He was commissioned as an officer in 1903 and was promoted to lieutenant in the Life Regiment Dragoons (K 2) in 1906. Douglas was lieutenant in the General Staff in 1913 and served as general staff officer in the staff of the 1st Army Division ("I. arméfördelningen") from 1914 to 1917. He was promoted to captain in 1916 and did the same year a study trip to Galicia region in Central-Eastern Europe. Back in Sweden, Douglas served as a military history teacher at the Royal Swedish Army Staff College in Stockholm from 1917 to 1918 and as general staff officer at the Finnish headquarters during the Finnish Civil War in 1918. During his time in Finland in 1918, he became captain and lieutenant colonel in the Finnish Army and took part in the Battle of Länkipohja and Battle of Tampere. Back in Sweden, Douglas became "ryttmästare" and squadron commander in the Life Regiment Dragoons in 1919 and taught ground warfare at the Royal Swedish Naval Staff College on and off between 1919 and 1929.