On 10 December 1916, his twenty-fifth birthday, Alexander became second-in-command (2-i-c) of the 1st Battalion, Irish Guards, as an acting major. By May, he was briefly acting CO of the 1st Battalion, as an acting lieutenant colonel, while still only a substantive captain. He became a permanent major on 1 August 1917, and was again promoted acting lieutenant colonel, this time confirmed as CO of the 2nd Battalion, Irish Guards, on 15 October. Alexander commanded his battalion at Third Ypres, where he was slightly wounded, then at Bourlon Wood (part of the battle of Cambrai), where his battalion suffered 320 casualties out of 400 men. Alexander, between 23 and 30 March 1918, had to assume command of the 4th Guards Brigade, during the British retreat from the German Army's Spring Offensive. He once again commanded the 2nd Battalion, Irish Guards, at Hazebrouck in April 1918, where it took such severe casualties that it saw no further action. Still an acting lieutenant colonel, he then commanded a corps infantry school in October 1918, a month before the war ended on 11 November 1918.