I wasn't sure what, or perhaps whom, to expect as the door opened at Christopher Hitchens's top-floor apartment in downtown Washington. The last time I had interviewed the renowned polemicist, author, literary critic and new resident in the medical state he's called "Tumortown" was in 2005. On that occasion, after a 5am finish to our extravagantly lubricated conversation, it was I who had felt the pressing need of hospital attention.

Since then there have been two dramatic changes in his circumstances. The first was the international bestselling success of his 2007 anti-theist tome God is Not Great. After decades of acclaimed but essentially confined labour, Hitchens suddenly broke out to a mass audience, becoming arguably the global figurehead of the so-called New Atheists. Almost overnight he was upgraded from intellectual notoriety, as an outspoken supporter of the invasion of Iraq, to the business end of mainstream fame. In America, in particular, he has reached that rare position for a journalist of becoming a news story himself.

Unfortunately the news, which provided the second personal transformation, was that in June he was diagnosed with cancer of the oesophagus, a malignancy whose survival ratings do not make soothing bedtime reading. As restraint is a quality for which neither Hitchens nor his critics are known, the ironies proved irresistible to many commentators. For the religiously zealous, the arch atheist suffering a mortal illness spoke of divine retribution – the unacknowledged irony being that belief in such a vindictive god served only to endorse Hitchens's thesis.

For more secular moralists, a different kind of cosmic accountancy was at work. The celebrated drinker and smoker who once claimed that "booze and fags are happiness" had succumbed to a cancer most often associated with drinking and smoking. Having previously gone so far as to promote the benefits of teenage smoking, he offered a public recantation of sorts. "I might as well say to anyone watching," he announced in a TV interview, "if you can hold it down on the smokes and the cocktails you may be well advised to do so."

Hitchens had already impregnated the story with preemptive meaning in his prologue to his recent memoir, Hitch-22, in which he meditated on the unpredictable incursion of death. One motivation to undertake the book, he confessed, was the need to do so before it was "too 'late'". As he wrote those words, he had no knowledge of the tumour growing in his oesophagus which has metastasised in his lymph nodes and lung. It was not until he was on a promotional tour for the book that he fell ill and was diagnosed.

There followed the minor spectacle of prayer groups invoking the unbeliever in their spiritual communications and even, in September, the informal designation of an "Everybody pray for Hitchens day". While avoiding any direct involvement, the writer professed himself touched by the attention. But for such an astute connoisseur of the form, these ironies were no doubt just a little too resounding for Hitchens to appreciate.

On top of which there was the distant thud of literary biography. One of Hitchens's great heroes is George Orwell, who no sooner produced his signature achievements (Animal Farm, Nineteen Eighty-Four) than was confronted with the prospect of his premature demise. Orwell cut a frail, bed-ridden figure towards the end of his short life, a fate that seemed to confirm his ascetic form of courage.

All very well for the man who nursed his TB alone in the damp reclusion of Jura, but infirmity was never going to be an image that sat well with Hitchens, a walking definition of the cosmopolitan bon viveur. The early post-chemo photographs and appearances were not encouraging. Without his trademark foppish fringe, he seemed to have undergone an Samsonian reduction. The few wisps of hair remaining mocked the mischievously boyish countenance that had duly withdrawn behind gloomy eyes. He looked old and battered. He looked like a 61-year-old man with stage-four cancer.

He'd told me by email that he had good and bad days. As I'd pushed him for the interview, it was with a certain trepidation that I stood clutching an effete bottle of white wine – I assumed that his usual tipple of whisky had been removed from the menu. At first, when he greeted me, I wasn't entirely convinced it was one of his better days. Although he looked fitter and sharper than he had in those earlier images, having lost a little weight and with his head now more flatteringly bald, the apartment was in darkness and he invited me in to watch the sunset. It all seemed rather forlorn.

If it triggered his cancer, burning the candle at both ends, as he recently remarked, also produced a "lovely light". The golden twilight over the American capital possessed its own illuminating charm, not least in the way that it seemed to recharge Hitchens. So it was that for a considerable part of the next 24 hours he held forth with unhesitating eloquence on a wealth of subjects, including the Iraq war and its likely aftermath, the global jihadist threat, his love of debate, romance, his illness and, of course, the persistently intrusive claims of religion.

When he initially became ill, Hitchens thought that he was suffering from exhaustion, brought on by overwork and age. "I knew something was wrong," he says. "I kept saying to myself, when the book tour is over I really will go and see a doctor."

