The Plan XVII was a French military plan drafted in 1911 by General Ferdinand Foch and adopted by the French General Staff in 1913. It was conceived during a period of rising political tensions in Europe known as the Armed Peace and as a result of the Franco-Prussian War of 1871, in which France had been invaded and thoroughly defeated by Prussia (Germany). Thus, the Plan XVII would be put into effect by the French Army if an armed conflict between France and Germany broke out again. In the event of war against France and Russia, the German Army had also devised a military plan called the Schlieffen Plan, which was much more aggressive than the French.
The Plan XVII consisted of an attack into Alsace-Lorraine on either side of the Metz-Thionville fortresses, which was occupied by the Germans since 1871. This offensive would be carried out by four French Armies. The right wing of the French invasion forces would first capture Alsace and Lorraine, while the left wing would advance into Germany via the southern Ardennes forests, or else move north-east into Luxembourg and Belgium. The Plan XVII was based on the naive and prejudiced belief that the French soldier was imbued with the mystical vital élan, or a fighting spirit capable of turning back any enemy by its sheer power and which was assumed to be instilled within every Frenchman. The French Generals erroniously assumed that the average French soldier was more than a match for its German counterpart.