World War I broke out while Brooks was in France, and he became an ambulance driver and subsequently a newspaper correspondent for "The New York Times" and "Collier's". He eventually took up duty as an ambulance driver for American troops on the front line. He was eager to join the A.E.F and thought the quickest way would be to study in a French artillery school. He served with the French Army and rose to the rank of lieutenant of a field battery, after his petition for transfer to the American forces was turned down on the grounds of poor eyesight. He saw action at Marne, Chemin-des-Dames, Chateau-Thierry and Meuse-Argonne, and was awarded the "Croix de Guerre" with a silver star for gallantry while engaged in special missions in France on July 15 and 16, 1918. He deplored much of what he saw, including how General Robert Lee Bullard sent American troops to fight and die even though the Armistice was due to be declared in a few hours, and wrote of war's folly: