The attacks on New York and Washington on 11 September 2001, so extreme and audacious, left many of us speechless: the monumentality of what had happened seemed to lie on the other side of language, beyond rational articulation. These events were “unimaginable” and “unspeakable”. They were nothing of the kind. In fact, the novelist Ian McEwan, writing on 12 September in the Guardian, offered an immediate and articulate narrative response to this world-historical moment, free from the cliches of mere horror and outrage. Three days later, he was back, haunted by how the passengers on the hijacked airliners, confronted by the certainty of their own deaths, had from high in the sky used their mobile phones to call their loved ones far below. “A new technology has shown us an ancient, human universal . . . There was really only one thing for her to say, those three words that all the terrible art, the worst pop songs and movies, the most seductive lies, can somehow never cheapen. I love you . . . There is only love, and then oblivion. Love was all they had to set against the hatred of their murderers.”

He wrote of how one of the chief crimes of the hi-jackers was a failure of imagination: to imagine how it must have felt to be a prisoner on one of those doomed planes. “Imagining what it is like to be someone other than yourself is at the core of our humanity,” he wrote. “It is the essence of compassion, and it is the begin- ning of morality.”

On Friday 8 July, the morning after the terrorist outrages on London, McEwan could be found once more in the Guardian, this time reporting from the bomb-shaken streets of the city in and around Bloomsbury. He wrote with clarity and imagination and empathy. What we are witnessing here is the emergence of McEwan as the closest thing this country has to a national novelist. He is the literary novelist-as-bestseller: the writer who, because of his continuous, imaginative engagement with the shifting complexities of the present, brings news and can speak to and for the nation at times of crisis and shock.

How has this happened? How has Ian McEwan come to occupy such an exalted position of public influence? It has been a long journey.

One June evening in 1988, as the high tide of Thatcherism swept across the country, a group of writers and media intellectuals gathered at Harold Pinter and Lady Antonia Fraser’s grand house in Campden Hill Square, Holland Park, west London. They wanted to discuss how best to respond to the hegemony of the Thatcher government. The response to the creation of this “informal discussion group” was derisive, notably from Andrew Neil’s neoliberal Sunday Times: what did this preposterous group of bien-pensants liberals know about politics and the state of the world?

Among those present at Pinter’s house that evening was the young McEwan, in the process of remaking himself as a writer. Since the publication of his first book of stories, First Love, Last Rites (1975), he had seemed to be more a chronicler of personal obsession than a writer of public engagement: the disturbed, isolated, amoral protagonists of his early fictions pursued their fantasies with vigour: killing, maiming, bullying, masturbating.

The early McEwan was no pornographer: he already wrote too well, his intentions were too serious, and his fictions, despite their splatter effects, were really about the disturbed imagination and about power - the power parents have over children, a brother has over a sister, a man over a woman, and the state has over the individual. Many of them, which he wrote under the guidance of Malcolm Bradbury as the first student of creative writing at the University of East Anglia, were experiments in form: the interior monologue, the fractured confession. Yet those stories, with their insistent linking of sex with death, were mostly cold - love was always absent from them, as if their very coldness were an expression of an entire, bleakly despairing world-view.

Born in Aldershot in 1948, the son of a remote middle-ranking Scottish soldier, McEwan grew up in North Africa and Singapore, where his father was posted. He later attended a state-run military boarding school in Ipswich where, as he has said, he felt cold, hungry and alone. From there, he went to Sussex University and then, after following the hippie trail in North Africa, he moved to Norwich to work with Bradbury at UEA. “I arrived in this lovely foreign town [Norwich], took a room and had a meeting every three or four weeks with Malcolm,” McEwan once told me over breakfast in a quiet upstairs room of a pub in north London. “He offered no strictures and showed no shock at the content of my stories. He was remarkably tolerant. It was a great piece of luck; having a reader with no egotistical wish to shape me to some preconceived end meant that I was free to find my own voice.”

