Plan XVII was the name of a "scheme of mobilization and concentration" that was adopted by the French
Conseil Supérieur de la Guerre (the peacetime title of the French
General Staff) 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 the Russians' 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 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.