“The Illuminations,” by Andrew O’Hagan. (FSG)

Most Americans’ lives are totally unaffected by the fact that, a world away, soldiers bearing our flag are fighting an enemy we know little about, using methods that don’t bear close inspection, for a purpose that no one seems able to define. Compared with Vietnam or World War II, wars that involved or convulsed all of American society, our “forever war” seems like an anomaly. But it would have been quite familiar to a 19th-century Briton: For these are the border wars of an empire, which can never be won because no empire is ever free from threats.

It makes sense, then, that a British writer, Andrew O’Hagan, should write so perceptively about the way we fight now in “The Illuminations,” his fifth novel. The structure of the book itself reflects the schizophrenia of Britain’s war in Afghanistan, which like its war in Iraq was begun under American auspices. Half of “The Illuminations” deals with the misadventures of a platoon of British infantry in Afghanistan; the other half tells the story of an aging woman in Britain who is slowly falling prey to dementia. Each plot is, in its own way, an affair of life or death, survival and devastation.

As it turns out, the connection between these two stories is as close as can be. Luke Campbell, the captain leading the squad in Afghanistan, is the beloved grandson of Anne Quirk, the woman losing her memory in the Scottish town of Saltcoats. Luke comes from a military family. His father was a British soldier killed in Northern Ireland during the Troubles, and he enlisted as a way of connecting to a man he barely knew. Yet from Anne, who in her youth was a professional photographer, Luke also inherits a sensibility — artistic, inward and, in the novel’s terms, feminine — that sets him apart from the army’s world.

In alternating sections, O’Hagan unfolds Luke’s and Anne’s stories, each of which is heavy with the sense of impending disaster. In Anne’s case, the decline is gentle but inexorable. Increasingly confused, she has begun to retreat mentally into the period of her life that meant most to her, when she lived in the British resort town of Blackpool with the love of her life, a man named Harry Blake. Thanks to her neighbor Maureen — a finely drawn character whose solicitude for Anne goes hand in hand with indifference toward her own children — Anne has so far been able to keep living on her own. But she can no longer use the stove, she has begun to feed bowls of soup to a ceramic rabbit, and clearly the next steps in her life will be the nursing home and the grave.

With Luke, on the other hand, O’Hagan conjures the chaotic sense that absolutely anything could happen. Luke is the protege of Major Scullion, a commanding officer who represents the best of the contemporary military. His career has been spent in humanitarian peacekeeping missions in places such as Sierra Leone and Kosovo. Yet he is visibly breaking down, having lost both his wife and his nerve. In his decay, Scullion embodies the West’s own loss of confidence in the power of its military to make any real difference in the world. For the new generation of soldiers, O’Hagan writes in one of the novel’s persistent themes, actual warfare seems less real than the combat video games they grew up playing: “Younger soldiers often thought they knew the battleground; they saw graphics, screens, solid cover and . . . guns you could swap. . . . They saw cheats and levels . . . and the kinds of marksmen who jump up after they’re dead.”

In such a war, where the privates get stoned on Afghan hash with their commanding officers and speed through the desert blasting heavy metal in their SUVs, it seems only a matter of time before disaster strikes. Once it does, Luke returns to Scotland for one last reunion with his grandmother, helping her to relive the Blackpool of her youth. In the course of their visit, family secrets are uncovered that help to explain why their history turned out as it did — in particular, why Anne’s career as a photographer never blossomed. An artist himself, O’Hagan exalts Anne’s evanescent truths of vision and perception, the ones that she captured in her old pictures, over the suspect truths of ideology and patriotism. “We got a lot of things wrong but we got a lot of things right,” Anne muses near the end of the book. “Some of them said there was . . . that’s right . . . justice in it.” The photographer’s flash, rather than the red and green tracer fire that lights up the Afghan sky, is the illumination in which O’Hagan finally asks us to place our trust.

Kirsch is the author most recently of “Rocket and Lightship: Essays on Literature and Ideas.”

Ron Charles is on vacation.

THE ILLUMINATIONS

By Andrew O’Hagan

Farrar Straus Giroux.
293 pp. $26