France: Plan XVII
The chief aim of Plan XVII, devised by Ferdinand Foch in the wake of the humiliation of the Franco-Prussian War, and taken up by French Commander-in-Chief Joseph Joffre in 1913, was the recapture of the territory of Alsace and Lorraine.
Entirely offensive in nature, Plan XVII made extensive use of the belief in the mystical élan vital assumed to be instilled within every Frenchman - a fighting spirit capable of turning back any enemy by its sheer power. It assumed the average French soldier to be more than a match for its German counterpart. Indeed, numerous French officers were dismissed from the army during the early stage of the war for a want of fighting spirit, including General Lanrezac following the French army's failure at Charleroi.
More technically, Plan XVII called for an advance by four French Armies into Alsace-Lorraine on either side of the Metz-Thionville fortresses, occupied by the Germans since 1871. The southern wing of the invasion forces would first capture Alsace and Lorraine (in that order), whilst the northern wing would - depending upon German movements - advance into Germany via the southern Ardennes forests, or else move north-east into Luxembourg and Belgium.
The architects of Plan XVII, which included Joseph Joffre, took little account of a possible German invasion of France through Belgium until just before war was declared; and in modifying the plan to deploy troops to meet such an eventuality, actual French activity to meet an invasion via Belgium was lacklustre at best in August 1914.
Before war broke out Joffre and his advisers were convinced that the threat of British involvement would keep Germany from invading through Belgium (with whom Britain had a treaty guaranteeing its neutrality; Germany regarded this as a mere "scrap of paper").
Whilst the French had accurately estimated the strength of the German army at the opening of the war, they did not place much emphasis on Germany's extensive use of reserve troops, having little faith in their own. This proved a serious miscalculation which, in conjunction with an underestimation of the Schlieffen Plan, almost led to France's undoing within a month of the outbreak of war.
Within weeks of the war's start, the French attack into Alsace and Lorraine had proved a debacle, effectively repelled by the German defences. With the inevitable advance of the Schlieffen Plan meanwhile, the French were thrown very much on the defensive.