The Fortified Sector of the Jura (Secteur Fortifiée du Jura) was the French military organization that in 1940 controlled the French frontier with Switzerland in the general area of Besançon. The area was lightly fortified, since the Swiss border was regarded as a low-risk frontier, save for the possibility of an advance from Germany through Switzerland into France. The Jura Mountains and the Doubs River presented natural barriers to invasion, allowing defenses to concentrate on a few points of potential access. The SF Jura was bordered on the east by the Fortified Sector of Montbéliard.