Upon the outbreak of the First World War in August 1914, a brigade of five battalions known as the "Brigade des Chasseurs Indigènes" was formed from Moroccan troops and sent to the Western Front in France. Juin joined "Chef de Bataillon" Joseph-François Poeymirau's "2e Régiment des Chasseurs Indigènes" as a "lieutenant". On 5 September, the brigade joined the fighting in the First Battle of the Marne. Juin was wounded in his left hand the following day, but refused evacuation to hospital, remaining at the front with his arm in a sling. He was awarded the Cross of the Legion of Honour. The brigade was withdrawn from the line in January 1915, but committed to battle again in March in the First Battle of Champagne. In this battle Juin was again wounded, this time in his upper right arm. The damage was permanent, and he was given permission to henceforth salute with his left.