Things go better with Coke, as some of us found ourselves singing in 1963. And who among us didn't dream of building the world a home and furnishing it with love? As brand awareness goes, you have to admit Coca-Cola is 'it'. Each day, 1.3bn thirsts are slaked with a Coca-Cola product. They made $4.8bn in profits last year. The company's sloganeers may have been overscrupulous in their efforts to avoid too much creative sturm und drang down the ages (their latest paean to wholesomeness is 'Welcome to the Coke Side of Life!'), but since the dawn of consumerism they have been singularly successful in stringing their bunting across the galaxy - from Aberystwyth to Zanzibar and those out-of-the-way places where reworking that iconic handwritten logo into an alphabet recognisable to speakers of Gurmukhi or Inuktitut is all part of teaching the world to sing from the same spreadsheet.

But this is no time to be thinking in Gurmukhi. Coca-Cola has a new product coming out in Britain. Like Diet Coke, Coke Zero contains no sugar, but unlike Diet Coke it tastes like the real thing - that is, like ordinary Coke. At least that's the verdict in the US and Australia, where Coke Zero is already up and running and doing good business. The global Coca-Cola Company - which even when nothing much is happening manages to spend $2bn a year on marketing - is pushing the boat out. This is the most important British launch since Diet Coke 22 years ago. It's a big deal, with noughts on. Everyone is hugely excited.

Will Coke Zero make as big a splash as the launch in 2004 of the company's ill-starred bottled water Dasani, which, it was belatedly discovered, came straight out of the tap and contained prohibited levels of bromate? Or the festival of schadenfreude surrounding the wretched failure of re-flavoured New Coke in 1985, when 40,000 letters of complaint a week poured into Coca-Cola HQ, and big-screen commercials were openly booed at baseball matches? The trouble with the kind of media coverage money can't buy is the difficulty of turning it off when you've had enough.

The World of Coca-Cola museum in Atlanta, Georgia - birthplace of Coca-Cola and home to its current headquarters - is a monument to things going better. Affable archives director Phil Mooney is showing me round. He says people come here expecting to see antiquated cogs and pulleys and assembly lines, but there's none of that. The showcases around us are devoted largely to advertising artwork, designs, old correspondence, sponsorship paraphernalia and merchandising items that eBay would call 'collectables'.

What most people don't realise is that for all the vast industrial panoply of Coke's operation - the busy factories, the Niagara of product, the billions of bottles and cans and crates and multipacks, the fleets of liveried trucks and vans, the display coolers and dispensing taps and vending machines and paper cups - the Coca-Cola Company itself manufactures nothing but syrup. Everything else, Phil explains, is down to Coke's franchise 'partners' worldwide - the bottling plants.

Whoever decided to cut in the middleman during the early 1900s did so for sound financial reasons - they didn't have the money to build factories. Today, that decision looks like a licence to print money. But how can Coca-Cola guarantee that its 'partners' are not watering down the syrup too much? Or that they're not going to print the logo upside down?

Phil laughs. 'We're giving the world's most valuable trademark to a partner and saying, "We're relying on you to deliver." No one wants to lose that.'

We wander around the exhibits. Phil tells me about John Pemberton, the small-time pharmacist who invented Coca-Cola in 1886, selling the cocaine-laced beverage as a remedy for depression, hysteria and anxiety. In his first year, Pemberton spent $78 on advertising and earned $50. It was his successors who finished the job. Phil points to the face of a coquettish young woman from the silent era, looking out from a tea tray, a postcard, a book of sheet music. 'See how they used one image across a range of things just to get the message out there.' Here now are the Coke calendars, the Coke barometers, lampshades and tea sets. I've never seen so many smiles under one roof. Even during the war years, soldiers, sailors and factory girls were chasing away the blues with Coke. 'He's Coming Home Tomorrow' says one image - a GI's apple-cheeked wife fetching a six-pack from the store. There are laughing servicemen at a soda fountain; a Norman Rockwell of a southern gent with girl, dog and Coke; a beautiful nurse carrying bottles of Coke on a tray. 'It's What They Want' is the tagline. It might easily have been: 'Hey, Who Needs Morphine?'

Phil is telling me about the early bottlers - hundreds of horse-and-buggy operations - then about the need to patent the famous 'contour' bottle, and the secret formula for Coke, which is locked in a bank vault a block from here.

The point is, I realise, we're only seeing the marketing side of things because that's the only side there is. The history of Coca-Cola is the history of marketing. It's also the history of the American century, from soda fountains to the Atlanta Olympics and beyond. A photograph from 1965 shows a line of Coke trucks crossing the Bosporus like liberating troops. A Martian browsing here would marvel at Coca-Cola's role in space flight. He would surmise that, during the Sixties, black people were discovered living in America and enjoying it immensely. Certainly he would assume that Coca-Cola was a force for good in the world.

