| Plan XVII | |
|---|---|
| Part of First World War | |
.jpg) | |
| Operational scope | Strategic |
| Location | Lorraine, northern France and Belgium 48°45′15.84″N 05°51′6.12″E / 48.7544000°N 5.8517000°E |
| Planned | 1912–1914 |
| Planned by | Joseph Joffre and the Conseil Supérieur de la Guerre |
| Commanded by | Joseph Joffre |
| Objective | Decisive defeat of Imperial German Army |
| Date | 7 August 1914 |
| Executed by | French Army |
| Outcome | Failure |
| Casualties | 329,000 |
  Grand Est Grand Est, the modern French administrative region of north-eastern France (including Alsace and Lorraine) | |
Plan XVII (pronounced [plɑ̃ dis.sɛt]) was the name of a "scheme of mobilisation and concentration" which the French Conseil Supérieur de la Guerre (the peacetime title of the French Grand Quartier Général) developed from 1912 to 1914, to be put into effect by the French Army in the event of war between France and Germany. The plan was for the mobilisation, concentration and deployment of the French armies to make possible an invasion either of Germany or of (neutral) Belgium or of both, before Germany completed the mobilisation of its reserves simultaneous with an expected Russian offensive.
The French generals implemented the plan from 7 August 1914, with disastrous consequences for their armies, which suffered defeat in the Battle of the Frontiers (7 August – 13 September) at a cost of 329,000 casualties. 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 (5–12 September), the German armies were defeated and forced to retreat to the Aisne river, eventually leading to the Race to the Sea.