Gallagher first gained a readership for his blog about life as a US cavalry officer in the dog days of the Iraq war. It was shut down by those up higher, but formed the basis of his vivid, profanely beautiful combat memoir Kaboom.

Kaboom was full of raw literary promise, and Youngblood delivers on it. While hewing close to the memoir — narrated by a young, middle-class officer, in the same area and era of Iraq — Youngblood is structured around a central mystery. Lieutenant Jack Porter is an army officer stationed in Ashuriyah, a fictional Iraqi village. There, he commands a “platoon of infantrymen, young, silly, fierce men from the country and the ghettos, marching into the outposts of hell”. He strives to be worthy of his men’s loyalty, and to “know idealism as more than a word”.

But Porter’s principled command is disrupted by his new sergeant, Daniel Chambers, an iron-hard veteran whom Porter suspects of war crimes. With his sergeant’s ferocious approach undermining Porter’s authority and disrupting the local peace, Porter secretly investigates Chambers’s past. But the investigation uncovers something more complex: a forbidden love between Chambers’s dead best friend and a local sheik’s daughter, Rana. As combat corrodes Porter’s ideals, he pledges to do “one good thing” before leaving Iraq: find Rana and bring her to safety, a quest that pits him against the army and his duty.

Youngblood’s central narrative is generally effective, delivering a well-crafted mystery with a poignant finish. But its real strength lies in the flesh around this spine: the experience of a millennial combat leader: a reflective, questioning soldier, archly conscious of the ironies of postmodern warfare and the “ambiguities of the desert”, who must lead while processing his first experience of combat. As Porter’s principles are challenged by his sergeant’s fierce effectiveness in the kill-or-die of battle, he begins to embrace “the beast in the heart of every fighting man”. With echoes of Heart of Darkness via Apocalypse Now, Gallagher adroitly charts how war frays the morals of a good man.

Similarly accomplished is Gallagher’s darkly humoured observation of war’s absurdities: the inane “fobbit” brass and their PowerPoints, safe in air-conditioned bases while Porter’s men bleed in the sand; the journalists who’ve left for bigger stories; the indignities of violent death — “most of Fat Mukthar had been scooped into pots and pans for burial. Belly guts hung from a palm tree like red banana clusters. An old woman was trying to dislodge them with a broomstick that didn’t reach. Across the road a pair of dogs fought over a tibia bone wrapped in calf muscle.”

Gallagher’s deft literary voice lets the barbs sting — offering sharp critique of the war machine — while simultaneously undermining Porter’s adrenalin-brittle anger with an intellect that seeks perspective and moral nuance. Porter finds understanding for the sergeant he considers barbaric, the “fobbits” he resents, the sheiks who doublecross him, and, through this, Gallagher gives a textured, empathetic view of a messy war.

Most impressive, though, is Gallagher’s prose. He writes lyrically about violence, recording the war with visceral immediacy but letting its rare beauties glimmer through. On patrol, Lieutenant Porter breathes air that “was windless and smelled of wildflowers”, sees “irrigation ditches [that] zigzagged through hamlets with gridded care, a network of blue forcing structure upon dusty bedlam”. We feel the “mad heat” as Porter takes off his helmet — “My scalp gasped” — and the night chill as his soldiers stand “in front of a Humvee, machinegun barrel pointing up at the muddy stars”.

This muscular lyricism is realised in combat scenes, where the poetry Gallagher finds in violence conveys the primal excitement stirred by lethal power: “I locked and loaded”; “the bolt chambering a round with an anvil’s grace”. Confronting an angry mob, Porter “smelled the loose flesh of violence … all hot sweat and young rage … As hands started reaching for us, trying to pull us into the mass of the riot, three simple words hung on my tongue like a scythe: light them up.” His descriptions of combat’s ­aftermath — quiet and taught with pathos — remind me of Siegfried Sassoon: “I turned around and looked back at what we’d wrought. Under sad yellow stars, billows of smoke swirled in the wind, and a sheet of wildfire tore through what had been the field of poppies. Flakes of ash drifted through the air. I stuck out my tongue and caught one.”

Violent, intelligent, and beautiful, Youngblood is one of the best novels to come out of America’s 21st-century wars, with an authority that eclipses most debuts and a literary talent that announces a commanding writer. There’s nothing coffee-shop about it.