Stubbs enlisted in the 3rd Battalion, South Lancashire Regiment to serve in World War I.[2] On 26 January 1916, he was promoted to temporary lieutenant.[3] On 4 January 1917, he was appointed a Flying Officer with orders to remain seconded from the Garrison Battalion, Liverpool Regiment.[4]
Stubbs served in France with No. 27 Squadron RFC (later No. 27 Squadron RAF) from 24 February until 9 May 1917 when he was wounded. He was repatriated on medical grounds in July 1917.[5][6]
After he recovered from his injuries, he was posted to No 2 Training Depot, Lake Down, near Salisbury, 19 September 1917 where he worked as an instructor until 24 March 1918. He was promoted to Acting Flight Commander on 9 October 1917. While stationed at Lake Down, he was posted to No. 107 Squadron RAF on 4 December 1917.
Stubbs was appointed Captain in the newly formed Royal Air Force on 21 April 1918 (the RAF was formed on 1 April 1918). Stubbs was posted to No. 103 Squadron RAF in the RAF as an Airco DH.9 pilot on the same date that he was appointed Captain on 21 April 1918. He left Lake Down for active service in France on 9 May 1918.[5][7][8] He scored his first aerial victory–a highly unusual one– on 20 May 1918; despite flying a bomber, he and his observer destroyed an enemy observation balloon over Seclin.[2]
On 6 June 1918, Stubbs and his observer, along with two other Royal Air Force air crews, set a German Fokker D.VII fighter aflame and sent another down out of control southwest of Ham, Surrey. On 4 July, Stubbs and his observer drove a Pfalz D.III fighter down out of control. On the 31st, with John Bernard Russell manning the observer's guns, Stubbs was credited with driving down two more enemy planes out of control.[2]
On 25 August, Stubbs and Russell repeated the feat of destroying one Fokker D.VII and driving another down out of combat. Five days later, Stubbs drove down one more Fokker D.VII. He would go on to destroy two more Fokker D.VIIs, on 6 September and 30 October 1918.[2] Stubbs had succeeded in becoming the ranking ace on the lightly regarded Airco DH.9 bomber.[9]
On 2 November 1918, Stubbs was awarded the Air Force Cross.[10] On that same day, his Distinguished Flying Cross was gazetted:
"Captain Stubbs is a fine leader and a skilful tactician, who during the last few months, has led fifty-one reconnaissances and raids over enemy lines with marked success, frequently extricating his formation, when attacked by large numbers of scouts, by his coolness and judgment. One evening this officer, with Lt. Russell as Observer, in company with another machine, encountered ten enemy aeroplanes. Regardless of their superiority in numbers, he at once attacked and shot down one. By skilful manoeuvring he enabled his Observer to bring down another; the remainder of the enemy were driven down to their lines; he then completed his reconnaissance and returned home. Leaving the other machine behind, he again crossed the enemy lines; he bombed a trail and attacked some mechanical transport at 1,500 ft. altitude. This particular exploit is highly creditable to both these officers, the machine in which they flew being unsuitable for low bombing attacks; moreover, they were subjected to very heavy anti-aircraft and machine-gun fire."[11]