After partition in 1947, the western half of the province of Bengal was ceded to India, and the east to Pakistan. "Pakistan was to be for the Muslims, and India for the rest," writes Philip Hensher. "To the left was West Pakistan, where they ruled, and spoke Urdu, and wrote in an alphabet that flowed like water under wind. To the right was East Pakistan, where the Bengalis lived. They spoke Bengali, which chatters like a falling xylophone, and is written in an alphabet that looks like a madman trying to remember a table's shape."

The Pakistanis didn't like the Bengalis, and the Bengalis did not enjoy being ruled by the Pakistanis. Attempts were made to re-educate the reluctant new citizens of East Pakistan  to get them to speak Urdu, and to discard their faith in Bengali poetry and music. It didn't work. In 1970 war between the two broke out and after a nine-month bloodbath, a new country was born: Bangladesh, the country of Bengal.

Philip Hensher's husband, Zaved Mahmood (fictionalised as Saadi), was born in late 1970 into a large, and defiantly Bengali, family in Dhaka. His grandfather kept poems and books walled up in the cellar, awaiting some theoretical moment when people would be free to read again. When the war begins, the whole Mahmood clan gather in the darkness. Most of the children are old enough to understand that any noise might well prove fatal, but Saadi is just a baby. In order to keep him from crying, he is fed and passed from person to person all day: by the time the war ends, "Months of feeding, of keeping me quiet with mishti doi [sweet yoghurt], had produced a gargantuan infant."

Hensher's fictionalisation of Saadi's early years flits from time to time, starting in the years shortly after the war ended. Saadi and his friends are playing street games based on TV characters: the girls are fond of Dallas, the boys like Kojak. But the most popular game is based on the American series Roots, since it allows for a large cast, despotic slave owners, and  most thrilling of all  imaginary slave auctions. Except that the game they're playing isn't about southern black history, but more local events. One boy, Assad, is the son of someone who had supposedly "taken money from the Pakistanis, and had told them where they could find intellectuals  musicians, poets, scholars, professors, schoolteachers  to kill".

The book slides backwards, pulling the reader back to the time before Mahmood's birth. There is the wider family but there are also those who are connected to it: the two musicians Amit and Altaf, his father's chauffeur, Rustum, and their neighbours the Khandekars.

As tensions increase, many people make the decision to flee to Bengal's capital, Kolkata. When, after a long journey, Amit reaches the border  without Altaf, and without his tabla  he "saw all at once his future He saw his life in India, arriving at his cousin's house with nothing but a small grip with a slashed lining and an apologetic face.He saw himself working at what he could get, sleeping in the corners of rooms, negotiating and explaining with Indian officials, getting nowhere in the course of weeks. He saw no end to the war that was coming."

It's a mark of Hensher's skill that the frequent leaps between dates read so seamlessly as to be unnoticeable. Instead, what snags in the mind are his pictures  of Saadi's father in the back of a rickshaw wrestling legal papers "like a large escaping fish", or Mrs Khandekar passing tiffin-pails to Altaf filled not with food but with something strangely heavy, wrapped in muslin. He has an ear for the ways in which stories glue people together: legends, calcified old anecdotes, necessary falsifications.

"Saadi" tells his story well, but it's the relationship between Amit and Altaf which really sings off the page; chaste, but made of love and music. In the five years that Amit is away, he supposes that Altaf will marry or move away. Instead, when he returns after the war to the same flat in the same street, he finds Altaf there drinking dusty firewater in the afternoon light, the tabla untouched. It will take a while, Amit understands, "to master his understanding of the parts of Altaf's life that had been abandoned, fallen into disuse, and the parts such as breathing which had continued nevertheless".

It's said that when a writer is born into a family, that family is doomed. They don't say what happens when a writer is imaginatively born into his partner's family. By coopting Mahmood's history and making of it something that is neither memoir nor novel nor history but a synthesis of all three, Hensher has created a greater thing than just a record of childhood, or war. It probably isn't Zaved's story any more, but it's great just the same.

It is a fervent hope for the children of violent civil war that their most keenly remembered pain should be of the death of a pet chicken. Mercifully for the reader, Piklu's tragic journey from idly scratching around in the garden to steaming on the family dinner table doesn't describe the extent of his boy-owner's memories; but it is, clearly, a formative moment. "No, thank you," he replies, when offered a serving. "I don't care to eat a friend of mine."

