From hero of the left to neocon turncoat, and still battling on: Christopher Hitchens talks to Decca Aitkenhead about old arguments and his new memoir

I'm not sure what a legend should look like exactly, but I'm pretty sure it's not this. The paunchy, middle-aged figure who opens the door at 10am has a crust of dried toothpaste around his mouth, an air of bleary dishevelment and the stooped shuffle of a man just out of bed and wishing he'd postponed the appointment to a less ungodly hour. Expecting to meet a sort of rakish Russell Crowe, I appear to have found a hungover Timothy Spall.

Where is the celebrated rhetorician, famed for speaking in perfect paragraphs sculpted from flawless sentences? Gruff, vague and nursing a cup of tea, he clasps one hand discreetly over the other in a manner suggestive of some practice in taming the morning shakes. Having flown in from America only the previous afternoon, he explains that he had been out with his old friend Martin Amis until 3am. Gradually – fortified by two packets of cigarettes – he begins to reconstitute himself, looking less and less like Spall but, strangely, more and more like Terry Wogan. He can't really manage eye contact. Once noon arrives, though, he brightens up, proposing the first scotch of the day with one of those bluff jokes about rules for drinking so dear to saloon bar bores the world over.

This, then, is the legendary Hitch – one-time titan and hero of the left, latterly post-9/11 neocon turncoat – the man who took on Henry Kissinger, Mother Teresa, George Galloway, God and Saddam Hussein. Really? This guy?

Hitchens is in London to talk about his memoir, Hitch-22, a hefty work charting his path from Trotskyist agitator at Oxford in the 60s, to glamorous young New Statesman colleague of Amis and Ian McEwan in the 70s, to prolific Washington pamphleteer and columnist throughout the 80s and 90s, and international darling of the intellectual left. There can scarcely have been a war zone or salon in the world untroubled by Hitchens' attendance, his ubiquity matched only by his prodigious output, moving Gore Vidal to anoint him his "dauphin".

Then came 9/11, and his dramatic political realignment, which reinvented him as the surprise poster boy for the neocon right. The memoir, as you might expect, goes into quite a lot of detail about why he thought we should invade Iraq, and why he was right. What the left just does not get, Hitchens argues, is that "Islamofascism" is hellbent on destroying our civilisation, and unless you fancy being bombed back into pre-Enlightenment times, you should bloody well be out there on the barricades, fighting the good fight beside him. Anyone who disagrees is either stupid, cowardly, naive or too lazy to have bothered updating their political faculties "since Woodstock".

Hitchens wants to make it clear that he has not written an autobiography, but the clarification is unnecessary given the book's striking omissions. He's not terrifically interested in emotional self-examination, and there is barely a mention of his three children, one of whom was still in his first wife's womb when he left her for the second, neither of whom gets much of a look-in. There is nothing, either, about his years of estrangement from his younger brother Peter, the rightwing British columnist. "Don't have copyright on other people," he explains rather unsatisfactorily.

Yet he found the memoir more of a challenge than anything else he's ever written. "I found that, well, what's the argument here? And, of course, there isn't one. You're not making a case." To the great polemicist, this was a problem. His solution was to write "not a confessional", but what he calls "the context of the battle of ideas".

Some readers will doubtless be gripped by his tales of assignations with guerrillas and presidents, and judge at face value the case Hitchens makes for his ideological journey. But it is the chapters on his parents, both now dead, and the other private reflections that are more illuminating – more so, probably, than the author intended. Because, for all Hitchens' stated contempt for the personal as political, I would say his opinions today owe more to his emotional world than to any amount of argument.

When the invasion of Iraq was first debated, one couldn't fail to notice the preponderance of left-wing men of a certain age who came out in support of the war. Radicals as adults, but often from conservative backgrounds, now beginning to confront their own mortality, and preoccupied by masculinity and legacy, their palpable thrill about military might suggested that, deep down, they secretly feared progressive principles were for pussies. Now here was their chance, before it was too late, to prove their manhood.

In 2006, Hitchens' wife, the American writer Carol Blue, told the New Yorker her husband was one of "those men who were never really in battle and wished they had been. There's a whole tough-guy, 'I am violent, I will use violence, I will take some of these people out before I die' talk, which is key to his psychology – I don't care what he says. I think it is partly to do with his upbringing."

Is there any truth in what his wife said? He pauses for a second. Then, unexpectedly: "Yeah. Yes. One of the things I've realised, writing the book, is that it has to be true."

Born in 1949, the second world war was "the entire subject of conversation" during Hitchens' childhood. He was the eldest son of a naval officer – "the Commander" – a quietly conservative, blimpish character in the Denis Thatcher mould, who would often say that war was the one time in his life when he "knew what he was doing". Hitchens' mother was a much more colourful character, and on the face of it the dominant parental force; he was her favourite son, and he adored her. "If there is going to be an upper class in this country," she told her husband, "then Christopher is going to be in it", packing him off to boarding school at the age of eight. "The one unforgivable sin," she used to say, "is to be boring", an injunction her son has observed faithfully. His father, by contrast, was a dreadful bore. And yet it is quite clearly the Commander's legacy that haunts Hitchens today.

