FICTION: The AbsolutistBy John Boyne Doubleday, 309pp. £16.99

IN HIS PARIS memoir, A Moveable Feast,Ernest Hemingway recalls a conversation with Gertrude Stein in which she refers to the surviving soldiers of the first World War as a "génération perdue":

“That’s what you all are,” Miss Stein said. “All of you young people who served in the war . . . You are a lost generation.”

Although Stein's lost generation are the traumatised postwar men of the indulgent 1920s Paris scene, who "drink themselves to death", the phrase is an equally resonant description of the soldiers we meet in The Absolutist.These are the men for whom the first World War is neither a rite of passage nor a right cause. They are the so-called white-feather men who see no justice in military struggle and who refuse to bear arms for their country. These are men who belong to an unspoken history of shame rather than one of heroic celebration.

“Objectors, they call themselves,” explains Sgt Clayton as he singles one of them out amid a new group of conscripts in the opening war-set scenes of John Boyne’s sensitive novel. “Chaps who examine their conscience and find nothing there to satisfy the call of duty. They look like other men, of course. They have two eyes and two ears, two arms and two legs. No balls, though, that’s a given . . . Unless you whip their pants off and make the necessary inquiries it can be fairly difficult to distinguish them from real men. But they’re out there. They surround us. And they would bring us down if they could.”

It is this conflation of physical violence with masculinity that is at the heart of Boyne's fiercely interrogative novel, but The Absolutistasks not just what it means to be a man but also what it means to be a human being in the extreme circumstances of war.

Boyne’s protagonist, Tristan Sadler, is not the absolutist of the title, but he casts himself as a white-feather man nonetheless. Sadler’s attitudes to the conflict are a complex blend of necessity and regret. On the battlefield he proves himself a good soldier. He follows orders and accepts the new moral codes of wartime Europe, with their underlying imperative to kill or be killed. And yet if Sadler’s best friend, Will Bancroft, is branded a coward, only half a man, when he finally lays down arms and trumpets the pacifist cause, Sadler sees himself as a coward too. In refusing to stand up for Will he condemns his friend to death.

Sadler doesn't believe in abstract political principles; he believes only in the immediate circumstances of the fight ahead. And yet this is not the reason for his actions in the end, or for the tremendous regret that throws a compelling melancholic pall on The Absolutist.Sadler is another type of emasculated soldier, a homosexual, cast out by family and friends as a deviant in a social climate where masculinity is as narrowly defined as heroism. Echoing the cultural repression of the time, Sadler's sexuality becomes the subtle psychological drive of the novel, enabling Boyne to avoid overt polemic while doubling the sense of lost histories – " la génération perdue"– exposed.

Over the course of his six previous novels Boyne has forged a niche in the genre of the literary historical novel. In previous works such as Next of Kin, The House of Special Purposeand Crippen,he has meticulously captured both the sensibility and the atmosphere of early 20th-century Europe, particularly Britain. The Absolutist follows suit, placing historical detail against a sensitive exploration of contemporary psychology. Using different timelines and the device of the writer-narrator, he enables a simultaneously immediate and backwards glance on history to emerge. In the mournful Sadler he finds a man more affected by history's broad-stroke processes than most, and what results is a plaintive lament for forgiveness from a desperately lonely man.

Boyne's most successful work to date, however, has been the young-adult novel The Boy in the Striped Pyjamas,whose unusual angle on the second World War made an oft-told story eminently new. The Boy in the Striped Pyjamaswas made into a successful film by the director Mark Herman in 2008, with David Thewlis and Vera Farmiga in starring roles, and The Absolutisthas similar cinematic potential.

It presents the reader with a vivid visual landscape, and a compelling flawed hero, who is at once central and peripheral to the forceful, forwards-thrusting narrative of revelation. As Boyne gives us Sadler’s subjective gaze through the first-person perspective, so the camera might play with the ambiguity of what Sadler sees as truth and an unwavering morality on the battlefield, and what the film viewer might perceive as a more objective reality.

But The Absolutistis also, like The Boy in the Striped Pyjamas, at once political and personal in a powerful way. The stories of Europe's world wars need not be confined to the trenches. Will's father, the Rev Bancroft, remarks on the changed nature of humanity in the war's aftermath, where the surviving soldiers he meets "aren't entirely convinced that the world returned to is the one left behind . . . Many of them half believe they died there and that this is all some kind of strange dream. Or purgatory. Or even hell." In the devastating denouement of The Absolutistthe reader is not quite sure which of these liminal spaces Sadler has condemned himself to.

Sara Keating is a freelance journalist and lecturer specialising in 20th-century Irish culture

John Boyne takes part in Dublin Writers Festival on Friday at 6pm at Liberty Hall, sharing a platform with the American novelist Paul Harding at an event called Legacy, Craft, Devotion. See dublinwritersfestival.com