Buchanan joined the Royal Naval Air Service in 1917. On 11 June 1918, he was assigned to 210 Squadron, scoring seven confirmed victories over Belgium. On 30 October 1918, in a battle with seven Fokker D.VIIs, Buchanan's Sopwith Camel was shot down and he was captured, spending the last twelve days of the war as a prisoner.

Distinguished Flying Cross (DFC)
"On 29 September [1918], this officer displayed great gallantry in an engagement with fifteen Fokker biplanes. Owing to engine trouble, he was compelled to remain under his flight; he nevertheless accounted for two enemy machines, attacking one under its tail causing it to crash, and driving another down out of control. In addition to the foregoing, this officer has destroyed three machines and driven down two out of control." DFC citation, London Gazette, 3 December 1918