By Jeremy McDermott
BBC News, Colombia
Colombia's 44-year civil conflict has largely been confined to the countryside. But now the country's main port, Buenaventura, is finding itself in the middle of a new, urban war.
Buenaventura sits on a series of islands amid swampy mangroves, the Pacific Ocean washing up to the slums that perch precariously on the edges of the city.
The crack of gunfire is heard every day here as the security forces seek to take control of this strategic city.
Colombia's civil conflict, now raging for more than four decades, has for the most part been conducted in the jungles in the south of the country, far from the cities where 70% of Colombians live.
Yet now Buenaventura finds itself in the midst of an urban war hitherto unknown in the country.
The actors are the same as in the rest of Colombia: the state, the Marxist rebels of the Farc or Revolutionary Armed Forces of Colombia and the drugs cartels allied with right-wing paramilitaries.
What makes Buenaventura such a sought-after prize is the fact that it is the perfect departure point for all exports, legal and illegal.
The city and the latticework of tributaries that surround it, spilling into the Pacific, have long been the superhighways for drugs heading up to Mexico on their journey to the US.
For some time the guerrillas and drug cartels have fought over control of the port where the thousands of containers leave every day, providing ideal hiding places for drugs consignments.
What has changed is that the security forces have finally decided to wrest back control from the warring factions and are fighting to establish their presence, street by street.
The huge profits from cocaine fuel Colombia's civil war
They want to gain the trust of the wary inhabitants and convince them that they are here to stay.
Spearheading this fight are the secret police - known as the DAS - and the Marines. The first provide the intelligence and the second the muscle and firepower.
The DAS have a list of Farc militiamen and the foot soldiers of the drug cartels. They have addresses where they believe many live.
But in the slums of Buenaventura, effecting an arrest means flooding the place with troops.
So the Marines in pick-ups and on motorcycles sweep into the slums, their tyres kicking up the dust from the dirt streets. Only once the area has been secured will the secret police spread out among the shacks.
They carry with them fingerprint machines, linked to a database in Bogota. Every young man in the target areas is rounded up and fingerprinted.
Those with arrest warrants outstanding are whisked away, the rest are catalogued.
"This is harassment," said one mother as her teenage son was rounded up to be fingerprinted. "This is all we see from the government. No help, just repression."
Turning to crime
With unemployment levels in the slums running at up to 70%, the guerrillas and drugs lords find there is no shortage of recruits when they pull up in their luxury 4x4s with tinted windows and offer wages of just over $200 (£100) a month.
The Farc is the oldest and largest of Colombia's left-wing rebel groups
People will literally kill for that amount in Buenaventura.
"These guys are expendable to the cartels and the guerrillas," said Agent Hernandez of the DAS. "Their life expectancy once they enter the ranks of crime here in Buenaventura is as low as two years."
"This is not just a problem of law and order," said Father John Reina, a priest who navigates the streets in the slums where heavily armed soldiers fear to go, wandering into the wooden shacks, most bare, apart from some hammocks to sleep in and a gas burner to cook on.
"This is a social problem. Without education and job opportunities, people are going to turn to crime to feed their families."
Hearts and minds
At night the security forces do not dare to venture in, and boats can be seen pulling up to the waterside slums. These boats then weave through the waterways snaking through the jungle swamps around the city.
It is here that the armed traffickers and guerrillas pass, carrying cocaine or the chemicals needed to process the drugs, moving to their laboratories set deep in the jungle.
The security forces are learning new skills: how to patrol and dominate urban areas; how to conduct hearts and minds campaigns to win the trust of the civilian population.
But the guerrillas have also been learning, and there are fears that they plan to bring their war into the cities.
Three Irishmen with IRA connections were arrested here in 2001, convicted of helping to train the rebels in explosives and of possessing false documents.
They were sentenced to 17 years in prison but were released on bail while an appeal was heard, and skipped the country, returning to Ireland, which has no extradition treaty with Colombia.
But their legacy is still felt every day, as mortars used by the guerrillas have improved, and there is an increase in the deployment of snipers and a quantum leap in the use of explosives.
The future of Colombia's conflict lies in the cities - the guerrillas and the security forces know that. And Buenaventura may be a taste of thing to come.
From Our Own Correspondent was broadcast on Saturday, 18 August, 2007 at 1130 BST on BBC Radio 4. Please check the programme schedules for World Service transmission times.