Image caption China hailed the Kunming trial as a model of cross-border law-enforcement

A notorious ethnic Shan warlord, Naw Kham, has gone on trial in the southern Chinese city of Kunming, charged with the murder of 13 Chinese citizens on board two boats on the Mekong River last October.

Naw Kham, who was widely believed to run much of the smuggling and narcotics business in the Golden Triangle region where the borders of Laos, Burma and Thailand meet, was detained in Laos in April after a joint operation by the security forces from the three countries and China.

Five of Naw Kham's associates are also on trial.

The prosecution's case is simple: that the two boats were suspected of trafficking narcotics, bypassing Naw Kham, so he killed the crew to set an example to the hundreds of other ships that ply the Mekong, carrying the expanding river trade, both legal and illegal, between the four countries.

Naw Kham is accused by the Chinese authorities of terrorizing Chinese ships along the Mekong for many years. Convicting him for a crime which outraged public opinion in China would be a popular move.

After reportedly confessing and apologizing to the Chinese police, he has now denied the charge in court and blamed the killings on the Thai security forces.

'Solid evidence'

Image caption Reports from China say Naw Kham "overturned" his earlier confessions

But there is a great deal about this case that is not clear or simple.

The two boats, the Hua Ping and the Yu Xing 8, were found on the Thai side of the Mekong, not far from the river port of Chiang Saen, some time in early October.

The Chinese authorities say the boats were attacked on 5th October. It took several days to discover the bodies of all 13 victims, most of whom had their hands tied and had been shot.

At the end of October the Thai police announced that after speaking to more than 100 witnesses, they were naming nine soldiers from a Thai army unit responsible for security along the Mekong, as suspects in the killings.

Some of the witnesses described seeing the soldiers open fire on the boats. The Thai military responded by saying its troops had found the two boats, already riddled with bullets, on the river bank, with the captain of one slumped dead over his gun.

They reported finding 920,000 methamphetamine pills on board.

The nine Thai soldiers are still viewed by the police as suspects, although they have not been charged.

They have denied killing the victims, and the commander of the Thai Army, General Prayuth Chanocha has called for them to be allowed to clear their names.

But the Interior Minister, Chalerm Yubumrang, has insisted that there is ''solid evidence'' that Thai soldiers fired on the boats. He has repeatedly stated that the case is close to being complete on the Thai side, but there is little sign of this.

Thai forces involved?

Image caption Drug-related crime is active in the stretch of Mekong River bordering Thailand, Laos and Burma

So what really happened? We will probably never know.

Violent crimes are routinely never solved in Thailand, especially in the lawless border regions, where the security forces are widely believed to collaborate with underworld figures like Naw Kham as often as they try to combat them.

Add to that the rivalry and mistrust between the Thai police and military, fuelled by the political polarization of the country in recent years.

"The evidence suggests the Thai security forces were involved", says Sunai Chulapongsathorn, chairman of the Parliamentary Foreign Affairs Committee, which conducted its own inquiry into the killings.

"But the investigation on our side not resulted in any charges. Perhaps the result of the Chinese trial will influence what happens here."

In fact we would probably have learned very little about this case, had it not been for the involvement of China.

Chinese 'arm-twisting'

Senior Chinese officials arrived in Thailand soon after the boats were found, and demanded action be taken.

Within a month, the four countries had agreed to beef up river patrols, although an initial proposal to allow Chinese gunboats to operate along Thai stretches of the Mekong was firmly rejected by General Prayuth Chanocha.

The speed with which Naw Kham - previously an untouchable warlord who moved with impunity for many years along the Mekong - was captured, demonstrates the impact of Chinese pressure.

The fact that he is being tried in China, even though the crime took place far from Chinese territory, is another indication of how powerful China's voice now is in the region.

Thailand has sent ten police officers to testify at the trial. The Chinese authorities have described this case as a model of cross-border law-enforcement.

In truth, it has been an example of arm-twisting by the regional superpower.

Whether Chinese power can impose order on the infamously lawless Mekong is another matter. Attacks on river boats have continued this year, though on a smaller scale.