Plan XVII

Plan XVII was the name of a "scheme of mobilization and concentration" that was adopted by the French Conseil Supérieur de la Guerre from 1912–1914, to be put into effect by the French Army in the event of war between France and Germany. Though it was not "a prescribed narrative for the campaign" or battle plan, the deployment made possible a prompt invasion of Germany and/or Belgium before Germany could mobilise its reserves, simultaneous to a Russian invasion of East Prussia. The plan was implemented from 7 August 1914, with disastrous consequences for the French, who were defeated in the Battle of the Frontiers at a cost of 329,000 casualties. The French northern armies were forced into a retreat as far as the Marne river, where in the First Battle of the Marne , the German armies were defeated and forced back to the Aisne river.

From 1874–1880, General Raymond Adolphe Séré de Rivières oversaw the construction of the Séré de Rivières system, a line of fortresses 65 km long from Belfort to Épinal and another line of similar length from Toul to Verdun, about 40 km back from the frontier. The River Meuse flows northwards from Toul to Verdun, Mézières and Givet on the Belgian border and a tributary of the Moselle between Belfort and Épinal, near parallel to the 1871–1919 French-German border. The Trouée de Charmes , 70 km wide, between Épinal and Toul was left unfortified and the fortified city of Nancy was to the east, 12 km from the German frontier. A second series of fortifications, to prevent the main line being outflanked was built in the south, from Langres to Dijon and in the north from La Fère to Rheims and from Valenciennes to Maubeuge, although for financial reasons these defences were not complete in 1914.

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