Vaughan entered the Indian Army in December 1896 and was made captain in 1904, having served in the 78th Moplah Rifles. He transferred to the 113th Infantry in July 1907 when the 78th Moplah Rifles was disbanded.[4] He attended the Staff College, Camberley from 1908 to 1910, and served as a General Staff Officer (GSO) Grade 3, in India from 1910 to 1912. He served in the War Office from 1912 to 1914 (GSO 2nd Grade), and was promoted to major in 1913, by then having transferred to the 7th Gurkha Rifles.[2] Vaughan married Emilie Kate Desmond Deane on 16 April 1913.[2]
Vaughan was on active service as a staff officer from 1914 to 1918, and was mentioned in despatches nine times. He was GSO Grade 2 in 1914, and GSO Grade 1 in 1915. Following this was a series of temporary (brevet) promotions, including temporary brigadier general in 1916, and temporary major general in May 1917.[5] Taking over from Major General Louis Bols, he was effectively chief of staff to General Sir Julian Byng, commander of the Third Army from May 1917 until the end of the war. He was made a substantive major general in 1919.[2]
Vaughan's brother, Percy Cecil Vaughan, who was a London solicitor pre-war, was killed at Ypres in September 1917.[6]
In his 1920 book The Realities of War, the British war correspondent Sir Philip Gibbs described Vaughan as: "That charming man, with his professional manner, sweetness of speech, gentleness of voice and gesture, like an Oxford Don analysing the war correspondence of Xenophon."[2] Vaughan was nicknamed 'Father' by the troops that served under him. John Bourne, of the University of Birmingham's Centre for First World War Studies, who has researched the nicknames given to British Army generals in the First World War, attributes to this nickname a priestly meaning, rather than any patriarchal meaning.[7]
