1939 , when a belated program was initiated to deal with a potential invasion through Swiss territory . The French were aware that the Swiss were fortifying their National Redoubt in the high Alps that would bar most invasion routes , but the French command thought it prudent to undertake their own measures . An initial 1937 plan envisioned three areas of effort , building Maginot - style " ouvrages " at strategic locations : Four " ouvrages " were to be situated to the south of the lake , three armed with 75mm guns in casemates and one with a 75mm gun turret : A final location in the upper Arve valley , armed with 75mm guns in casemates : Since funding was more urgently needed on fronts that were obviously exposed , these proposals bore no fruit . A 1938 revision scaled the proposal back , replacing " ouvrage " Crêt Mourex with a fortified position at the bottom of the Col de la Faucille , along with five blockhouses at Crêt Mourex , Riamont , Le Pailly , Petit - Montrond and Montets , and a demountable turret at Arbère . The positions in Haute - Savoie were replaced by a fortification at the Saint - Gingolph frontier with four demountable turrets , as well as a center of resistance with six blockhouses at Les Dranses covering the Les Epinches - Pont de la Douceur , Morzine - Les Cluses , and Martigney - Chamonix axes , each with two blockhouses . In the final event , even this program was considered unaffordable and unjustified , and was scaled back yet further . Command . The Rhône sector was under the overall command of the Army of the Alps , headquartered at Valence , under the command of General René Olry . The SF Rhône was commanded by General Michal . The command post moved several times . It was first at Annecy in the Hôtel Beau - Rivage , then the Hôtel Jeanne - d ' Arc from 2 September 1939 . The command post moved to Sevrier , in the Hôtel de la Payde on 20 June 1940 , then to Marthod on 24 June , then back to Sevrier the next day . The field army unit assigned the sector was the First North African Infantry Division . From October 1939 , some units were replaced by the 64th Infantry Division , then in turn by the 66th Infantry Division . The SD Rhône was disestablished on 15 July 1940 , after the French surrender . Description . The sector was manned by the 230th " Demi - brigade Alpin de Forteresse " under the command of Colonel Lanoyerie . Prior to mobilization in 1939 , no active units were stationed in the sector . The sector includes , in order from north to south , the following sub - sectors and fortifications : Sub - sector of Faucille - Fort - l ' Écluse . At a narrow point in the Rhône valley , the sector consists only of the Fort l ' Écluse , built in two stages in the first half of the nineteenth century . The fort was modernized with the Casemate du Tunnel , which guarded a road tunnel passing near the fort . The casemate was armed with two anti - tank gun , never installed , and automatic rifles . The upper portion of the fort mounted four 90mm guns . Sub - sector of Chablais . The Chablais sub - sector covered the territory around Lac Léman with a series of blockhouses defending strategic routes . The sub - sector was further subdivided . Sub - sector of the Arve . The Arve sub - sector faced Italy with one blockhouse at Magland , two at Saint - Roch , and an " abri " at Vallorcine . Battle of France . No action was taken by Italian forces directly against the SD Rhône during the Italian invasion of France in June 1940 . However , German forces advanced along the Rhône valley from the north . In early June the 230th DBAF ( a " demi - brigade " of fivebattalions ) was placed to block the German advance along a front between Frangy and Annecy . The German force was no less than Panzergruppe von Kleist , composed of the 3rd and 4th Panzer Divisions and the 13th Motorized Infantry Division . The Fort l ' Écluse blocked the German advance toward Albertville , manned by a company of the 179th Alpine Fortress Battalion along with personnel from several miscellaneous units of artillery and regional reserve forces . Contact was made between French and German forces at l ' Écluse on 22 June . The French held off the Germans , destroying the railway viaduct in front of the fort on 24 June . The Germans did not press further action , with the armistice taking place on the 25th . Since no orders had been given to the fort ' s garrison following the armistice , they remained in place , barring passage . The standoff continued until 30 June , when General Charles Huntziger , in charge of post - surrender French forces , ordered the unit to permit passage to the Germans , and to surrender . Units . The 230th DBAF was made up of five battalions , the 179th and the 189th " batallions alpins de forteresse " and the 199th BCHM , as well as the attached II / 440th Pioneer battalion and the II / 281st infantry battalion . Apart from the unit taken prisoner at the Fort l ' Écluse , the remainder of the 230th moved to the Rumilly area to regroup after the armistice . They ended up at Annecy on 17 July , and the regiment was dissolved on 31 July 1940 . Active personnel were assigned to the Vichy French " bataillon de chasseurs de la Haute - Savoie " . Present status . The Fort l ' Écluse was purchased by a syndicate of nineteen communities of the " Pays de Gex " and is operated as a museum . It has been open to the public since 1995 .