Plan XVII was the name of a "scheme of mobilization and concentration" that was adopted by the French General Staff in 1913, to be put into effect by the French Army in the event of war between France and Germany but was not ‘a prescribed narrative for the campaign’ or battle plan.[1]
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Following the defeat of the French armies during the Franco-Prussian War of 1870-71, the French military had to adapt to a new balance of power in Europe. The emergence of the German Empire on the other side of the Rhine, combined with the loss of Alsace and Lorraine, seriously weakened France's strategic position.
In 1898, the French General Staff adopted Plan XIV. Taking into account the numerical inferiority of the French Army, Plan XIV was an offensive strategy along the Franco-German border. Besides the increasing disparity in population (by the turn of the century France had a stagnant population of around forty million, compared to fifty million Germans) there was also the problem of reserves. The war of 1870-71 had demonstrated the ability of the German General Staff to make use of the German railroad network to deploy its armies and its capability to quickly mobilize its reservists into front-line units. While the French General Staff began to apply the lessons of the use of railways, the question of using reservists in frontline units was not resolved. Plan XIV didn't take reserves into account.
In 1903, Plan XIV gave way to Plan XV. While still defensive in character, Plan XV did consider using reserve formations, although in a subordinate role.
Plan XVI of March 1909 placed greater emphasis on the risk of a German attack through Belgium, and was modified in 1911 by Général Joseph Joffre.[2]
Joffre presented plan XVII in 1913. Unlike other plans, it was a plan of concentration but did not include a fixed military strategy,[3] which remained flexible to permit an offensive into Belgium or Lorraine.[4]
When Germany declared war in 1914, France began the execution of Plan XVII with five initiatives, now collectively known as the Battle of the Frontiers:
The German strategy, the Schlieffen plan, included an attack through Belgium and northern France to encircle Paris.
Plan XVII was implemented as an offensive into Alsace-Lorraine. The German defense of Alsace-Lorraine turned out to be much better than expected and force proved to be a more and more meaningless concept in wars fought by modern, huge armies supplied by industrialized countries.
Within a few weeks, the French were back in their starting positions, having suffered great losses. Meanwhile the Germans had advanced against resistance through Belgium and northern France and were threatening Paris. Only the fact that the German attack ran out of steam, and that Joffre was able to re-organise his armies across France, allowed the French and their British allies to halt the German advance in the First Battle of the Marne.
The original Schlieffen Plan had little defense in Alsace-Lorraine in order to lure French forces away from Paris into Germany, then to be double-enveloped and destroyed. In this perspective the failure of the French in Alsace-Lorraine has contributed to their success at the Marne and it would have been even worse for France if it had been more successful.