In her teenage years, Chimamanda Ngozi Adichie recalls with wry amusement that she developed a rather unusual crush. While her friends were infatuated with film stars, she was obsessed by Chukwuemeka Odumegwu Ojukwu, leader of the breakaway eastern Nigerian state of Biafra during the country's 1967-70 civil war. Ojukwu - who, like Adichie, is from the region's Igbo ethnic group - said secession would create a safe haven at a time when Igbos were being slaughtered by the thousand elsewhere in the country.

Adichie's yearning eventually went the way of other adolescent simplicities, another small reflection of her lifelong curiosity and maturing consciousness about the story of Biafra's birth and death. The conflict ended before she was born but it resonates through the histories of her family and countless others who lived there at the time. More than 1m people are thought to have died, some in pogroms against Igbos, some in the fighting and many from starvation in a new state that was economically blockaded but managed to survive on its own resources.

"As I got older, I developed this sense of sadness at what happened but also pride at what we could have done," she says. "I still feel that way: that if we could be so self-sufficient under such trying circumstances, where has it gone?"

This bittersweetness is at the heart of the 28-year-old Adichie's second novel, Half of a Yellow Sun, a love story set during the war. The book, following on from her much-admired debut Purple Hibiscus, has marked her out as one of the most notable young literary talents at work today. It has already received the imprimatur of her fellow Nigerian Chinua Achebe, who has said Adichie the author "came almost fully made". "That meant so much to me," Adichie says. "I was in tears for the whole day."

Over afternoon tea in the lobby of her Kensington Hotel, Adichie describes the strength of personal feeling that underpins Half of a Yellow Sun. A few days earlier, at a party to mark its launch, she had talked unaffectedly and movingly about how happy she was to see publication of a book that she had worked on her whole life. Both her grandfathers died during the civil war; one in particular has, she says, always been a "presence", through her father's memories of him. One of her five siblings, Chukwunwike, was born "in the thick of war": his near-death as a baby, after being given some palm wine by a friend of her father's, is commemorated in the experiences of an infant in the book. "I always knew the novel would come," Adichie says. "But I also knew that it had to come when I was ready."

Another reason why she chose to write about the war is that she considers it "so central" to the way her country is today. Nigeria is a nation of epic diversity, comprising well over 100m mostly poor people from hundreds of ethnic groups speaking hundreds of language, with all the yearnings for autonomy that implies. The book's title refers to one of the components of the flag of independent Biafra: a sun that was either midway through rising, or - in hindsight - presciently destined never fully to breach the horizon.

Since the end of the war, Nigeria has stayed restlessly together, its post-independence oil wealth for the most part ruthlessly carved up by multinationals, officials in military and civilian governments and other expatriate and Nigerian elites. Many see it as a predictable fate for a fractious nation created in 1914 by British imperialists accomplished in the politics of divide and rule. Adichie says people within Nigeria "hadn't been given the tools" to handle it after independence in 1960. "Nigeria was set up to fail," she says. "The only thing we Nigerians should take responsibility for, in my opinion, is the extent of the failure."

Adichie says her book is also an attempt to explore the belief among some Igbo people that their experiences of marginalisation and persecution in Nigeria have made them the "Jews of Africa". Half of a Yellow Sun mentions the Holocaust, although sparingly and with some subtlety. Without overdoing the historical comparisons, Adichie says, she saw parallels in the idea that "by creating the state then you become secure, you no longer want to be killed".

Half of a Yellow Sun avoids the pat conclusion that the masters of a nation forged from a people's suffering would necessarily behave better than their former persecutors. A character from the Efik minority ethnic group asks explicitly whether the new state of Biafra will be fairer than what went before. The author admits this is a question that still hangs in her own mind, although she wants to believe a victorious Biafra "wouldn't have been too bad" for non-Igbos to live in.

"I also like to think that we wouldn't have" - "I say we, goodness!" she notes, in a characteristically thoughtful and self-parodic aside - "I mean the Igbo people, wouldn't have been too oppressive. I hope not."

In its combination of passionate yet nuanced reflectiveness, Half of a Yellow Sun is perhaps a book that could have been written only by someone not directly involved in the horrific events it describes. At the same time, as Adichie acknowledges, this distance creates an extra level of responsibility to provide an account that will seem truthful to those who there at the time. Disarmingly, she admits to being "quite often terrified" and sometimes "overwhelmed" while working on the book. It was her father, a retired academic - he was Nigeria's first professor of statistics - who really made it possible for her to write it, she says. "I would read something in a book and then I would ask him what he remembered of it," she says. "And he then put a human face to it."

Adichie says she has "played with history" a bit, although only in terms of minor details, such as shortening the distance between towns and creating a train station where none existed. She does not think anyone could say she lied about the war but she is in any case looking forward to presenting her book in Nigeria in December, where people can heckle her and say she was wrong. "It will be so exciting," she says.

Adichie, who grew up in Nigeria before getting her university degree in the US, says she will probably always "live in both places". She thinks her next big literary project will focus on Nigerian immigrants, although she says she probably shouldn't talk too much about it. She has just started a Masters course in African studies at Yale, which she thinks will help her writing. "I really feel so strongly about Africa, west Africa particularly," she says. "And I want to be knowledgeable."

In her more idealistic moments, Adichie says she thinks about moving back to Nigeria full time. The country has begun a programme of economic reforms and an anti-corruption drive, although the results are much argued over and many people are deeply worried about the destabilising capacity of elections due in April next year. On her visits, Adichie says she sometimes criticises people she knows for being too pessimistic, although she acknowledges her views may be coloured by the possibility of escape that she enjoys. "I realise I am certainly one to talk," she says. "I can leave, and have options outside of Nigeria."

For all Adichie's fondness for the US, which she compares to a "rich uncle" who gives her pocket money, she detects a certain blandness there in some of the responses to her work. She finds this less the case in Britain, where the knowledge of Nigeria - while hardly very deep or wide - is perhaps greater. In the US, she says, a little wistfully, she will always considered an African writer, whereas in Britain she is more likely to be viewed simply as a writer.

Adichie sees the contrast partly in historical terms. Britain, which dismayed the Biafrans and many others by arming the Nigerian military government during the war, has far more colonial baggage. The plus side of this, says Adichie, is that there is "more to engage with and more to fight about, which is more fun. I prefer more colour," she says, "whether good or bad, to a certain extent."

An intriguing character in Half of a Yellow Sun is a needy young Englishman called Richard, who finds love and a cause in Biafra. Near the end, Richard, of whom Adichie says she is "very fond", lapses into sorrowful, impotent racism against a military officer whom he fears is a rival for his Igbo lover. I ask Adichie if this reflects a pessimism on her part that there is something forever racist in the white European mind and education. She laughs and talks around the subject a bit, ending up by outlining the idea - relevant also, she agrees, to some of the activities during last year's "Year of Africa" in Britain - that it is possible to simultaneously love something and condescend to it.

"I feel that is a way a lot of liberal whiteness looks at African blackness," she says. "I don't doubt the love . . . but I also often detect that there is something condescending about it."

It is a typically calibrated remark, sharp but not pious, from a young author who has come a long way in a short time since her obsession with Biafra's head of state. Adichie's ability to see and acknowledge failure in the good and good in the failing make her a sane and compassionate new voice in an often strident world. Half of a Yellow Sun is full of awful happenings but the whole is somehow oddly uplifting; it also manages to be a deeply political book while simultaneously celebrating the spiritual and the sexual. Above all, she says, it is "really a story of love".

'Half of a Yellow Sun' is published by Fourth Estate at £14.99