At the outbreak of World War I in August 1914, de Langle de Cary was placed in command of the Fourth Army. In concert with general Pierre Ruffey and his Third Army and general Charles Lanrezac and his Fifth Army, he was ordered by Joffre to attack the approaching Germans advancing south through the heavily forested and ravined Ardennes. They were badly outnumbered by the Germans facing them attacking from above and were defeated with very heavy losses in the Battle of the Ardennes but managed to withdraw and form a line of defense strong enough to halt the German counter-offensive. Unlike generals Ruffey and Lanrezac, Langle de Cary was not relieved of his command. He continued at the head of Fourth Army (though its strength was greatly reduced for the benefit of Foch's newly created Sixth Army) in the Marne and Aisne operations and in the trench warfare fighting of 1915. He commanded the French forces in the Second Battle of Champagne, another failed and costly French offensive ordered by Joffre.