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Plan XVII was the name of a war plan adopted by the French General Staff in 1913, to be put into effect by the French Army in case of a war between France and Germany.

Following the defeat of the French armies during the Franco-Prussian War of 1870-71, the French military had to adapt itself 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 the provinces of Alsace and Lorraine, had the combined effect of putting France at a critical disadvantage.

In 1898, the French General Staff adopted Plan XIV. Taking into account the numerical inferiority in which the French Army found itself at the time, plan XIV called for a purely defensive strategy along the entire Franco-German border. Besides the ever increasing disparity in terms of total population—by the turn of the century France had a stagnant population of around forty million, compared to fifty million Germans, and rising — there was also the problem of the reserves. The war of 1870-71 had demonstrated not only the ability of the German General Staff to make full use of the German railroad network to deploy its armies, but also its capability to mobilize and incorporate its pool of reservists into the front-line units in a short amount of time. While the French General Staff began to apply the lessons learned from experience regarding the adequate use of the railways, the question of using the reservists in frontline units was not adequately resolved. As such, plan XIV didn't take into account the reserves.

In 1903, Plan XIV gave way to Plan XV. While defensive in character, plan XV did consider using the reserve formations, but only in a subordinate role.

The offensive French military strategy in World War I known as Plan XVII was initially created by Ferdinand Foch. The offensive plan used brute force and a mystical belief in the French "élan" or "fighting spirit." General Joseph Joffre adopted this plan upon becoming commander-in-chief in 1911.

After the Franco-Prussian war of 1870, the French had lost the provinces of Alsace-Lorraine to the German empire. This created a spirit of revanchism in France, and one of the main aims of Plan XVII was to recapture Alsace and Lorraine. In order to do this, four French armies would advance on either side of Metz and Thionville. This left only one army to defend northern France, but French planners were convinced that Germany would not invade through Belgium, as this would lead to British involvement (in the Treaty of London, the United Kingdom had guaranteed Belgian neutrality).

Unfortunately for the French, the Germans regarded the Treaty of London as a mere "scrap of paper" (and thought the British would do so as well), and their Schlieffen plan called for an attack through Belgium and northern France in order to encircle Paris.

When the war broke out in 1914, the execution of Plan XVII ended in total failure. The German defense of Alsace-Lorraine turned out to be of much better quality than expected, and within a few weeks, the French were back in their starting positions, while the Germans had advanced almost unopposed through Belgium and northern France and were threatening Paris. Only the fact that the German high command diverted troops to the Eastern Front and to a counterattack in Alsace-Lorraine (which was in turn repulsed by the French), allowed the French and their British allies (who had adhered to the Treaty of London and thus declared war on Germany after the German invasion of Belgium) to halt the German advance in the First Battle of the Marne.

• First World War.com A map of the French and German war plans