Frew enlisted as a private in the
Highland Light Infantry in 1914, and after serving on the front lines in France, transferred to the
Royal Flying Corps in August 1916. was appointed a flying officer on 6 April 1917, and confirmed in his rank on 11 April. On 28 April 1917 Frew was posted to
No. 45 Squadron RFC to serve on the
Western Front in France, flying the
Sopwith 1½ Strutter. He gained his first two victories on 5 June, flying with
observer Second Lieutenant M. J. Dalton, then gained three more between 16 July and 10 August with observer Second Lieutenant George Brooke, all over
Albatros D.IIIs, to attain flying ace status. and on 24 October, though still only a second lieutenant, was appointed a
flight commander with the temporary rank of captain. On 17 December he was awarded a Bar to his Military Cross. No. 45 Squadron was then transferred to the
Italian Front, and Frew gained another seven victories between 11 January and 4 February, bringing his confirmed total to 23. On 4 March he was awarded the Distinguished Service Order, and was promoted to lieutenant on 26 March. On 1 April 1918, the day that the Royal Flying Corps and Royal Naval Air Service were merged to form the Royal Air Force, Frew was again appointed a temporary captain. He received
mentions in dispatches on 18 April and 30 May, On 5 May 1919 Frew was transferred to the unemployed list, and his award of the
Air Force Cross was
gazetted shortly after on 30 May. ==Inter-war career==