In 1915–16 Armstrong was assistant organist in Peterborough for a year before being elected organ scholar of Keble College, Oxford. His studies were interrupted by service in France during the First World War; he enlisted with the Royal Artillery in 1916, before being commissioned towards the end of the war. During his war service he met Ralph Vaughan Williams, whom Armstrong later described as "the greatest influence of anybody on my life". Armstrong remained in France for some months after the cessation of hostilities, not returning to England until 1919; he rented a room with a piano in Cambrai to get himself back in practice.