With hindsight, there was an early clue to his appetite for combat in the ferocity of Hitchens' support for the Falklands Royal Naval task force, shared by few on the left. "I couldn't possibly see the UK defeated by those insanitary riffraff!" he exclaims. "This was a diabolical liberty." But Islamic fundamentalism presented a more promisingly meaty foe than a tinpot Argentine dictator, and ever since the 1989 fatwa against Salman Rushdie, Hitchens says, "I knew there would be some huge intrusion into the heart of civilisation from barbarism."

And so chief among Hitchens' emotions by the end of the day on 11 September was "exhilaration. Because I thought, now we have a very clearly drawn confrontation between everything I hate and everything I love. There is something exhilarating about that. Because, OK, now I know what I'm doing." Just as his father had felt during the second world war? "Yes, exactly," he agrees.

"Anyone who's studied the 20th century wants to know how they would have shaped up. And you don't usually get the chance to find out. My son considered joining up, and I'm the first Hitchens for quite a while who hasn't put on a uniform – and it would have been quite amazing if he had."

To say that Hitchens is stirred – even obsessed – by the question of courage would be to state the obvious. It seems therefore highly likely that his longing for the great Orwellian test – the momentous moral challenge to match the 1930s – might tempt him to overstate the threat of Islamic fundamentalism. "Do I ask myself," he replies, "do I think our civilisation is superior to theirs? Yes, I do. Do I think it's worth fighting for? Most certainly."

That wasn't really the question.

"Well, I know how to correct atavism in myself, yes, I do," he retorts. As evidence, he offers the fact that after 9/11 he voted against his Washington apartment block flying the American flag. "So I was pleased to find out I wasn't in the mob majority in that way." But then he adds, bizarrely, "The other thing is, what happens when this ebbs, and the flag gets tatty? And you start seeing cabs with flags worn to rags, making the flag look crappy."

For someone feted for his adversarial prowess, I'm surprised by how often he sabotages an argument with a lurch into self-indulgence. For example, he has written at length about the failings of Guantánamo Bay. But then he says to me, "Guantánamo slightly threatened at one point to change my attitude towards capital punishment. I thought it would have been good if some of those people could have been taken out and shot. Yeah, put up against a wall. Lincoln would have done it. Of course, I would have been against it if they had. But that's how I felt."

Similarly, his memoir contains an extended and frankly sentimental tribute to a US soldier killed in Iraq, an intelligent young man with deep misgivings about the war, who had been persuaded to sign up only by reading Hitchens. ("Sentimental? I was hoping to avoid sentimentality," Hitchens objects – and to his credit it's the only criticism that visibly offends him.) If he really wanted to win the argument – that the war was worth it, however tragic the loss of life – surely a shrewd polemicist would honour Iraqi civilian deaths with the same grief he accords one American soldier. He blames his former comrades for failing to be persuaded by his case for invading Iraq: "Yes, absolutely. I was right and they were wrong, that's pretty much it in a nutshell." He laughs.

But another way of looking at it would be to blame himself for failing to convince them.

"What you're trying to say is, couldn't I be a bit more," his lip faintly curls, "sales conscious."

Where's the point in engaging in a battle of ideas if you have no interest in being persuasive? "That seems like an invitation to soften the tone and be more agreeable." Doesn't he want to win the battle? "Sure." Why, then, did he tell an interviewer in 2001: "I don't really care whether people agree with me"? He looks momentarily surprised. "Oh, that's too bald. What I mean is that I'm not going to soften a case in order to make it more presentable. When I've flung down the pen, I want to be sure that I've made the strongest possible case I can make – and also," he adds tellingly, "really had fun doing it."

Isn't the measure of its strength how many people he persuades? "No, it's interesting because I don't want to make that the test. It isn't what gets me out of bed – how many people can I win over today? No, it isn't. It's the enjoyment of the combat – in part for its own sake, sure – but also to give a good representation, I hope, to the people whose principles are in common with mine."

Do you think, I ask him, you're a show-off?

"No, I don't. I don't. But obviously I wouldn't be the judge."

Hitchens would be the first to agree he cuts a less dashing figure than the beautiful young firebrand who illuminated the pages of the New Statesman in the 70s. As he dodders round his ex-wife's kitchen in west London, I wonder what that young self would have made of the man he is today.

"Well, I've done better than I thought I would. I've made more money than I ever thought I would. I've got more readers than I ever thought I would, and more esteem." He now earns "several hundred thousand dollars a year" – but claims his wealth hasn't influenced his opinions at all.

Does he think wealth ever affects people's opinions? "Well, yes, I'm a Marxist, after all." So why would his own opinions be mysteriously immune to his bank balance? "Well, because I can't trace any connection." Doesn't he find that unusual? He pauses to consider. "Well, no, because I think that comes in with inherited wealth."

