With the army expanding for
World War I, Grant served on the operations staff at the
United States Department of War. On August 5, he was promoted to temporary
major, and
lieutenant colonel, after which he was assigned to the operations section (G-3) on the staff of the
American Expeditionary Forces in France. During his service on the AEF staff, Grant took part in the
Battle of Cantigny. As a member of the
2nd Division staff, he participated in the
Battle of Château-Thierry. While on the
42nd Division staff, Grant was a participant in the
Champagne–Marne campaign. As a member of the
First Army staff, he was a participant in the
Battle of Saint-Mihiel. While a member of the
I Corps staff, Grant took part in the
Meuse–Argonne offensive. He was promoted to temporary
colonel in August 1918, and at the end of the war he was serving as First Army's deputy chief of staff. After the war, Grant served as I Corps chief of staff during the U.S.
Occupation of the Rhineland. Grant received the
Army Distinguished Service Medal for his wartime service. The citation read: After returning to the United States in early 1919, Grant was assigned to the War Department staff. In March 1920 he was reduced to his permanent rank of major. In July 1920, he was promoted to permanent lieutenant colonel. In 1921, he was posted to the Command and General Staff College, where he served as an instructor, chief of the faculty's command section, and assistant director of the school. In 1923, he began attendance at the
United States Army War College, after which he remained at the school as an instructor, then was assigned as director of the school's command section. From July 1926 to June 1927, Grant was director of the War Plans Division on the army staff. He was then assigned as executive officer of the 3rd Cavalry at
Fort Ethan Allen, Vermont. When the
Great Vermont Flood of 1927 took place in early November, Grant helped coordinate federal participation in relief efforts, for which he was commended by the commander of the
First Corps Area. In 1928, he was assigned to command the regiment's 1st Squadron. In January 1929, he was promoted to permanent colonel and assigned to command
Fort Adams, Rhode Island; during the summer of 1929, he commanded the
Citizens' Military Training Camp that was held at Fort Adams. In September 1929, he was assigned to command the
13th Cavalry Regiment at
Fort Riley, Kansas. ==Later career==