On the strength of this, he was sent to work in the information section of the British War Mission in Washington, D.C. After the war, Beith's novels did not achieve the popularity of his earlier work, but he made a considerable career as a dramatist, writing light comedies, often in collaboration with other authors including P. G. Wodehouse and Guy Bolton.

[5] Beith was educated at a Manchester preparatory school, Lady Barn House,[6] and then at Fettes College, Edinburgh.

Unable to secure a permanent position teaching Classics he returned to Cambridge and spent what a biographer called "a laborious year in acquiring sufficient knowledge of chemistry and physics to fit him for a Senior Mastership".

[7] Beith joined Durham School in 1902 as junior science master; he also coached the rugby and boating crews.

"[9] Beith's cautiously adopted pen name was redundant for its original purpose of camouflage in case of failure, but he decided to stick with it; he remained "Ian Hay" in all his published work thereafter.

[4] Between 1908 and 1914 he followed Pip with five more novels, characterised by The Times as "of the right stuff and happy-go-lucky, their good feeling saved from insipidity by its seasoning of piquant humour".

[10] He was unable to contest the seat as there was no general election until 1918, when the Unionists did not oppose the sitting member, a coalition Liberal.

It was assembled with the help of his publisher from a series of articles written for Blackwood's Magazine, describing with wry humour life in his battalion.

[4] It became one of the most popular books of the time, with multiple editions in Britain (including a 1940 Penguin paperback) and the US, and was published as Les premiers cent mille in France.

[2] The Irish Times called it "a book which was read eagerly not only by the civilian public but also found its way into countless dugouts and redoubts in France, Mesopotamia, Palestine and Salonika.

[13] Later that year, on the strength of the impact The First Hundred Thousand was making in neutral America, Beith was sent to Washington to join the information bureau of the British War Mission.

[2] His biographer Patrick Murray writes, "[Beith's] energy and success were rewarded by a CBE (1918) and promotion to the rank of major.

His co-authors were Anthony Armstrong, Guy Bolton, Seymour Hicks, Stephen King-Hall, A E W Mason, Edgar Wallace and P. G.

[20] Between the wars he wrote or co-wrote original screenplays, such as Keep Your Seats, Please (1936);[21] and adapted his own and other authors' works for the screen, including Tommy Atkins (1928),[22] and The 39 Steps (1935).

[25] In 1938 Beith published The King's Service, described by The Times as "an attempt to give an informal history of the British infantry soldier in peace and war".

[31] Beith died in the Hillbrow Nursing Home in Liss, near Petersfield, Hampshire, on 22 September 1952 after several weeks' illness.