He collapsed first on the New York leg of his tour, and had fluid drained from around his heart. Then an oncologist performed a biopsy. Most authors would have called off the rest of the dates at this juncture. But Hitchens went on to Florida, Chicago and Philadelphia, before collapsing again, this time at Boston airport, a venue he recommends for a speedy medical response.

The surgeons took a lump of tumour for analysis and opened what's known as a "window" in the pericardium to allow fluid drainage. Since when he's been undergoing chemotherapy at three-week intervals. The tumour has shrunk but not sufficiently for radiotherapy, and he was about to start a new course of a different cocktail of drugs. At the same time he's been having his genome sequenced and that of the tumour – though, since this sci-fi approach to cancer treatment is still in its infancy, it is unlikely to offer any help to Hitchens.

"The worst days," he says, "are when you feel foggy in the head – chemo-brain they call it. It's awful because you feel boring. As well as bored. And stupid. And resigned. You don't have any motive, which is bad. You don't care what's going to happen to you. That lasts sometimes two days. And when that comes with nausea – even if you have eaten, you have to go and be sick – it's very upsetting."

But there are also some more favourable signs. For instance, he hasn't yet had any trouble swallowing, a problem frequently associated with oesophageal cancer. And his immune system remains strong: "I haven't picked up a sneeze or anything like that."

Which is just as well, because Hitchens was never going to be a natural candidate for a prune-juice and brown-rice lifestyle. He was grateful for my white wine, but only because that's what I drink and he was low on stocks. For himself he poured a generous measure of Johnnie Walker. None of his doctors, he says, has issued any kind of fatwa on booze, though you sense he probably hasn't demanded a ruling.

Whatever his health regime, it doesn't appear to have affected his work levels. The pugnacious weekly Slate column, the far-reaching literary essays for the Atlantic Monthly, and the Vanity Fair dispatches are all filed on time. And he continues to participate in public debates. Last month in New York he sliced through the Islamic scholar Tariq Ramadan's problematising evasions with stinging precision, and later this month he's taking on Tony Blair in Toronto.

He disputes his industriousness on the grounds that he's not currently writing a book. "I spend a lot of time asleep or lying down." Yet the spacious apartment he shares with his wife, the writer Carol Blue, and their 18-year-old daughter, Antonia, shows few signs of recuperative comforts. With its bare floorboards, sparse furniture, baby grand piano and teetering stalagmites of books, it resembles something a Hollywood set designer might have come up with had she been asked to create an intellectual's lair.

He's said before that his life is his writing, which includes priority over his family, but perhaps it would be more accurate to say that his life is an argument in which writing takes the lead role. It's notable that there's scarcely any mention of his wife or his children or former lovers in his memoir. He stated in the book that this was out of respect for others' privacy. If he had named one woman, he adds, he would have have had to go into detail about them all. "Couldn't do it selectively, it would lead to nothing but pain."

One of the women he didn't name was Anna Wintour, the legendarily glacial editor of American Vogue. They had a year-long affair in the mid-70s.

"It was transatlantic, very stormy, tempestuous, passionate," he says, "all of this enhanced by having to fly the Atlantic the whole time. Otherwise it might not have gone on for so long because really, although she's amazing, we didn't have much in common."

Other than fashion, I joke. Among his many struggles, the one waged against the tyranny of the pressed and laundered outfit should not be overlooked. Still, the absence of women in the foreground of his memoir leaves the impression of a life consciously determined by the intellect, rather than haphazardly shaped by emotions. It's an effect that's underlined in his writing by a Zelig-like presence at the great moments of history, as though they are the dots that join together to form an inexorable continuity.

He shakes his head. "Even with all the advantages of retrospect, and a lot of witnesses dead and gone, you can't make your life look as if you intended it or you were consistent. All you can show is how you dealt with various hands."

In America it's been suggested by some religious types that his condition could prompt a revision of his atheism. It's not a hypothesis to which he grants much respect.

"So now I know that there's another life in my body that can't outlive me but can kill me, it's the perfect moment to gratefully acknowledge that I'm a product of a cosmic design? Who thinks up these arguments? Actually it's an insulting question: 'I hear you're dying. Well wouldn't it be a good time to get rid of your beliefs?' Try it on them and see how they would like it. 'Christian, right? Cancer of the tits?' 'Well, yes, since you ask.' 'Well, can I suggest you now drop all that tripe?'"

In Britain, I say, the notion that he will undergo some kind of deathbed conversion has minimal traction. What you find more often here is the accusation that the New Atheism, as expounded by Hitchens and his fellow bestsellers, Richard Dawkins, Sam Harris and Daniel Dennett, is a "militant" or even "fundamentalist" attack on the numinous and the unknowable.

