In the long struggle against sex trafficking, we finally have a breakthrough!

It didn’t come from Congress, or the White House, from the courts or the police. Rather, it came from credit card companies: Pimps can no longer easily use American Express, Visa or MasterCard to pay for prostitution ads in which they sell 15-year-old girls as if they were pizzas.

That upended the business model of sex trafficking. Pimps all over the country are reduced to figuring out how to pay to promote their ads with, yes, bitcoin!Human trafficking is one of the most insidious human rights abuses in the United States — some 100,000 minors are trafficked into the sex trade each year in the U.S. So let me explain how we came to enjoy a triumph over traffickers.

A website called Backpage.com has for years dominated the sex trade advertising business. In April alone it published more than 1.4 million ads in its adult services section in the United States. Almost every time a girl is rescued from traffickers, it turns out that she was peddled on Backpage.

Last year I wrote about a missing 15-year-old Boston girl whose parents were beside themselves with worry. In their living room, I pulled out my laptop, opened up Backpage and quickly found seminude advertisements for the girl, who turned out to be in a hotel room with an armed pimp.

Backpage is allowed to operate because of a loophole in the Communications Decency Act. Attorneys general from 48 states have pleaded with Backpage to stop this exploitation, to no effect. Girls who have been sold on Backpage when they were as young as 13 have sued the company, but haven’t succeeded because of the loophole.

Then suddenly this summer, the miracle of the market intervened.

Sheriff Tom Dart of Cook County, Illinois, wrote tough letters to Visa and MasterCard, calling on them to stop allowing their cards to pay for sex ads on Backpage. Both companies effectively agreed. To its great credit, American Express in April stopped working with Backpage for adult ads, so as of the beginning of July pimps had no easy way to pay for advertisements.

Flummoxed, Backpage responded by making its basic sex ads free, but, even with a fee to promote a free ad, that’s not a business model that can sustain it. Backpage is suing Dart, but my sense is that pimps won’t be using their credit cards again on the site any time soon.

“If it’s down for six months, that’s six months of children who aren’t raped,” says Yiota Souras of the National Center for Missing and Exploited Children.

So bravo to American Express, MasterCard and Visa — and to Dart — for getting results where Congress failed.

There will still be human trafficking, of course, and pimps will find other ways to peddle kids. But it may not be quite so easy for traffickers as it was.

“When on Backpage, I was advertised in the same way as a car or a phone, but with even less value than a bike,” one girl told me late last year. She said she was advertised at the age of 15 and 16 and raped 1,000 times as a result.

My guess is that a majority of sex ads on Backpage are for consenting adults. But a significant minority are for sex with children or with women who are coerced — representing some of the largest and most mistreated classes of human rights victims in America. We don’t have the moral authority to tell other countries to end modern forms of slavery when we don’t clean up our own act.

There has also been progress in other areas. The police in the United States are going after pimps more, and sometimes johns, as well (that still needs to happen more).

The Nordic model to combat trafficking and exploitation, pioneered in Sweden, has been gaining ground, too. It provides for the arrest of johns while offering help rebuilding the lives of women who were selling sex. Nothing works all that well in curbing sex trafficking, but this model has succeeded better than other approaches.

Yet in some quarters, there’s still a myopia about the degree to which this is a human rights issue. Amnesty International will consider a proposal in the coming days that would call for full decriminalization of the sex trade, including for johns, on the theory that this would benefit sex workers. Nice theory, but a failed one. It has been tried repeatedly and it invariably benefited johns while exacerbating abuse of women and girls: A parallel underground market emerges for underage girls.

Let’s hope Amnesty comes to its senses and, as Swanee Hunt of Harvard put it, avoids “endorsing one of the most exploitative human rights abuses of our time.” Then we can go back to celebrating the struggles of America’s sex traffickers as their business model is upended.