Despite the glut of beverages here, I need a drink. The World Cup has just started. Out in the heat of the day, I am directed to Hooters, a vulgar but welcoming place where I order a cold beer and watch the second half of Germany versus Costa Rica. Coca-Cola has a big piece of the World Cup, of course. But then the measure of a brand's success is precisely this sort of ubiquity. Coca-Cola, the world's most recognised brand, has long reached a point of saturation at which its signature in public spaces is omnipresent and yet as invisible as telegraph poles or people wearing shoes. Like Costa Rica's defence, Coca-Cola is all over the place.

The domination of high streets by multinational interests is viewed unsympathetically, and not just by people who throw dustbins through the windows of McDonald's restaurants. A 2006 poll of British consumers found that Coca-Cola was considered the sixth most unethical big brand. The 'Boycott Coca-Cola' campaign was prominent at this year's Winter Olympics in Italy, undermining the company's $66m sponsorship. We are encouraged to see global commerce as imperialism by other means, though in fact it is nothing new - at least not for Coca-Cola. Writing in the New Yorker magazine as long ago as 1959, EJ Kahn noted that overseas critics of the brand's growth had come up with the term 'Coca-Colonialism'. He quotes a vice president of the company, who wrote in a memo to an associate: 'Apparently some of our friends overseas have difficulty distinguishing between the United States and Coca-Cola. Perhaps we should not complain too much about this.'

Coca-Cola's world tour has continued. Indeed, elsewhere seems the only place to go. There's a dwindling market for fizzy drinks in the developed world (in the UK, sales are down more than 5 per cent). With 85 per cent of Coca-Cola's profits made in the fizzy sector (the other 15 is from juice drinks and water), developing countries in particular are openly targeted, raising the hackles of critics who question the morality of creating a demand for fizzy drinks in poor countries.

Globalisation is one thing, but Coca-Cola has not been helped by concerns over obesity - tarred and feathered alongside McDonald's in Eric Schlosser's bestselling 2001 Fast Food Nation and Morgan Spurlock's 2004 movie Super Size Me, and buffeted by perennial health scares over aspartame, the sweetener used in Diet Coke. Then there's the ongoing story about the Indian farmers who complained that the local Coca-Cola plant was exhausting water supplies. Last year, the Today programme ran a story about a Mexican shopkeeper who was suing Coke because the local bully-boy distributor wouldn't let her keep a rival drink in her promotional Coca-Cola fridge.

The worst horror, though, involved the murders of union officials at a bottling plant in Colombia - allegedly with Coca-Cola's connivance. Successive legal inquiries have cleared Coca-Cola and its associates of all involvement in the latter - though not before US college students started a website (www.killercoke.org) outlining the company's perfidy, detailing operations in Colombia and India in particular. Within weeks, college campuses nationwide were boycotting the company's products. Coca-Cola is the sort of company with rebuttal procedures in place, and it issues a report each year detailing its good work in labour relations, environmental issues and community initiatives. But mud sticks. More people, for example, will have read headlines about the Italian study last year suggesting links between aspartame and cancer in rats than the European Food Safety Authority's (EFSA) response to it last month, concluding that, on the contrary, there is in fact no link between aspartame and cancer in rats.

But all is cheer when I meet Dr Kelly Sepcic, who heads the flavour research team at Coke HQ. She talks of 'creative tasting' and 'improved mouthfeel'. She won't be drawn on the worst-ever drink she has invented in her 12 years here, but she is passionate about Coke Zero. 'It's as close to regular Coke as we could get,' she says.

We crack open some cans. It does taste like Coke, though I'm no judge. Maybe this was what they were after when they invented Diet Coke, I suggest.

'Maybe,' she says.

So why not launch it as New Diet Coke?

Am I kidding? People love Diet Coke. Well, not everyone. Which is why this one is aimed at men. The marketing teams are already calling it Bloke Coke. It's for younger guys who are starting to get calorie conscious but refuse to drink Diet Coke in case their mates think they're a wuss.

'I drink Diet Coke,' I say.

In the long pause that follows I ask about aspartame. Weren't they rattled by the recent Italian study? Weren't they relieved to get the all-clear from the EFSA? Dr Sepcic shrugs. 'We have total confidence in aspartame. It's had more studies carried out on it than any ingredient in the food-supply chain. Everything we use has been tested for consumption in 200 countries.'

And yet it keeps coming back, I say.

'Coca-Cola sells two things. Products and newspapers. We're an easy target. A big story.'

