I thought he was going to cry, and more bizarrely I thought I was going to laugh: laugh because I was quite scared and possibly because he was wearing a Burberry baseball cap. That gave the whole episode a faintly comic dimension. There was nothing funny about it, though, really. He was a contract killer and he’d come straight from work to meet me at my boutique hotel in Bogota.

“It was a young kid. He borrowed a gun and he didn’t give it back. Then he went out on a bender. He was a liability,” said the killer, still possessed by the atrocity he’d just perpetrated.

Surely only mad people kill in cold blood, and he wasn’t mad. Heavy, heavy black bags under his slow, stoned eyes, he was the ghoul doppelganger of the man that I’d got to know a few days earlier.

I’ve met a lot of people I’ve liked less. He was good-looking and bright. I’d asked him how many people he’d killed and he said you’d have to be mad to count, but that he was busy.

According to a recent poll, Colombians are the second happiest people on earth. I fell in love with the place in the two weeks that I spent there at the invitation of the president, Alvaro Uribe. The invitation was extended to me because in my recently published autobiography I claimed to have spent £1m on champagne and cocaine. That’s all behind me. I’m a farmer now, and it was as a farmer that I wanted to go there.

My heart beat faster from the moment I arrived. We were bundled through the airport with armed security into a bombproof brick of a car that shot along white lines and hard shoulders with great speed and finesse.

I’ve had armed security in South America before, but these guys were on the edge of their seats, eyes probing mirrors and fingers twitching on triggers.

They were Holman’s guys. Holman’s a journalist and human-rights activist (which often amounts to the same thing in Colombia) and he’d just had a death threat that was being taken seriously by everyone except himself.

“If they’re going to kill you, they don’t warn you,” he said. “They just kill you.”

He told me this when we arrived at the hotel, which was tucked away in what might have passed for a quiet enclave of Madrid. The guards watched the doors while we sipped milkshakes.

Journalists are treated like rock stars in Colombia. I’ve been to South America as both now and I can say with confidence that journalists there receive and deserve more respect.

The big story in Colombia is cocaine. There are many intricacies, back stories and subplots, but cocaine is at the beginning, middle and end of all of it. Eighty per cent of the world’s cocaine is produced in Colombia. It’s an industry worth more than Kellogg’s, Microsoft and Coca-Cola combined. It’s big, big business in a developing economy. I wanted to try to understand exactly what’s happening on both sides of the law.

Early in the morning I flew south in a twin turboprop to a narcotics police air-base in San Jose del Guaviare. It was nice there. Think South Pacific: all technicolour and songbirds. The Colombian rainforest has greater biodiversity than anywhere else on the planet. There are thought to be undiscovered small mammals under the canopy as well as many birds, plants and insects unknown to science. It’s very precious, the proverbial lungs of the earth.

Cocaine is manufactured from the coca leaves that thrive in this climate and I’d come here to observe the destruction of the coca plantations that proliferate throughout the jungle. The vast amounts of cash generated by the drug fund a confusing number of paramilitary organisations and factions throughout the wilderness.

From the air, the beauty of the rainforest is breathtaking. I’ve never seen anything as beautiful: a vast, cosmic Eden of moundy, feminine hills, but it’s a very masculine struggle that’s raging there.

Three spray planes flew low and neat like skaters, dusting the easy-to-spot coca fields with herbicide – glyphosate – as I circled above in a heavily armed Black Hawk helicopter.

The US Treasury currently gives Colombia more than $700m a year in aid to fight cocaine production – more than it gives to any country outside the Middle East and Afghanistan – and 80% of that money goes to the military, to spray herbicide on rainforest.

Last year was a record year for spraying, yet production was up for the third year running. The Colombian government estimates at least 388,000 acres of national parkland is being used for growing coca. There was an obvious flaw in dumping weedkiller on this much rainforest. Avoiding deforestation is at the top of every eco-warrior’s shopping list. And it seems that every time a coca plantation is sprayed, another one is hacked out of the jungle.

It’s more effective to pull the plants up by hand but that’s time-consuming and risky: 10% of the manual eradication work-force died last year. We landed at a coca plantation that had been secured by the army in the Macarena national park, in the heart of rebel country, and was now being pulled up by hand.

The area was heavily mined and we had only a 10-minute safety window. I wondered if they were being overcautious, but the next day one of the policemen in my helicopter was killed on a similar mission at another coca plantation nearby.