His position on private education is even lamer. All three of his children were privately educated – to which, he says, he has no principled objection, "because private school was very good for me". That's not the measure of the principle, I object. "Oh, but it is for me. It was my big chance."

The march of time certainly hasn't altered one thing about Hitchens, which is, alas, his unaccountable pleasure in word games of the most puerile variety. Page after page is devoted to the infinite hilarity derived by Amis, Rushdie, McEwan and Hitchens from substituting in the titles of well-known books, films and songs the word "dick" for "heart", or "fuck" for "love", or "cunt" for "man".

"Oh, I know," he chortles, when I bring this up. "Shameful." He surely can't still find these jokes funny, can he? "Oh yeah, I do. I sometimes wake up laughing at them. Yup. Never get bored of it." And this from a man who once wrote that women weren't funny.

"No, come on," he grins cheerfully, "you have to admit some of them [word games] are funny." Emphatically not. He giggles, looking boyishly delighted. "Sometimes I'm sitting on a plane and start laughing when I think of another. And then I email it to Martin."

His long friendships with Rushdie, McEwan, James Fenton and, above all, Amis are the great love story of his life, and memoir. Hitchens says he has no heroes, but it's hard to escape the impression of the author as an archetypal hero worshipper. He agrees – "Yes, I do have [hero worship]" – but adds, "I just think one should be suspicious of hero worship, and have it under control."

Would he have slept with Amis, had his friend been agreeable? "Oh, I wouldn't have been able to refuse him anything." Hitchens smiles. Would he sleep with him now, if Amis wanted to? "No. There would be something grotesque about that. But then, I still say I couldn't refuse him anything, so make of that what you will."

We must also make what we will of his claim to have slept with two unnamed young men at Oxford who later joined Thatcher's government. Hitchens was exuberantly bisexual in his younger days – until his looks "declined to the point where only women would go to bed with me" – and is quite candid on the matter, so his refusal to name the future ministers looks at best coy and at worst like teasing up a bit of publicity for the book. "Oh no," he says. "To the contrary, I'd rather not discuss it." So why mention them at all? "You may look in vain for logic or consistency," he concedes.

Poetry, he does volunteer, always played an important part in his impressive sexual success. "You're disarming yourself in an important struggle if you can't produce a fucking sonnet. What if I had to try on my own merits? You've got to have some sort of reserve arsenal." He looks incredulous when the photographer, a very beautiful young woman, expresses doubt about the efficacy of this seduction technique.

"Oh no, not if it's done right," he says knowingly. Go on then, I say. Give us a demonstration. "Maybe at lunch?" he suggests, cheering up immediately. "Let's have lunch, and make a day of it." And so, inevitably, we adjourn to the pub.

It seems to me so evidently the case that Hitchens is an alcoholic that to say much more feels unnecessary. But for the record, he trots out all the usual self-serving, defensive evasions: "For me, an alcoholic is someone who can't hold his drink" or, "I'm not dependent, but I'd prefer not to be without it." The longest he has ever been was a dry weekend "in fucking Libya", and he claims he drinks only to make other people less boring. So, presumably, he doesn't drink when he's with Amis? "Er, yuh, I do."

I wouldn't say he's exactly boring himself when dry, but drink certainly makes him livelier company than the 10am sober version, and we pass a highly enjoyable few hours in a pub garden, during which he tries out successive renditions of a Shakespearean sonnet, Being Your Slave, What Should I Do But Tend, on the photographer.

"Well?", I ask her.

"Give her time to let it sink in!" he objects.

"Um," she ventures. "I'm feeling something like blind panic."

"Really? No!" And he's off again. "Being your slave what should I do but tend/Upon the hours and times of your desire?"

"My feeling," she reports kindly after he finishes, "is that I would be more seduced by argument."

"Well, I've got arguments!" he exclaims, laughing. "You want arguments? I've got arguments!"

He certainly has. Quite how they've earned Hitchens his status as a legend, however, I am at a loss to say. Posing himself the Proust questionnaire in his memoir, he answered the question, "What is your most marked characteristic?" with "Insecurity" and says this surprised him, but I'm not sure why, because all the decades of showy erudition and aggressive rhetoric look a lot like camouflage for a deep and heartbreaking fear of not quite measuring up.

Why does he say to the barmaid, "Put a Xerox in that" when he wants another drink? He's meant to be an international sophisticate, not a home counties golf club bore.

"I think it's rather ingenious." He beams. "You don't want to say, 'Same again', like everyone else. It works like a sonnet. It gets them every time."

On our way out of the pub, a man leaps to his feet with a smile. "Ah, Mr Hitchens! I'm a great fan. Still am! Just not politically." He gestures to the empty seat beside him. I leave, and Hitchens settles down happily beside the stranger, ready for another Xerox.

• Hitch-22: A Memoir by Christopher Hitchens is published by Atlantic Books at £20. To order a copy for £17.99 (including UK mainland p&p), go to theguardian.com/bookshop or call 0330 333 6846. Hitchens will be speaking at the Guardian Hay festival on Sunday 30 May at 8.30pm. For more information and tickets, visit hayfestival.com.