Popularly known as the Plymouth Division and its shoulder insignia was a blue diamond with a red center and the figure "12" in white pierced by a bayonet. Organized at Camp Devens, Mass., July 12, 1918. The 36th and 42nd Regular Army Infantry Regiments were ordered to Camp Devens in the latter part of July as a part of the 12th Division. There were taken from each company of these two regiments a certain number of non-commissioned officers and privates who were assigned to the 73rd and 74th National Army Regiments as a nucleus. The 12th Field Artillery Brigade, the divisional artillery of this division, was organized and trained at Camp McClellan, Ala., and never actually joined the division at Camp Devens. By the 1st of September the training of the division for overseas service was well under way. At the time the armistice was signed the division had received for the demobilization of the division, and by January 31st the entire commissioned and enlisted personnel not in the regular establishment had been discharged.

Major-General Henry P. McCain commanded this division from the time of its organization until it was demobilized.

This division was composed of the following organizations: 12th Div. Hqs. Troop, 34th Div. Machine Gun Bn., 23rd Inf. Brig. (36th and 73rd Inf. Regts, 35th Machine Gun Bn.,) 24th Inf. Brig. (42d and 74th Inf. Regts, 36th Machine Gun Bn.), 12th Fld. Arty. Brig. (34th, 35th, 36th Fld. Arty Regts., 12th Trench Mortar Battery), 212th Engr. Regt. And Train, 212th Fld. Sig. Bn., 12th Hqs. Train, and M.P., 12th Supply Train, 12th Amm. Train, 12th Sanitary Train (consisting of 245th, 246th, 247th and 248th Amb. Cos. And Field Hospitals).