I watched farm workers toil to pull up a crop that had probably been planted by their relatives. I was starting to get a feel for what an intractable situation Colombia finds itself in. From Britain, it’s easy to see cocaine as a problem that starts in Colombia. When you’re standing on Colombian soil, it looks a lot more like a problem generated by the vast markets in the West.

Next day I went to see Vice-President Francisco Santos, who was kidnapped by the former cocaine king Pablo Escobar and held for eight months. Others taken with him were killed, but he survived.

A warm and gregarious man, he has recently been to London to promote the idea of shared responsibility, the suggestion that trying to eradicate coca production is fairly fruitless in the face of the worldwide demand and the unsinkable glamour of a bag of charlie among customers to whom cocaine goes hand in hand with yoga and organic vegetables.

The vice-president showed me maps of rainforest with thousands of red dots representing deforestation by coca fields. I asked him why it’s illegal. It’s been legal before and civilisation didn’t collapse.

“I think once the door is opened, you can never close it again,” he said. “No government in the world is seriously considering legalising it anyway.”

Having seen how much the American government is spending here, I personally don’t think America would allow any other government to legalise it.

“If we don’t find a solution,” said the vice-president, “if we can’t work it out together, it doesn’t really matter what happens here – production will move to the African rainforest. The climate there is perfect. They have weak governments and corruptible police.” He’s absolutely right.

I went back to the hotel and met a gangster, a dealer. It was hard to hate him. He wasn’t an obvious jerk. He offered me some of the “national product” and produced a large bag. He never took the stuff himself. As in any other business, people who take coke are seen as a bit of a liability by the suits.

He was a two-dimensional character, happiest, he said, hanging out at casinos with prostitutes. There was nothing else to him. He said he made people happy and he obviously believed it.

“I never offer people cocaine,” he said. “They ask me for it, and if they want it I will sell it to them.”

Think of the smell of coffee and try to imagine what the place that it comes from looks like. Medellin is about the prettiest city I’ve ever seen, surrounded by green mountains in brilliant sunshine. I will always remember it for being the place where I discovered guanabana.

Colombia has many fruits, but guanabana was the nicest one that I found. It’s a cross between vanilla and banana, and when it’s mixed with milk it’s nicer than chocolate. It was hard to believe, as I blissfully gurgled down just one more guanabana shake in the Medellin sunshine, that 10 years ago this was the murder capital of the world.

There are still walk-in hire-a-hitman agencies, but things have calmed down a lot since Escobar was killed. Killing Pablo was a massive operation that took the combined might of the SAS, American special forces and intelligence, the Colombian police, army, government and all the other cocaine cartels put together.

In the last few days of his life, when he was on the run from absolutely everybody, Escobar was looked after by his aunt, Luz Mila. He is such a key figure in the story of cocaine in Colombia, I wanted to know more about him, and since she was the last woman to see him alive, she merited a visit.

All these very grown-up struggles were peppered with the sweet drinks of my childhood. Pablo’s auntie served a pink fizzy pop similar to Tizer and told me what a lovely boy he was really, and that people shouldn’t have made him so angry.

She lived modestly in an upstairs flat with a statue of the Virgin Mary and a couple of photograph albums. She was a nice lady and she clucked away about the good old days. So many of the people in her photograph albums were dead now, murdered.

We went to Pablo’s grave, which was drawing a crowd, and said farewell. I was more confused than when I arrived.

From Medellin, I flew to Cali and then by light aircraft to El Charco, a settlement in Nariño, the biggest coca-producing region in the country.

El Charco was charming, an enchanting place. I sat around slurping sugar cane with Sotero, a farmer, before going to take a look at his crops. His farm was sprayed a month ago and three-quarters of his harvest had been destroyed. A little before that, the entire settlement had been displaced by rebel fighting.

All of Sotero’s food crops had had it from the spraying. But some of his coca had survived and was being harvested; the leaves can be cropped up to six times a year.

When I asked him why he grew coca, he said that no one would buy his bananas, but there was a big market for coca leaves. He started growing coca six years ago. Prior to that he earned $132 a month; now he earns $500 a month. Not exactly a fortune. His house had no plumbing or electricity.

Farmers here process their own base cocaine, cocaine sulphate, known as pasta, to sell on to middle men and illegal jungle factories that turn it into cocaine hydrochloride: coke. We made our way upriver and then swashbuckled through thick jungle to a pasta laboratory.

The going was hard and heavy. This dense tropical forest is the battleground of the civil war that has been raging here since the 1960s: the only war in the Americas. On the left is Farc, the Revolutionary Armed Forces of Colombia, evenly matched and opposed by the ELN, the National Liberation Army. Both groups are being fought by the Colombian army, funded by American money. Then there are paramilitary organisations fighting everybody, and the cash generated by cocaine continues to feed the war machine.

