for France in June, McNair shared quarters aboard ship with the division's assistant chief of staff for operations, George C. Marshall. During their long ocean voyage, they forged a personal and professional bond that they maintained for the rest of their careers. McNair was promoted to temporary lieutenant colonel in August 1917, soon after his arrival in France, and was assigned to the American Expeditionary Forces (AEF) headquarters as chief of artillery training and tactics in the AEF staff's training division (G-5). He was promoted to temporary colonel in June 1918, and temporary brigadier general in October; at age 35, he was the youngest general officer in the Army. He continued to impress his superiors with his technical and tactical expertise, and at the end of the war he received the Army Distinguished Service Medal from John J. Pershing and the French Legion of Honor (Officer) from Philippe Pétain. The citation for the Army DSM reads: In June 1919, seven months after the Armistice with Germany which brought an end to the war, McNair was named to the AEF board that was charged with studying the problem of how to provide adequate mobile indirect fire support to infantry during combat. This panel, called the Lassiter Board after its chairman, Major General William Lassiter, was one of several formed by the AEF to review wartime plans and activities, and make recommendations for the Army's future equipment, doctrine, and training. Post-World War I School of the Line McNair remained with the Lassiter Board only briefly because he received orders assigning him as one of the faculty members of the Army's School of the Line. The school had ceased operations during the war and was being re-formed at Fort Leavenworth to offer professional education to field grade officers on planning and overseeing the execution of operations at division level and above. McNair reverted to his permanent rank of major, and remained on the faculty until 1921. In addition to receiving accolades for his work to help design and field the course curriculum, McNair also played a key role in another task traditionally assigned to the School of the Line: developing and promulgating the Field Service Regulations, the Army's main document for codifying training and readiness doctrine. While on the School of the Line's faculty, he was one of the main authors of the 1923 revision of Field Service Regulations. The officers who were