Patrols on the Mekong River by the Laotian army and Myanmar police have subdued pirates who once robbed the Mekong's cargo ships with impunity.
But drug production and trafficking in the region, known as the Golden Triangle, is booming - despite the presence of Chinese gunboats and Chinese armed police.
1 Mar 2016 . Mekong River, Laos. Reuters/Jorge Silva
Soldiers from Laos (left) and China stand at an army post on the Mekong river at the Laos port of Xieng Kok.
The United Nations Office on Drugs and Crime (UNODC) estimates that Southeast Asia's trade in heroin and methamphetamine was worth $31 billion in 2013.
"That's bigger than the economies of some Southeast Asian countries," says Jeremy Douglas, the UNODC's Asia-Pacific chief. "It's like having an undeclared sovereign state in your midst with no borders and lots of money."
3 Mar 2016 . Mekong River, Thailand. Reuters/Jorge Silva
Thai soldiers stand guard at Ban Kaen Kai operation base on the Mekong river.
The Lao People's Army patrol boat was custom-made in China with night-vision capability and two of the most powerful engines on this remote stretch of the Mekong River.
Today, like most days, it sits idle for lack of gasoline, guarded by a single Laotian soldier in flip-flops.
Enter another Mekong boat, looking at first glance like a pleasure cruiser filled with middle-aged tourists. In fact, they are senior police and drugs experts from five countries, among them one of China's top anti-narcotics officials, Wei Xiaojun.
1 Mar 2016 . Mekong River, China. Reuters/Jorge Silva
Arranged by the UNODC and lent further clout by Wei's involvement, their recent voyage down the Mekong was aimed at mustering the regional collaboration needed to tame the Golden Triangle.
Reuters was invited to join the four-day trip from the Chinese port of Jinghong through the heart of the Golden Triangle.
Wei, who is deputy secretary general of China's National Narcotics Control Commission (NNCC), called drugs the "main threat" along the Mekong.
"All other types of organised crime are rooted in the drug business, like human trafficking, money laundering and the illegal wildlife trade," he said.
3 Mar 2016 . Mekong River, Thailand. Reuters/Jorge Silva
A long boat sails near the port of Chiang Saen on the Thai side of the Golden Triangle.
Many factors combine to help the Golden Triangle's drug industry prosper.
The Myanmar-Laos border, which the Mekong delineates, is mostly unguarded. The terrain is rugged and hostile, with rebel armies holding sway in some areas and drugs and money-laundering flourishing in lawless enclaves on both sides of the river.
Regional law enforcement agencies are often underfunded and ill-trained, and the intelligence they gather is not effectively shared with neighbouring countries.
2 Mar 2016 . Mekong River, Thailand. Reuters/Jorge Silva
A cargo ship where Chinese sailors were killed in 2011 is seen at the port of Chiang Saen on the Thai side of the Golden Triangle.
In October 2011, a gang led by a Mekong pirate called Naw Kham murdered 13 Chinese sailors. He was hunted down in Laos, then taken back to China to be tried and executed.
Afterwards, Chinese gunboats began patrolling further downriver, extending China's security reach far beyond its borders.
2 Mar 2016 . Mekong River, Laos. Reuters/Jorge Silva
A Laotian soldier walks up a stairway in front of an army patrol boat on the Mekong river port of Mouang Mom.
This includes a riverside facility in Muang Mom in Laos, which Reuters visited, run and guarded by a 25-strong unit of Chinese People's Armed Police.
China conducts monthly joint patrols with its Laotian and Myanmar counterparts, who - gasoline permitting - do additional patrols by themselves.
There have been successes. In 2013, a Chinese-Laotian patrol found 580 kg (1,280 lbs) of ya ba, worth more than 100 million yuan ($15 million), hidden in a cargo ship.
But more patrols were needed, said the UNODC's Douglas, and Mekong countries also needed to coordinate and share intelligence to interdict more drugs.