Hitchens once wrote a line that has almost gained the status of philosophical epigram or even scientific dictum: "What can be asserted without evidence can be dismissed without evidence." Although it echoes Wittgenstein's famous injunction regarding the ineffable – "Whereof we cannot speak, thereof we must be silent" – Hitchens's version is less a "no entry" sign than a civic reminder to place rubbish in the bin.

In fact, you could say that in God is Not Great Hitchens ignored his own advice by conducting lengthy theological and historical research to assemble his case. His beef, in any event, is not really against faith itself, but against the way that all faiths are compelled to make irrational demands on believers and non-believers alike.

Hitchens dislikes the "New Atheist" title. "It isn't really new," he says, "except it coincides with huge advances made in the natural sciences. And there's been an unusually violent challenge to pluralist values by the supporters of at least one monotheism apologised for quite often by the sympathisers of others. Then they say we're fundamentalists. A stupid idea like that is hard to kill because any moron can learn it in 10 seconds and repeat it as if for the first time. But since there isn't a single position that any of us holds on anything that depends upon an assertion that can't be challenged, I guess that will die out or they'll get bored of it."

As for the notion that his brand of atheism is reductive or joyless, it's religion, he contests, that is "cosmically hopeless, as is all the related masochism that goes with it – you've got to spend your entire life making up for the vermin you are. What is that if not degrading? We don't do that to people. We say you may as well know you're a primate, but take heart, primates are capable of great things."

Nonetheless, Hitchens mentions a "narrow but quite deep difference" between himself and Dawkins. Unlike the evangelical biologist, he has no wish to convert everyone in the world to his point of view, even if it were possible. In other words, he savours the counterargument. Like John Stuart Mill, he is aware of the empty end of achieved objectives. The true satisfaction lies in the means. Although Hitchens is often seen as a provocateur or a contrarian, and both are indeed aspects of his character, at heart he's incurably in love with the dialectic.

He cut his teeth on dialectical materialism as a teenage Trotskyist, and it was the analytical method that eventually put paid to any allegiance, as it were, to the political madness. The past 40 years have amounted to a long and serpentine political journey. As he relates in his memoir, it started out at Oxford with his keeping "two sets of book", one for the puritanical group of revolutionary socialists with whom he campaigned against the Vietnam war, and the other for the conservative socialites with whom he caroused at black-tie balls. And it reached its furthest distance from origin with his support for George W Bush and the second Iraq war.

Along the way, he says, "I learned that very often the most intolerant and narrow-minded people are the ones who congratulate themselves on their tolerance and open-mindedness. Amazing. My conservative friends look at me and say, 'Welcome to the club. What took you so long?' Well that's what it took and I think it's worth recording."

The hinge events, of course, were the 9/11 attacks on New York and Washington. He had previously held positions that were unpopular on the left – preferring the British government to the Argentinian fascist junta during the Falklands conflict, and calling for American intervention to stop the ethnic cleansing in Bosnia – but his support for the toppling of the Taliban in Afghanistan proved to be a step too far for his anti-imperialist comrades.

Hitchens genuinely believes radical or jihadist Islam to be an existential threat to civilisation. First because it is a pronounced enemy of free speech and social liberty and has succeeded in intimidating and silencing civilians across "an extraordinary number of countries in Europe" and the rest of the world. And second, he says, "because it has potential access to weapons of mass destruction." In the end, he argues, there are no pain-free options. You have to choose which future regret you're going to have.

"I was at a Hezbollah rally in Beirut about two and a half years ago," he says. "Very striking. Everyone should go. But of the many things that impressed me about it, having the mushroom cloud as the party flag in an election campaign was the main one. You wouldn't want to look back and think, I wish I'd noticed that being run up. Now I can give you all the reasons that it's bombast on their part. Still, I know which regret I'd rather have."

There appear to be two main criticisms of this stance. Either people think he's a bonkers Islamophobe – though many who do were content enough to leave Muslims to their bloody fate in Bosnia – or they believe such antagonistic talk only serves to create the problem it seeks to prevent. Hitchens is contemptuous of the former, but scathing of the latter. He says that those who tell him to tread more softly believe that the price of not doing so is more violence. "Oh I see, so you're always aware when you're contesting the holders of this view of the threat that lies behind it? Would you care for their opinions if it wasn't for that? Or are you telling me you'd be reading their stuff just for the sheer pleasure of it. I don't think so. If you say that this looks like war, you're accused of liking it. Not true. Demonstrably not true."

