Tidmarsh was commissioned as a second lieutenant in the 4th Battalion, Royal Irish Regiment (Special Reserve) on 23 April 1915. He began flying training at Shoreham on 27 August 1915, and transferred into the Royal Flying Corps on 13 January 1916 when he was appointed a Flying Officer in 24 Squadron. He was piloting an Airco DH.2 on 2 April 1916 when he scored his—and his squadron's—first victory, destroying a German Albatros two-seater and killing its crew of Karl Oscar Breibisch-Guthmann and Paul Wein. On 21 April, a dud antiaircraft shell blew through the nacelle of his plane without harming him. On 25 April, Tidmarsh was flying Airco DH.2 No. 5965, escorting a mission of Royal Aircraft Factory FE.2s, when he dived on an approaching Fokker Eindekker fighter. It fled. He pursued. The Ger