text: Early career On 22 May 1895, Lord Castlereagh was appointed a second lieutenant in the 2nd (Seaham) Durham Artillery Volunteer Corps, a corps within the Volunteer Force attached to the Royal Garrison Artillery (Western Division) and at the time commanded by his father who owned Seaham Colliery from which many of the part-time gunners were recruited. After passing out from Sandhurst, he was commissioned into the Royal Horse Guards as a second lieutenant on 8 September 1897. He was promoted lieutenant on 30 August 1899, and appointed adjutant on 9 May 1900.
  In early 1901 he was appointed by King Edward VII to take part in a special diplomatic mission to announce the King's accession to the governments of Austria-Hungary, Romania, Serbia, and Turkey. In August 1903, following the King's visit to Ireland, he was appointed a Member Fourth Class (present-day Lieutenant) of the Royal Victorian Order, his father being honoured with the Knight Grand Cross of the Order at the same time. He resigned his position of adjutant in the Royal Horse Guards on 24 March 1904, and was promoted to captain on 6 April.
  Castlereagh was subsequently pressed by his parents to stand for election to the House of Commons at the 1906 general election for Maidstone. He retained his army commission but was placed on the half-pay list from January 1910. His relatively unsuccessful career on the depleted Unionist backbenches was broken by a return to the British Army during the First World War.