Western men who visit red-light districts in poor countries often find themselves surrounded by coquettish teenage girls laughingly tugging them toward the brothels. The men assume that the girls are there voluntarily, and in some cases they are right.

But anyone inclined to take the girls’ smiles at face value should talk to Sina Vann, who was once one of those smiling girls.

Sina is Vietnamese but was kidnapped at the age of 13 and taken to Cambodia, where she was drugged. She said she woke up naked and bloody on a bed with a white man — she doesn’t know his nationality — who had purchased her virginity.

After that, she was locked on the upper floors of a nice hotel and offered to Western men and wealthy Cambodians. She said she was beaten ferociously to force her to smile and act seductive.

“My first phrase in Khmer,” the Cambodian language, “was, ‘I want to sleep with you,’ ” she said. “My first phrase in English was” — well, it’s unprintable.

Sina mostly followed instructions and smiled alluringly at men because she would have been beaten if men didn’t choose her. But sometimes she was in such pain that she resisted, and then she said she would be dragged down to a torture chamber in the basement.

“Many of the brothels have these torture chambers,” she said. “They are underground because then the girls’ screams are muffled.”

As in many brothels, the torture of choice was electric shocks. Sina would be tied down, doused in water and then prodded with wires running from the 220-volt wall outlet. The jolt causes intense pain, sometimes evacuation of the bladder and bowel — and even unconsciousness.

Shocks fit well into the brothel business model because they cause agonizing pain and terrify the girls without damaging their looks or undermining their market value.

After the beatings and shocks, Sina said she would be locked naked in a wooden coffin full of biting ants. The coffin was dark, suffocating and so tight that she could not move her hands up to her face to brush off the ants. Her tears washed the ants out of her eyes.

She was locked in the coffin for a day or two at a time, and she said this happened many, many times.

Finally, Sina was freed in a police raid, and found herself blinded by the first daylight she had seen in years. The raid was organized by Somaly Mam, a Cambodian woman who herself had been sold into the brothels but managed to escape, educate herself and now heads a foundation fighting forced prostitution.

After being freed, Sina began studying and eventually became one of Somaly’s trusted lieutenants. They now work together, in defiance of death threats from brothel owners, to free other girls. To get at Somaly, the brothel owners kidnapped and brutalized her 14-year-old daughter. And six months ago, the daughter of another anti-trafficking activist (my interpreter when I interviewed Sina) went missing.

I had heard about torture chambers under the brothels but had never seen one, so a few days ago Sina took me to the red-light district here where she once was imprisoned. A brothel had been torn down, revealing a warren of dungeons underneath.

“I was in a room just like those,” she said, pointing. “There must be many girls who died in those rooms.” She grew distressed and added: “I’m cold and afraid. Tonight I won’t sleep.”

“Photograph quickly,” she added, and pointed to brothels lining the street. “It’s not safe to stay here long.”

Sina and Somaly sustain themselves with a wicked sense of humor. They tease each other mercilessly, with Sina, who is single, mock-scolding Somaly: “At least I had plenty of men until you had to come along and rescue me!”

Sex trafficking is truly the 21st century’s version of slavery. One of the differences from 19th-century slavery is that many of these modern slaves will die of AIDS by their late 20s.

Whenever I report on sex trafficking, I come away less depressed by the atrocities than inspired by the courage of modern abolitionists like Somaly and Sina. They are risking their lives to help others still locked up in the brothels, and they have the credibility and experience to lead this fight. In my next column, I’ll introduce a girl that Sina is now helping to recover from mind-boggling torture in a brothel — and Sina’s own story gives hope to the girl in a way that an army of psychologists couldn’t.

I hope that Barack Obama and Hillary Clinton will recognize slavery as unfinished business on the foreign policy agenda. The abolitionist cause simply hasn’t been completed as long as 14-year-old girls are being jolted with electric shocks — right now, as you read this — to make them smile before oblivious tourists.