During World War I, in early 1915, at 43 years of age, and having refused an offer of the Viceroyalty of India, he enlisted as a private in the Royal Army Medical Corps, which was almost unheard of at that time as hereditary peers and their heirs or university graduates such as himself were generally commissioned as officers. Prior to the war he had held the rank of Captain in the 1st (Volunteer) Battalion, Manchester Regiment. He thus swapped palaces in India and the prospect of a comfortable administrative position for the reality of a front line clearing station's operating theatre. At times up to 1,000 casualties each day passed through the clearing station at Hazebrouck, where he was stationed. This was when he developed what were described by his granddaughter, Rose Luce, as 'mixed feelings' about members of the officer classes (his own 'class', of course). In 2013 his diaries of his experiences were published as the memoir "Private Lord Crawford's Great War Diaries: From Medical Orderly to Cabinet Minister", edited by his grandson Christopher Arnander.