Harold Hobson, in the Oxford Dictionary of National Biography, reports of Darlington‘s war service merely that:
He was commissioned in the 7th battalion of the Northumberland Fusiliers in 1915, and served in the trenches (being wounded at Arras) during the First World War, at the end of which he was a captain. He enjoyed himself even in such conditions, and in 1916 he began sending humorous sketches to Punch from the trenches.
However, Darlington’s own autobiography, I Do What I Like (1947), provides much more detail.
He had left Cambridge in 1913, and in 1914 was teaching. He carried on into the Autumn of 1914. “War, when I thought of it at all, seemed a relic of barbarism, a ridiculous way of settling international disputes which (surely) could never again be resorted to by great European powers” (IDWIL 162). He was not military-minded in the slightest, and had not joined the Cadet Corps at either Shrewsbury to Cambridge.
All this gives a reason—I am not suggesting that it gives an excuse for political naïveté—why I was slow to realise that it could be my duty to “go for a soldier”. There were other reasons. In the early months of the war authoritative voices were heard saying that it would take at least six months to train a civilian to be any kind of effective soldier, while others no less authoritative said that in six months the war must be over in obedience to the inevitable law of economics. […] But by the time that term started things were beginning to change. The voices that had sounded so authoritative at first wavered and fell silent, and instead of them were heard Kitchener’s uncompromising tones calling for more men. People stopped talking about economic laws and a short war, and it began to seem likely that the men now going into training would be in action long before the end (IDWIL 163).
Darlington thought that the Territorials would be a solution, a halfway house. He would get training, but would be in home defence and not in France. He wrote to a territorial officer who had married a cousin of his mother’s, and he was told that he should come and talk to the commander of the 7th Northumberland Fusiliers
—but there was no immediate hurry (I was afterwards to learn that the reason for this surprising lack of urgency was that the new unit had almost no equipment whatever.) In the meantime all of us [teachers] enrolled as special constables and went spy-hunting at night. On that coast, from which the guns in France could be heard, everybody was extremely spy-conscious, but all we ever actually achieved was to force rash or forgetful cyclists to put out their lights (IDWIL 164).
Just after Christmas 1914 he went up to Northumberland, and was accepted as an officer. He had to go up to Alnwick in uniform, with no prior hints as to how to behave when saluted (or when meeting a superior officer). The only officer who had any experience was the company commander, R.H. Hodgkin (the author of a celebrated history of the Anglo-Saxons and later the head of an Oxford college).
As a company officer he was much more don than soldier, and any kind of military smartness was beyond him. This was a never failing source of glee to his subalterns. I remember most vividly a day when Hodgkin, unshaven and with every visible button undone, reprimanded a private only one degree more untidy for presenting an unsoldierly appearance on parade, and all his junior officers with one accord turned about smartly for fear they would have to disgrace themselves by laughing aloud (IDWIL 166-67).
Not long after joining up, the Territorials lost their distinction of not being sent abroad, and Darlington signed the general service form “and began to feel like some kind of amateur soldier instead of a civilian in disguise” (IDWIL 167).
I suppose I became a useful soldier, but never throughout my four years’ service could I get rid of a conviction that war was futile and armies absurd. […] Yet if I had my time over again and could choose, I would not set those four years out of my life. They gave me something which I could not otherwise have got; the knowledge, common today but strange to the sheltered generation in which I grew up, that the power to conquer fear, or at least to control it, is not the prerogative of the fighting man but the common human heritage—that there are more brave men than there are cowards. It is a good thing to know by experience, what nobody could believe from hearsay, that the moment when a soldier scrambles out of a trench to attack is very like that moment when a nervous batsman walks down the pavilion step on his way to the wicket; or that he watches a bomb lobbing towards him with precisely the same anxious attention as he watches a cricket ball coming high at him into the deep field, with the important difference that he proposes to move away from the one and towards the other at the psychological moment (IDWIL 167-168).
He also credits the war with turning him into a professional writer. One day on leave, he looked at a piece he had sent to Punch, and saw instantly why it had been rejected. He tidied it up, and it was accepted immediately. Thereafter he was able to place pieces not just in Punch but in other periodicals.
A subaltern’s expectation of life in France at that time was generally computed at three months; and it was three months after I went overseas, almost to the day, that my C.O. ordered me one April afternoon  to take eight men and drive a group of Germans out of the ruins of Wancourt Tower, on the crest of a ridge just outside Arras, and I got a bullet clean through me. Three months is just the length of a school term; and the whole experience was like a bleak but not unrewarding term at a very rough school. There was the same sense of having entered a new existence, self-contained and separate from home life, and of giving oneself up wholly to the purposes of that existence (IDWIL 179).
He arrived back in England on a hospital ship, and (after having to insist on it, threatening to go back to France otherwise) was sent to London: a label saying London was pinned on his chest, and he was pushed in a sort of perambulator onto the London train. He fond himself in Lady Mary Meynell’s in Lennox Gardens, near Harrods—which is near where his girl-friend Marjorie was working, packing parcels for sending to prisoners-of-war. She came to see him the following morning, on her way to work.
Darlington had lost a lot of blood, and nearly died. But the bullet had missed all vital organs, and the only serious repercussion was a collapsed lung. After a long time in bed, he was allowed to sit in an arm-chair, as long as he did not move. “I will sit in this chair till I get an idea,” he told himself. And he had the idea of writing a story about how a bit of Aladdin’s lamp found itself into a soldier’s button. The editor of Punch came to see him, and was very encouraging. Darlington went to near Sunningdale to recuperate, and there he wrote six short pieces about Alf’s button, and sent them off to Punch. The were rejected. But the editor of The Passing Show agreed to take them, if they could be doubled in length. Darlington sent him the first draft on 31 July 1917.
In the autumn, Darlington rejoined the regiment, on light duty, on Humber Garrison duty; in the winter he was stationed in Beverley. Early in 1918 he was interviewed by the head of the School of Oriental Studies, and he was chosen to go on an Arabic course, for army officers. In the summer of 1918 he was working very hard: six hours of Arabic in the day, and lengthening Alf’s Button yet again, for book publication by Herbert Jenkins.
One thing about it did give me satisfaction. The Arabian Nights part of the book was authentic. My fellow pupils and I had been labouring painfully through one of these stories in the original, and I had enlisted the interest of one of my teachers, who supplied me with names and local colour as I went on. As it turned out, that was the only practical use to which my six months’ hard slogging at Arabic was ever to be put (IDWIL 201).
It turned out that he did very well on the Arabic, and was ordered to report to the War Office rather than go East.
The work I then did was on the secret list, and I shall say no more about it here for fear of being thrown into prison; but a full detailed description of what my department did and how we did it was published after the war by an officer in the United States Intelligence (there is no Official Secrets Act in America). I reviewed that book, in its English edition, for The Daily Telegraph, and thought how very queer it was that as a reviewer I might discuss fully in public things which I had been at great pains in private never to mention (IDWIL 201).
He got leave from the first Friday of October to the following Tuesday, and got married to Marjorie in St James’s Piccadilly.
He left the army in February 1919, although for another month he was technically in the Army, on leave and on full pay. In April he was at a desk in Covent Garden, “at peace with all the world and delighted beyond words at being able to call myself a professional journalist” (IDWIL 205).