During World War I, France was one of the Triple Entente powers allied against the Central Powers.
Although fighting occurred worldwide, the bulk of the fighting in Europe occurred in Belgium, Luxembourg, France and Alsace-Lorraine along what came to be known as the Western Front, which consisted mainly of trench warfare.
Specific operational, tactical, and strategic decisions by the high command on both sides of the conflict led to shifts in organizational capacity, as the French Army tried to respond to day-to-day fighting and long-term strategic and operational agendas. In particular, many problems caused the French high command to re-evaluate standard procedures, revise its command structures, re-equip the army, and to develop different tactical approaches.
Upon mobilization, Joffre became Commander-in-Chief of the French Army. Most of his forces were concentrated in the north east of France, both to attack Alsace-Lorraine and to meet the expected German offensive through the Low Countries.
- First Army (7th, 8th, 13th, 14th, and 21st Army Corps), with the objective of capturing Mulhouse and Sarrebourg.
- Second Army (9th, 15th, 16th, 18th and 20th Army Corps), with the objective of capturing Morhange.
- Third Army (4th, 5th and 6th Army Corps), defending the region around Metz.
- Fourth Army (12th, 17th and Colonial Army Corps) held in reserve around the Forest of Argonne
- Fifth Army (1st, 2nd, 3rd, 10th and 11th Army Corps), defending the Ardennes.
Over the course of the First World War another five field armies would be raised. The war scare led to another 2.9 million men being mobilized in the summer of 1914 and the costly battles on the Western Front forced France to conscript men up to the age of 45. This was done by the mobilization in 1914 of the Territorial Army and its reserves; comprising men who had completed their peacetime service with the active and reserve armies (ages 20–34).
In June 1915, the Allied countries met in the first inter-Allied conference. Britain, France, Belgium, Italy, Serbia and Russia agreed to coordinate their attacks but the attempts were frustrated by German offensives on the Eastern Front and spoiling offensives at Ypres and in the hills west of Verdun.
By 1918, towards the end of the war, the composition and structure of the French army had changed. Forty percent of all French soldiers on the Western Front were operating artillery and 850,000 French troops were infantry in 1918, compared to 1.5 million in 1915. Causes for the drop in infantry include increased machine gun, armored car and tank usage, as well as the increasing significance of the French air force, the Service Aéronautique. At the end of the war on November 11, 1918, the French had called up 8,817,000 men, including 900,000 colonial troops. The French army suffered around 6 million casualties, including 1.4 million dead and 4.2 million wounded, roughly 71% of those who fought.