121st Siege Battery, RGA, was raised at Pembroke Dock on 22 March 1916 under Army Council Instruction 701 of 31 March 1916 with a cadre of 3 officers and 78 other ranks from the Glamorgan RGA. It went out to the Western Front in July 1916, manning four 9.2-inch howitzers. It joined I ANZAC Corps in Fifth Army on 15 July in time for the Battle of Pozières in the Somme Offensive. It transferred to 5th Heavy Artillery Group (HAG) in Second Army on 4 August, then returned to the Somme with 28th HAG in Fourth Army on 14 September. The Battle of Flers-Courcelette was launched the following day and largescale fighting continued on Fourth Army's front into November. 121st Siege Bty transferred to the command of 64th HAG within Fourth Army from 3 October.