The cruelly betrayed child is Saadi, the narrator of Scenes from Early Life; his full name is Zaved Mahmood, and he is the Bangladeshi husband of the novel's author, Philip Hensher. In a recent interview with Guardian Review, Hensher explained how he became aware that Mahmood, who is an international human rights lawyer, was unlikely to find the time to describe his rich and dramatic childhood in writing, and how he decided to take on the job himself, interviewing his partner about the tiniest details of his early life. The resulting book is consequently something of a hybrid: an autobiography written, as it were, by proxy, and finessed, by choice and by its creator's natural affinities, into a novel.

And how well that mash-up of fact and fiction works, coalescing into a narrative shaped by cataclysmic historical events but inflected with small-scale personal drama and individual idiosyncrasy. That the death of Piklu should occur so near the book's start seems no accident; its first 50 or so pages unfurl in a series of semi-comic, scene-setting vignettes that place us firmly in the heart of a well-to-do family in 1970s Dacca. Historical back-story  for example, the departure of Saadi's grandparents from Calcutta for East Pakistan after partition in 1947  plays second fiddle to more immediately human establishing details: a little boy's fascination with his grandfather's red Vauxhall, or with the sewing thimble belonging to one of his six aunts, or, indeed, his addiction to imported television programmes.

One of the highlights of this straightforwardly memoiristic opening section is an extended description of Saadi and his friends' devotion to the television blockbuster Roots, complete with their intense competition to take the part of the slave Kunta Kinte in their re-enactions. Saadi himself ends up as the slave-owner, on account of his comparatively pale skin ("He can be the white man," say his friends), but this is no bad thing, given that the slave auction is their favourite bit, the climax of the game that fills them "with a terrible, inexpressible excitement".

But the realities of the recent past lurk behind even the most childish and innocent play. Saadi is dissuaded from including one neighbourhood boy, Assad, in the games, with the portentous words: "You know why." Chillingly, he does: even at a young age, he's aware that Assad's parents must have sided with the Pakistan authorities during the war of independence that raged for eight months in 1971. "It was as if," his older self reflects, "there were two cities laid on top of one another, each quite invisible to the other, each engaging only with its own sort."

Gradually, the novel's domestic palette relaxes and expands to encompass larger social and political events. Now, alongside the affectionate rendering of the household  of Saadi's parents and grandparents, his aunts, Mira, Nadira, Mary, Era, Bubbly and Dahlia, each with their different talents and family role, and of his feckless uncle Laddu and artistic uncle Pultoo  come other stories.

There are tales of family friends  one of them Bangladesh's first leader, Sheikh Mujib  and servants, including a chauffeur charged with sealing up the household's forbidden Bengali books and music in a secret cellar, and a beautiful ayah exiled from her marriage because her father could not afford her dowry. There is a deeply painful, sparely rendered account of the quasi-romantic relationship between two musicians, Altaf and Amit, abruptly sundered when the Hindu Amit has to quit Dacca as the repression of the authorities begins to bite. Throughout, these stories illustrate the dreadful conflict that settles over India and West and East Pakistan, the hastily divided area that resembled on the map "a broad-shouldered ape with two coconuts, one on its right shoulder, one under its left armpit".

Hensher's skill in combining the story of a family  one with its own tensions and tiffs, separations and reconciliations  with the story of a country coming into being is highly impressive. And, as the novel travels from Saadi's childhood back to his birth, the two strands become horribly enmeshed.

Now we understand that the reason Saadi was allowed to eat anything he liked as a baby was not because he was indulged, or even because he was unwell or a poor eater, but because he had to be kept quiet as the family hid in their home while vicious battles overtook the streets outside; we realise that his grandmother's fondness for a particular recipe for steamed fish  and her fondness for repeating it, over and over again  proceeds from that same experience of besiegement in 1971. Both stories have become part of family folklore, trauma reinvented as humorous anecdote. The accounts of families who did not survive the war intact are inserted in and amongst, with all the terrifying dispassion of a newspaper report.

But for all Hensher's accomplished ventriloquism  his ability to inhabit the voice of a Muslim child and a history teacher at the same time  his own voice is not lost. As much of his previous work, The Northern Clemency in particular, has shown, he is drawn to describing the minutiae of everyday life, and the sillinesses and sadnesses and comedy of family life. This new work shows that he is able to co-opt somebody else's story with generosity and ease. But it also suggests that he was right not to turn biographer; even a life eventful enough to be only minimally garlanded with invention benefits from the novel's capacity to blend the absurd and the heart-breaking so effectively.