text: First World War As Captain Castlereagh MP he travelled to northern France in the first weeks of the war and reached Paris on 29 August 1914, having been gazetted ADC to General William Pulteney the previous day. Although a staff officer, Castlereagh immediately saw plenty of fighting and believed he had shot and killed one of the enemy on 2 September 1914. In the following months of 1914, Castlereagh extensively witnessed the destruction of war and the terrible suffering of the British wounded. He was promoted to the temporary rank of major in his old regiment on 1 November, and to the substantive rank on the 7th.
  Hitherto reluctant to involve himself, like his father, in Irish politics, the war prompted him to take up the cause of recruitment in Ireland. With his father's death in February 1915, he ceased to be MP for Maidstone and inherited the Londonderry title and the immense wealth and status that went with it. His exalted position helped his political career, not least in Ireland, which later brought him favourable attention from the British government. In 1915, Lord Londonderry (as he had now become) ll, was mentioned in despatches and rejoined his regiment, the Royal Horse Guards (The Blues). He saw in 1915 for the first time the horrific effects of gas attack upon human beings when visiting soldiers gassed at the first Battle of Ypres.
  In 1916 Londonderry was appointed second-in-command of The Blues, part of the 8th Cavalry Brigade.  He served at the front during the Battle of the Somme, witnessing the mass slaughter first-hand; his closest friend, Lieutenant Colonel Harold Brassey, best man at his wedding in 1899, was killed. He was an acting lieutenant colonel from 15 December 1916 to 20 January 1917.