Later we meet Marc Mathieu, Coke's vice president in charge of global brands. 'You see, we are really expanding our offering to cover the totality of the need states,' he says. 'Some people drink to complement their food, some to cover wellness concerns they may have. The question is: how do you complement the variety of need states under the one trademark - Coke?'

I've no idea. I'm still wondering what a 'need state' might be. Marc is ardent about the 'footprint' of the brand, comparing it with, say, BMW, which can fulfil all of our needs under one marque - the sedan, the SUV, the sports model, a family station wagon, something for the office, bigger or smaller engines, male, female. 'They serve all the needs - enable people to keep their loyalty to a brand. That's what we're trying to do with Coke.'

I suggest that maybe this flowery, bright optimism is just slightly out of kilter with the times. And the global thing - has the image of Coca-Cola not suffered from flying the flag too high for America?

'Whether we like it or not, we are the leader and we have a point of view. Coke has become a symbol of America because it has embodied a lot of the American dream. But when people buy a Coke, they are not buying just a fizzy drink, they're buying into a vision of world citizenship. Coke stands for much more than a refreshing beverage. It stands for an ideal: people feel Coke is part of their lives. It belongs to them. It accompanies them in their moments of joy, their moments of pleasure, their moments of connection with other people.'

I nod. We agree that Marc should maybe show me some amusing ads on his laptop.

Back on the ground a week later, in Hammersmith, London, the British Coca-Cola team is counting down to what they are calling Zero Hour. The launch starts after the World Cup, but the Spar in Colchester has already sold the first can in Britain. 'Last Sunday, five minutes after midnight,' says marketing director, Cathryn Sleight.

'A bit like Harry Potter,' I say.

'Exactly!'

It's been frantic but totally exhilarating. And a privilege, Cathryn says. The response has been amazing - from the taste panellists, from the sales people, from Australia. There's such belief. Some retailers have been sending their own lorries out to collect stock from the factory! 'It's unknown,' Cathryn says. 'Coke Zero could be as big as Diet Coke in 10 years' time.'

'Is that your target?' I ask.

She has a serious pause. 'I'd say it was a vision. Here, we talk about the completion of the trilogy - Coke, Diet Coke, Zero.'

So no plans for a fourth? She shakes her head. 'I don't think so.'

That's what they said about Star Wars, I say.

Cathryn moves on to 'need states', which I now dimly understand to mean that people are more likely to be in the mood for a cup of tea with their mid-morning flapjack rather than, say, a bottle of Jack Daniel's. Needless to say, it's not quite as simple as that. Coca-Cola's research army has identified a whole list of 'need states', which include 'comfort and relaxation', 'action replenishment', 'food enhancement', 'mental renewal'...

Are people ever just thirsty, I ask. Yes, she says. Anyway, they've had 3,600 people keeping drink diaries, and have come up with 40,000 different 'occasions' when only this drink or that will do. 'It's a great way of connecting with our consumers,' she says.

Hilary Langan, marketing manager for Diet Coke, is talking now about the cast of thousands behind the launch - the 'flavour people', R&D, the 'ground teams', outside agencies for PR, ads and graphics. She explains how Coke Zero's black tins and bottles which feature the silvery 'urban icons' of dice and girls' legs will appeal to their profile male - ie, someone of about 25 with the world at his feet, no mortgage, first job, who is just becoming health conscious. 'These are young guys who are maybe reducing their consumption of Coke, but not connecting with Diet Coke.

For them, Coke Zero is very male, no sugar - but with the great taste of Coke!'

They call their profile male Dan. The TV ad for British Dans is still being shot, but we sit and watch the Australian one, which features a man shouting on a bus. Blimey. Can he really just be talking about a fizzy drink?

'Yes, that's a bit too Australian for us,' Hilary says, but the core message will be the same with ours - 'Great things in life made a little bit better'. Then there'll be huge outdoor ads and lots of presence online, as well as Swat teams hitting the streets, jumping out of vans, giving stuff away and targeting sports events, big music shows, parks. No one is safe. 'In four weeks we'll have sampled four million consumers,' Hilary says. 'It will be unmissable.'

'So you're not just going up against Pepsi Max?' I say.

There's a blank look here. Pepsi who? No, says Cathryn. 'It's about breadth of choice and customer opportunities and need states. It's about anticipating consumer needs.'

But hasn't Pepsi already anticipated that one? They've already got two diet colas. No, no, Cathryn insists. It's much broader than that. Cola wars are something drummed up by the media. By now I am suffering from enthusiasm fatigue.

But a few days later, at Liverpool Street station, I find myself approaching a team of young people handing out mini-cans of... yes, Pepsi Max! I notice no one leaps out to offer me one. Am I a victim of age profiling? Or are they just a figment of my imagination? Sometimes you just don't know what to believe.