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.

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.

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.

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