Plan XVII () was the name of a "scheme of mobilization and concentration" that was adopted by the French (the peacetime title of the French ) from 1912 to 1914, to be put into effect by the French Army in the event of war between France and Germany. It was a plan for the mobilisation, concentration and deployment of the French armies and it made possible an invasion of Germany or Belgium or both before Germany completed the mobilisation of its reserves and simultaneous to an allied Russian offensive. 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 The French armies (and the British Expeditionary Force) in Belgium and northern France were forced into a retreat as far as the Marne river, where at the First Battle of the Marne the German armies were defeated and forced to retreat to the Aisne river, eventually leading to the Race to the Sea. Background Concentration plans 1871–1911 From 1874–1880, General Raymond Adolphe Séré de Rivières oversaw the construction of the Séré de Rivières system, a line of fortresses long from Belfort to Épinal and another line of similar length from Toul to Verdun, about 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 (Charmes Gap), wide, between Épinal and Toul was left unfortified and the fortress city of Nancy was to the east, 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 incomplete in 1914. During the 1870s, the French army drew up concentration plans according to a defensive strategy, which exploited the Meuse and branches of the Moselle parallel to the 1871 border. The completion of the fortress lines between Belfort and Verdun in the late 1880s and railway building from the interior to the border, then gave the French army the means to contemplate a defensive-offensive strategy, in which a German attack would be repulsed and then followed up by a counter-attack. In August 1891, Plan