Upon the outbreak of the war in August 1914, Hazell volunteered for service as a private with the
South Irish Horse. On 10 October he was commissioned as second lieutenant in the 7th Battalion,
Royal Inniskilling Fusiliers. As part of the
49th Brigade in the
16th (Irish) Division, the 7th Inniskillings were initially based at Tipperary, where Hazell was promoted to lieutenant on 4 June 1915. His regiment moved to
Finner Camp in August 1915, and moved to Woking in England in September. The regiment landed in France in February 1916. He was appointed a flying officer on 5 June, and survived a severe crash before completing his training, and eventually joined No. 1 Squadron on the
Western Front. Flying
Nieuport 17 Scouts, he shot down 20 enemy aircraft between March and August 1917, and was awarded the
Military Cross on 26 July. His citation read: Between 25 May and 25 June 1918 he served as an instructor at the
Central Flying School, with the acting rank of major. He then took command of "A" Flight, No. 24 Squadron, flying the
S.E.5a. On 22 August 1918, Hazell shot down an observation balloon despite its escort of seven
Fokker D.VIIs. The escort was led by German ace
Ernst Udet, who attacked and riddled Hazell's petrol tank, propeller, and two longerons with bullets. In spite of this Hazell fought his way back, eyes full of petrol, and landed safely. Udet thought that he had forced the British pilot to crash, and actually claimed him as his 60th victory. Hazell finished the war with 43 confirmed kills, the top British surviving ace of the 1914–18 war (excluding Dominion airmen). His claims tally consisted of one captured enemy aircraft; ten enemy observation
balloons destroyed (two of which were shared); 18 enemy aircraft destroyed (including one shared); and fourteen aircraft driven down 'out of control' (including three shared). Hazell was twice awarded the
Distinguished Flying Cross, and both were
gazetted on 2 November 1918. His citations read: On 22 October he was appointed a squadron commander with the acting rank of major, taking command of
No. 203 Squadron, flying
Sopwith Camels, remaining there until 2 April 1919. On 8 February 1919 he was awarded the
Distinguished Service Order. His citation read: ==Post-war career==