Today McEwan lives in London with his wife, the journalist Annalena McAfee, and his children from an earlier marriage, in a fine house near Regent’s Park, having lived for many years in Oxford. They are well connected and sociable. “They are great foodies and love to entertain,” says one of their close friends. “At their dinner parties you may encounter Martin Amis, Timothy Garton Ash, Julian Barnes or Salman Rushdie, as well as assorted newspaper editors, literary agents and celebrity academics.”

Yet for all his metropolitan wealth, influence and the comforts of hardened achievement, McEwan shows little willingness to retreat from the defining issues of his time. He is never complacent. Shortly before the American-led invasion of Iraq, he wrote an article for the openDemocracy website in which he expressed his ambivalence about the threatened conflict. Compared to the often strident and bellicose certainties of those arguing either for or against the war, his commentary was a model of restraint and uncertainty. “The hawks,” he wrote, “have my head, the doves my heart. At a push I count myself - just - in the camp of the latter. And yet my ambivalence remains . . . One can only hope now for the best outcome: that the regime, like all dictatorships, rootless in the affections of its people, will crumble like a rotten tooth; that the federal, democratic Iraq that the Iraqi National Congress committed itself to at its conference can be helped into existence by the UN, and that the US, in the flush of victory, will find in its oilman’s heart the energy and optimism to begin to address the Palestinian issue. These are fragile hopes. As things stand, it is easier to conceive of innumerable darker possibilities.”

In January this year McEwan published his fictional response to the attacks of 11 September 2001 and the wars that followed: Saturday, perhaps the most discussed and debated literary novel of recent times. It is set on a single day - 15 February 2003, when more than two million people marched in London in opposition to war in Iraq - and is told entirely from the point of view of one man, Henry Perowne, a 48-year-old neurosurgeon. So the novel, as well as being a political text, is a dramatic study in consciousness, in showing over a single day what Virginia Woolf called “the quick of the mind”.

Perowne, like McEwan, lives affluently in central London. He loves his wife and is the father of two well-adjusted young adults, one a published poet (by Faber, no less), the other a promising blues musician. There the similarities end: Perowne, though intelligent, is resolutely unliterary. He cannot understand the impulses of the artistic imagination: he is a scientific reductionist. He strips and reduces human behaviour to its biological essentials. “There is much in human affairs that can be accounted for at the level of the complex molecule,” he suggests.

Yet out on the inner-city streets and beyond in our interconnected, globalised world are true dangers and little control: not everyone is as calm and rational as Perowne, or thinks as he does. He encounters one such person when, on his way to play squash, his car collides with another driven by an aggressive thug called Baxter. There follows a menacing confrontation. Baxter, who Perowne immediately understands has a degenerative brain disorder, feels that he has been humiliated. Later, Baxter returns, violently, to confront Perowne at home, where he is preparing for a family dinner, with brutal and then surprising consequences.

McEwan has spent most of his writing life conceiving of darker possibilities - perhaps the darkest in contemporary fiction. Long before 11 September 2001, he would create entire novels out of randomness and a single, terrifying chance occurrence: the disappearance of a child from a supermarket (The Child in Time, 1987), which pre-echoed the Bulger case in Liverpool; a balloon crash-landing in a field (Enduring Love, 1997); or the malicious allegations of a little girl, in an England suspended between two world wars, that condemn an innocent man to imprisonment and perpetual separation from the woman he loves (Atonement, 2001).

Yet Saturday is his most fearful novel so far, perhaps because it is also his fullest expression of the times in which we live. It is a novel suffused with anxiety - about the coming conflict in Iraq and its likely aftershocks - but is also suffused with a sense of the new vulnerability of life in our terrorist-threatened cities, where bombs can explode at any moment and anywhere . . . and do. To read this novel - indeed, to read most of Ian McEwan’s fiction as well as his recent journalism - is to discover, as Shakespeare wrote of Hamlet, a writer who seeks “to show . . . the very age and body of the time his form and pressure”. In doing this, at least in Britain, he has no peer. He is our national novelist.