After his graduation, Douglas enrolled at Massachusetts Institute of Technology, where he took courses in preparation for a career as a mining engineer. When the United States joined the First World War, Douglas volunteered for service, receiving a commission as a Second Lieutenant in July 1917. Initially assigned to the field artillery, he later served as an assistant to General H. A. Greene, the commander of the 91st Infantry Division and was promoted to First Lieutenant in the spring of 1918. Deployed to France in the summer of 1918, he served as an assistant G-3 in the operations branch of division headquarters, where he directed communications. He experienced action at Saint-Mihiel and in the Meuse-Argonne Offensive and received the Belgian Croix de Guerre for heroism.