Popular success, 1914–1939

By 1914 Maugham was famous, with 10 plays produced and 10 published novels. Too old to enlist when World War I broke out, Maugham served in France as a member of the British Red Cross's so-called " Literary Ambulance Drivers", a group of some 23 well-known writers including Ernest Hemingway, John Dos Passos, and E. E. Cummings. During this time he met Frederick Gerald Haxton, a young San Franciscan who became his companion and lover until Haxton's death in 1944 (Haxton appears as Tony Paxton in Maugham's 1917 play, Our Betters). Throughout this period Maugham continued to write; indeed, he proof-read Of Human Bondage at a location near Dunkirk during a lull in his ambulance duties.

Of Human Bondage (1915) initially received adverse criticism both in England and America, with the New York World describing the subject of the main protagonist Philip Carey as the sentimental servitude of a poor fool. However the influential critic, and novelist, Theodore Dreiser rescued the novel referring to it as a work of genius, and comparing it to a Beethoven symphony. This criticism gave the book the lift it needed and it has since never been out of print.^[

The book appeared to be closely autobiographical (Maugham's stammer is transformed into Philip Carey's club foot, the vicar of Whitstable becomes the vicar of Blackstable, and Philip Carey is a doctor) although Maugham himself insisted it was more invention than fact. Nevertheless, the close relationship between fictional and non-fictional became Maugham's trademark, despite the legal requirement to state that " the characters in [this or that publication] are entirely imaginary". In 1938 he wrote: " Fact and fiction are so intermingled in my work that now, looking back on it, I can hardly distinguish one from the other."

Maugham returned to England from his ambulance unit duties to promote Of Human Bondage but once that was finalised, he became eager to assist the war effort once more. As he was unable to return to his ambulance unit, Syrie arranged for him to be introduced to a high ranking intelligence officer known only as " R", and in September 1915 he began work in Switzerland, secretly gathering and passing on intelligence while posing as himself — that is, as a writer.

In 1916, Maugham travelled to the Pacific to research his novel The Moon And Sixpence, based on the life of Paul Gauguin. This was the first of those journeys through the late-Imperial world of the 1920s and 1930s which were to establish Maugham forever in the popular imagination as the chronicler of the last days of colonialism in India, Southeast Asia, China and the Pacific, although the books on which this reputation rests represent only a fraction of his output.

In June, 1917 he was asked by Sir William Wiseman, chief of the British Secret Intelligence Service (later named MI6), to undertake a special mission in Russia to keep the Provisional Government in power and Russia in the war by countering German pacifist propaganda. Two and a half months later the Bolsheviks took control. The job was probably always impossible, but Maugham subsequently claimed that if he had been able to get there six months earlier, he might have succeeded.

Quiet and observant, Maugham had a good temperament for intelligence work; he believed he had inherited from his lawyer father a gift for cool judgement and the ability to be undeceived by facile appearances.

Never losing the chance to turn real life into a story, Maugham made his spying experiences into a collection of short stories about a gentlemanly, sophisticated, aloof spy, Ashenden, a volume that influenced the Ian Fleming James Bond series.

In 1922 Maugham dedicated On A Chinese Screen, a book of 58 ultra-short story sketches collected during his 1920 travels through China and Hong Kong, to Syrie, with the intention of later turning the sketches into a book.

Dramatised from a story which first appeared in his collection The Casuarina Tree published in 1924, Maugham's play The Letter, starring Gladys Cooper, had its premiere in London in 1927. The play was later turned into a film in 1929 and again in 1940.

In 1928, Maugham bought Villa Mauresque on twelve acres at Cap Ferrat on the French Riviera, which would be his home for most of the rest of his life, and one of the great literary and social salons of the 1920s and 30s. His output continued to be prodigious, including plays, short stories, novels, essays and travel books. By 1940, when the collapse of France forced Maugham to leave the French Riviera and become a well-heeled refugee, he was already one of the most famous writers in the English-speaking world, and one of the wealthiest.