text: In 1917, Londonderry took command of a composite battalion drawn from the 8th Cavalry Brigade with the brevet rank of Lt-Colonel, and the Royal Horse Guards (The Blues) took part in the massed mounted cavalry attacks on Monchy-le-Preux on the morning of 11 April 1917, during the Battle of Arras.  Monchy-le-Preux was one of the keys to the northern end of the Hindenburg Line. While reconnoitring the enemy near Monchy the GOC 8th Cavalry Brigade, Brigadier-General Charles Bulkeley-Johnson, was shot in the face; he fell with a piercing shriek, the thirtieth British General to be killed in action or to die of wounds on the Western Front. This put Brevet Lt-Colonel Londonderry temporarily in command of the 8th Cavalry Brigade during their charge in the Battle of Arras.  At Monchy 600 cavalrymen were casualties and many more horses died. The animals were tethered in the open, as their riders took cover; attempts to take them to the rear during a "box barrage" only increased the casualties. For Londonderry, the experiences of war and the carnage of his brother officers and the family and school friends he grew up with would, as Ian Kershaw commented, "leave an indelible mark on him".
  After serving in the Irish Convention of 1917–18, Lord Londonderry served on the short-lived Viceroy's Advisory Council, meeting at Dublin Castle in the autumn of 1918. Promoted to brevet lieutenant-colonel on 7 November 1918, he retired from the army on 10 September 1919 as a major and brevet lieutenant-colonel.
  On 13 August 1920, he was appointed Honorary Colonel of the 55th Medium Brigade, Royal Garrison Artillery in the Territorial Army, the successor unit to his father's 2nd (Seaham) Durham Artillery Volunteers. He continued in that role until World War II, after it had been converted into the 63rd (Northumbrian) Anti-Aircraft Regiment, Royal Artillery in Anti-Aircraft Command.
  In the Northern Ireland cabinet