Demonstrably? Certainly he can sound like he enjoys the conflict. He has said that he experienced "a feeling of exhilaration" while watching the World Trade Centre collapse on 11 September. "Here we are then," he later recalled thinking, "in a war to the finish between everything I love and everything I hate. Fine. We will win and they will lose."

He says the exhilaration was born of a sudden if overdue sense of clarity.

"What I felt is that we'd been suffering from all this for some time. And yet people's main interest seemed to be in ignoring it or denying it, or if they were politicians or soldiers, running away from it: abandoning Somalia, leaving Afghanistan to rot, trying to subsume Islamism into multiculturalism. I thought: until yesterday, they knew they were at war, and we didn't. And now we do: of course that's exhilarating. It was the feeling that the somnambulance was over. Of course it turned out to be a very brief wake-up call, followed by a very long nap: 'Turned over in bed briefly. It's 8.59? No, it can't possibly be!'"

If 9/11 was the decree nisi, then the divorce from the left was made absolute by the war in Iraq. He had been a persuasive opponent of the first Gulf war in 1991, arguing that Bush senior was not liberating Kuwait but simply restoring the status quo. Slowly he changed his mind, and he describes the process:

"I said that Bush [senior] may have used the rhetoric of anti-fascism but he didn't mean it. And then I said, yeah, but what if he had meant it? Would I therefore be obliged by my own argument to be in favour? The answer was 'yes'. And then I said, well what do you care how they argue? You should be arguing it yourself. And I found I couldn't get out of that."

It's an instructive piece of reasoning. For while it suggests he remained true to his dialectical word by examining the counterargument, it's also a kind of a first-principles assessment of principles. As such it could be applied to countless other situations. For example, should the west have intervened in Cambodia when stories first began to emerge of the genocide taking place there under the Khmer Rouge? "I don't think there was a policy," he says, a little lamely, "so I can't say that there should have been one. I don't think it's a real question."

His only major regret, as far as remaining silent goes, is what he didn't say about Robert Mugabe. "That makes me wince. More than wince. I'd met him a couple of times and I knew that he had in him a terrible capacity for fanaticism, absolutism, and I didn't say as much about that as I could have done. If I asked myself about why I didn't, I'm sure the answer is because I didn't want to give ammunition to the other side."

As an aside on Mugabe, he makes one of those observations that are so precisely to the point that you wonder why so few other commentators ever get round to coming to it. "Darfur, Zimbabwe, Burma, North Korea, anywhere that the concept of human rights doesn't exist, it's always the Chinese at backstop. And always for reasons that you could write down in three words: blood for oil."

But the problem with Hitchens's inductive reasoning about Iraq is that it didn't and couldn't take into account future outcomes – namely the relative likelihood of death and destruction. In the light of what unfolded in the wake of the occupation, these were the matters that I wanted to put to him the following day.

First, though, there was dinner. We walked to a local restaurant where Hitchens knows the barman and the barman knows what Hitchens drinks, and I asked whether his cancer diagnosis had altered his political outlook at all. He looked mystified at the question, but I explained that he used to say that he woke up angry, full of disgust at the world. Was it still possible to feel so strongly about external enemies when the internal one had taken such malevolent root in his body?

"It's the sort of alternative that doesn't present itself to you," he says. "You don't think, 'Why do I care when I could be thinking about my daunting nemesis?'"

The banality of cancer seems to irk him almost as much as its lethality. Lacking any dialectical substance, it affords few opportunities to escape platitude or avoid cliche. It's a big subject, but it's essentially small talk, and Hitchens's style requires the elevated registers of the epic and the ironic. Anything less is like asking a high-wire artist to perform his act at ground level.

Yet his engagement remains unusually engaging, in large part because with him it's never just about politics. His frame of cultural interests is far too large to be squeezed into the straitjacket of dogma and doctrine. He chided me a couple of times for not asking him about his first love, literature. "I wish people would put in a bit more of that because it's also what I think of when I say grand things like defending civilisation."

It's no coincidence that the political thinkers he most often references are also gifted writers: Orwell, Arthur Koestler, Thomas Paine, even Thomas Jefferson. The first two, like Hitchens, committed anti-totalitarians, and the second pair voices from the only revolution – the American – whose praises he continues to sing.

Hitchens's friend Ian Buruma wrote a damning review of Hitch-22, in which he noted that Hitchens displays "a tendency toward adulation and loathing [that] comes naturally with the weakness for great causes."

Was that fair?

"I think that probably is true of me, that I can be 110% for or against people," he says, pausing meaningfully. "I don't know whether that's meant as a criticism or not."

