The Wounaan Indians are used to newcomers. Over the past 500 years the Spanish conquistadors, freed African slaves and Marxist guerrillas have all tried to encroach on their ancestral land. But the Wounaan, one of the oldest indigenous peoples in the region, have held firm. And then came the coca leaf.

Living in small villages of wooden houses on stilts in riverside clearings in the jungles of Chocó, the Wounaan had largely succeeded in keeping out of Colombia’s bloody conflicts. They hunted, fished and farmed along the banks of the San Juan River, left alone by the battling rebels, paramilitaries and government troops.

Then, three years ago, the guerrillas came with an order at gunpoint. “They said we had to grow coca or else leave,” Fernando, a Wounaan leader, said. “And so we began planting it on our lands.”

The arrival of coca cultivation set in motion a chain of events that has left the Wounaan divided, displaced and at risk of extinction.

With cocaine increasingly the drug of choice in Europe – 1.2 million Britons are regular users – the plight of the Wounaan, and of Colombia’s three million other displaced, are prompting a major rethink in how to police the world’s most important drugs war.

It is more than four decades since the Revolutionary Armed Forces of Colombia, or Farc, began their armed struggle to install a Marxist regime. In the 1990s, with added pressure from right-wing paramilitaries, the guerrillas became increasingly dependent on cocaine to finance their campaign, to the point where it has superseded the Utopian vision.

Since 2000 the US – where more than 75 per cent of Colombia’s cocaine exports end up – has spent $700 million (£350 million) a year to help to root out drug traffickers and eliminate coca crops. Plan Colombia, which began as a mixture of social programmes, crop eradication and military action, quickly lost sight of its social elements.

Chocó, an extravagantly fertile eco-region along Colombia’s Pacific Coast, was never traditionally an area for coca cultivation. Although coca has grown wild in Colombia since time immemorial, and was chewed by indigenous people as a mild stimulant, like coffee, it did not naturally take root in Chocó.

As spraying began in provinces such as Caquetá and Putu-mayo to the east and south, the rebels who supervised the growing of the coca crops started moving into areas farther afield, like the rainforests of Chocó.

Under threat of death or expulsion, the Wounaan began planting coca, which the guerrillas then bought from them. The crop was lucrative: a three inch cube of pressed coca would fetch them 1.8 million pesos ($900). The rice they had grown before fetched only 4,000 pesos a small sack.

Not everyone liked what else the coca brought. “At the beginning there was money,” Fernando said. “But then it started to destroy the culture. Drinking and prostitution, all these things begin when the money comes.”

In March last year leaders in Union-Wounaan, the largest settlement, sent word to the guerrillas that their coca-grow-ing days were over. A day later guerrillas seized a teacher from his classroom. His mutilated body was found hours later. The next day a tribal leader was seized and beaten to death.

The killings caused panic in the community. More than 1,000 of the area’s 3,500 Wounaan fled upriver to the town of Istmina, with hundreds more fleeing elsewhere into the jungle. Hundreds more wanted to leave but those arriving in Istmina said that there were no boats to bring them and they remained trapped upriver.

The displaced tribespeople took shelter in a house by the river, crammed 30 to a room. Exile proved too hard for many and some climbed into boats and travelled back. Those who had opposed the guerrillas had no choice but to stay – 47 of them were given asylum in Panama and more than 70 remain in Istmina.

In recent weeks right-wing paramilitaries who have refused to disarm under a government programme have started to move onto Wounaan land, seeking control of the coca crops themselves.

José Llanos, a tribal leader who has been speaking out against coca cultivation, arrived in Istmina with his family last month after receiving death threats from the paramilitaries. He told of a community caught between two illegal armies.

“We cannot be neutral,” he said. “Wherever we live, the other group thinks we are with that group. And both groups want the coca.”

The Wounaan join a human tide of migration across Colombia. More than three million people have been forcibly displaced across the country, second only to Sudan. Every day, 850 more people are driven from their homes. Alvaro Uribe, Colombia’s President, has received plaudits for improving security, though many blame his policies for the new crises in far-flung corners of the country previously little touched by the war.

“It is true that the violence is no longer affecting the governing classes as much,” Marie-Helene Verney, spokesman for the United Nations refugee agency in Colombia, said. “But the situation is very different in the countryside where voiceless minorities are coming under threat.”

In Condoto, a riverside cathedral town, hundreds of the displaced inhabit rundown slums that they built after fleeing their villages. Afro-Colombi-ans, the marginalised descendants of former slaves, have been here as long as five years. “I don’t believe we will ever go back,” Maria Cesario, a heavily pregnant mother of six, says. She is probably right. The vast majority of displaced Colombians never return home.

For the Government, there is little political capital to be won from helping the displaced. An average displaced family receives just one quarter of the subsidy given to the family of an illegal militiaman who turns in his guns. But for the Wounaan, there is more at stake than for most.

“The conflict has put their way of life at risk,” Ms Verney said. “They have been forced to leave their ancestral lands and now they find themselves at risk of extinction.”

Many more of Colombia’s one million indigenous people are also under such a threat. A year ago, 80 members of the hunter-gatherer Nukak tribe walked out of the jungle after months on the run from armed groups who ordered them off their land to grow coca. Only 500 remain in the jungle and fears are growing for their survival. For the Wounaan, the future looks bleak. In Istmina, they live in their own small community by the river, surrounded by an alien Afro-Colombian culture of hard drinking and casual sex. An air of tragedy hangs over them. All have taken Spanish names, the ones used here, to hide the Wounaan names that appeared on the guerrillas’ hit lists.

With their teachers dead, they struggle to teach their children to read and write their own language. At school they are taught in Spanish, which few Wounaan can speak.

At night, they try to perform traditional dances to preserve their culture but they find little to celebrate. They live as close to the river as they can. “The river is like our blood. Just like somebody without blood cannot live, we cannot live without the river,” said Fernando.

But their ancestral lands are far away and they have no prospect of return. “The land is like our mother. To lose it is very hard.”

Plan Colombia, the programme that drove the coca crops deep into their forests, is nearly over. While the acreage of coca has shrunk, the yield has increased, so the amount of cocaine produced has not changed.

That fact, and revelations about links between the Government and illegal paramilitaries, have led to a suspension in the US aid money that funds it.

A new scheme, Plan Colombia II, has been hatched with less emphasis on military action and more on social programmes. But sceptics say they have seen this scheme before, and give warning that it may go the way of the original plan.

For the Wounaan, such geo-political manoeuvring is mean-ingless. But one thing they do know. If no one wanted the little green leaf, no one would plant it. As Colombia gears up for the next chapter in the war on drugs, a dying tribe has this plea for the outside world.

“If nobody bought the drug, it wouldn’t be produced,” José Llanos says. “To those who buy it, it’s just merchandise. For us, it is disaster. They have our blood on their hands.”

Trading in misery

80% of the world’s cocaine is produced in Colombia

$400bn - value of the global trade in the drug

500 hectares of rainforest cleared each year for new coca cultivation in Chocó province alone

3,000 people killed each year in Colombia’s cocaine-fuelled conflict

3m people displaced in what the UN describes as the worst humanitarian crisis outside Africa

60% of the world’s cocaine is consumed in the US

6.8% of Britons have tried the drug

5% of British banknotes have significant traces of cocaine indicating that they have been used to snort the drug

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