It’s impossible to imagine living and fighting in terrain like this. The spiders were scary enough. After half an hour we arrived at the lab. These makeshift huts known as chongosexist throughout Colombia by the thousand. This particular one belonged to the mother of one of the boat crew.

The extraction of the alkaloid from the leaf is a grotesque and extraordinary process. First the leaves are mixed with cement and ground up. Then petrol is added. This releases the active compound, which is captured in solution by the addition of cold water and sulphuric acid.

At this point, the chef nearly passed out and had to sit down. He discarded the petrol, which left a paste, which is boiled and added to another reactive to make the pasta basica that is sold on to the larger cocaine factories for further processing into the end product. The yield is high. A big sack of leaves makes about 50g of cocaine.

Back in Bogota I met the hitman for the first time. He picked me up from the hotel in a Noddy taxi with a whiff of slapstick about it. The sicarios, as they are known, often use taxis for cover when gathering intelligence.

As I said before, he was no cockroach. He was slick and handsome, in his early thirties. These guys are usually dead by the time they’re 35.

“Sure I’ll die, but I want to leave a legacy for my family,” he said. His story begins in the barrio, the neighbourhood.

The patron picks out the best boys, they work for him and all the work is linked to cocaine. There’s no money in kidnapping any more. His work involves debts, respect and territories. Errant wives and their unfortunate lovers also form quite a big slice of the pie.

“Not one gram of cocaine leaves this country without the right people knowing about it,” he said.

It was all very plausible and straightforward. I started to relax as we sped around in the taxi. He was an excellent driver. Then I asked him if he was carrying a gun, and he’d drawn it before I’d finished saying “gun”, and I remembered who I was talking to.

The saddest thing about the women’s prison, Buen Pastor – Good Shepherd – were the visitors, devastated despite their inculpability, and the children who were locked up with their mothers in the high-security wing that also houses the child-killers. A high percentage of the inmates are there because of drugs, one way or another.

I met an American called Angela, or maybe it was Sarah, and she told me her story, or maybe it was a story about how she'd been caught the first time she’d tried to smuggle cocaine, in a customised suitcase. She said she wouldn’t do it again, but it all seemed a bit hopeless – there’s just too much money involved for it not to be a very addictive gamble. If it’s not her, there are hundreds more like her willing to take the risk. The men’s prison was even darker and more hopeless.

I went straight from there to a Louis XIV-style state room in the presidential palace for an audience with the boss man, President Uribe, whose father was murdered by Farc.

“The Uribator”, as the newspapers dubbed him during my stay, Photoshopped into a kind of Arnie-style super-hero, was an otherworldly presence, monk-like, almost. A man of great dignity, he’s at the wheel of the most difficult to drive economy in the world. He was way above one-line soundbites. He spoke in flowing oratory. “We need to support our farmers,” he said.

“Yes, I spotted that,” I said. I just thought I’d check in with the hitman before we left. I didn’t expect to find him in such a bad way. “I want to get out,” he said. “You know I’m good at taking orders. I’m loyal to my boss. I’m professional. I’ll do a good job.”

He would, too, whatever it was. And he was echoing what so many people involved in this business seemed to be trying to tell me. Nearly everybody would rather be doing something else. Anything else.

I brushed with the human side of what comes across as a cold and distant problem in a far-off country – although wherever cocaine is available, it’s not really very far away at all. The purpose of my visit was not to moralise, merely to observe, but it’s transparent that every single line of cocaine is tainted in blood. If legalising it isn’t an option – and synthesising it about as far off as cold nuclear fusion – then we need to start buying more bananas.

Colombia is a uniquely wonderful place. Any less remarkable nation would have been completely torn apart by the terrible virtue of being the place where the best cocaine comes from. But it has a problem in selling its other wonderful produce, particularly in Europe.

There is a saying there: “A French cow is paid better than a Colombian farmer”, referring to European farm subsidies. Colombia produces the world’s best coffee and chocolate, in my opinion, and I can’t help thinking that if Colombian farmers were given a glimmer of a chance, and more of their amazing produce was available in Europe, they wouldn’t need to grow coca and we’d be well on the road to peace in Colombia. Something they deserve.

© Alex James 2007

A documentary of Alex’s trip, Alex James in Colombia, by Back2back Productions, will be broadcast on BBC1’s Panorama in January

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