We repair back to the apartment for a nightcap or two, and I fear it is I, the ostensibly well one, who crashes first. The spare room had only recently been vacated by Patrick Cockburn, the distinguished foreign correspondent, old friend of Hitchens, and savage critic of the Iraqi occupation. Clearly the breach with the "other side" is not quite so decisive up close and personal as it might sometimes appear from afar.

The following morning Hitchens rises late, as is his routine nowadays, and after working for an hour or two, reconvenes our discussion over lunch. We sit in the dining room with the window open on a distinctly chilly autumn afternoon. He's wearing just a thin shirt, while I shiver in a thick pullover. Not for the first time, I feel a twinge of pity for that tumour. Does it realise what it's up against?

A few days before we met, Tariq Aziz, Saddam's former deputy, was sentenced to death in Iraq for suppressing Shia religious parties during the Ba'ath rule. This surely wasn't the bright future for Iraq that the trenchant secularist and opponent of capital punishment had in mind back in 2003. He agrees that it's depressing news. Nor is he confident that things won't deteriorate after the coalition has departed.

"The best one can do is say that we've given the Iraqis the chance to produce a constitution, independent courts and a free press. They can keep it if they want but the parties of God may veto that. Unless we're directly requested by a functioning government backed by a functioning parliamentary vote to stay on, we have to leave it to them now."

So if the parties of God gain control, Iran's influence increases and human rights further decline, will it still all have been worth the loss of life and limb?

"Oh yes, I definitely think so," he replies without vacillation, "and not just for the humanitarian reasons. There are all kinds of reasons that don't get discussed and are harder to quantify."

He points out something he says opponents of the war always fail to mention: the success of an autonomous Kurdistan. But he also says that the discovery of oil around Baghdad has transformed the material basis for political control. "There's a petro-chemical reason for federalism now. If the oil laws were enforced properly by province, it could be as rich as Kuwait. The Saudis and the Iranians don't want a revived Iraqi oil industry because it will undercut them. You could have a modern Middle Eastern country or a parties-of-God failed state."

The point, I say, is that you can't make countries take the right path.

"No, again, since one is always going to regret something, you have to decide in advance what it will be. OK, I'm glad we're not having an inquest now, as we would be, into why we allowed a Rwanda or a Congo to develop on the Gulf, an imploding Iraq right in front of our eyes, a vortex of violence and meltdown, a whole society beggared and fractured and traumatised, waiting to fall to pieces."

The problem with this picture, of course, is that many people believe that it exactly describes what has taken place since the invasion. Hitchens maintains that the situation is better than it would otherwise have been, and to the extent that it's worse, the responsibility lies with al-Qaida.

"Trying to destroy the Christian community, strategy of tension, trying to start a civil war," he says, listing al-Qaida's nihilistic programme, "and people act as if those casualties should be on my conscience. I won't have that. For one thing, it absolves those who have done it of their guilt."

I try to argue that it overstates the case to suggest, as some of his more deranged critics claim, that he is somehow personally responsible for the tens of thousands of lost lives in Iraq. After all, had he not existed, would history have taken a different path?

To his credit, he refuses to accept that get-out. As he does in his memoir, he restates his role, along with people such as Peter Galbraith and Kenan Makiya (he insists Ahmed Chalabi's part has been "ridiculously exaggerated") in helping to persuade Washington of the need for regime change. But he also says that, even if he had negligible or no effect, "you should act as if your opinion might have made the difference. You don't have to be a megalomaniac to do that. And to say that you feel to that extent responsible, without making a parade of your feelings."

What's beyond doubt is that Hitchens's sense of optimism and purpose in 2002 in Washington was never going to be matched by the post-invasion plan in Iraq in 2003. He has no quarrel with the fact that the occupation was badly handled. "As Peter [Galbraith] said, you never get a second chance to make a good first impression. Tragic. But I mean tragic."

He leaves the room briefly to deal with a domestic issue and I take the opportunity to close the window. When he returns, he opens it with a knowing smile.

"I'll bring you an overcoat," he quips.

We continue talking for another couple of hours on everything from the Russian revolution to the Bay of Pigs, from the Spanish civil war to Tony Blair. In the end, it's only my need to catch a plane that brings the discussion to a close. With Hitchens, though, the argument will continue, first with himself and then, if need be, with the world. His intemperate style is not to everyone's tastes, but as he has often remarked, you can't produce light without heat. To those of us who admire his clarity of thought, if not always his conclusions, it is indeed a lovely light. And I'm pleased to say that on a cold November day in Tumortown, it showed little sign of fading.