Babies were snatched off the streets by strangers in passing cars. Or taken from day-care centers or church basements where they played. Or stolen from hospitals, right after birth, passed from doctor to nurse to a uniformed “social worker” — before vanishing in an instant.

Some were dropped into dismal orphanages; others were sent to a new family, their identities wiped, no questions asked. Most would never see their birth parents again.

While it sounds like something out of Dickens or the Brothers Grimm, this happened in the United States in the 20th century. Thousands of times.

She was the mastermind behind a black market for white babies.

It was the dark handiwork of the Memphis branch of the Tennessee Children’s Home Society, a supposedly charitable organization, led by a woman named Georgia Tann.

Tann was a pied piper without scruple; she was the mastermind behind a black market for white babies (especially blond, blue-eyed ones) that terrorized poor Southern families for almost three decades. It’s estimated that over 5,000 children were stolen by Tann and the society between 1924 and 1950 and that some 500 died at the society’s hands as a result of poor care, disease and, it is suspected, abuse.

Particularly vulnerable were newborns. In 1945 alone, as many as 50 children perished in a dysentery outbreak. The precise figure, like so many terrible details about the society, is not known.

Tann had various means of procuring babies and children for her wealthy customers. She bribed nurses and doctors in birthing wards, who would then tell new parents that their babies had been stillborn.

Her organization was quick to snatch babies born in prisons and mental wards. Older children were grabbed off the street by Tann’s agents and were told their parents had died. To cover their tracks, the society falsified adoption records and destroyed any trace of these children’s origins.

Lisa Wingate brings these shocking crimes and their long-term emotional impact to light in her affecting new novel, “Before We Were Yours” (Ballantine Books), out now. The book tells the story of two families — the wealthy, connected Staffords and the dirt-poor “river gypsy” Fosses.

Though her tale is fictional, it stems from the true, terrible events of the Tennessee tragedy. Tann and her associates would tear apart one family to benefit another, creating wounds not easily healed. The loss would linger, like a phantom limb, for generations.

Tann would tell adoptive parents that the children were “blank slates,” Wingate tells The Post. “What really resonated with me is that they’re not. Foster kids, adopted kids, they’re not blank slates. They’re people. And they have genetic tendencies and . . . talents and abilities that are all their own.”

Tann’s background was a privileged one. She was born in 1891 in Hickory, Miss., where her father was a district court judge. One of Judge Tann’s responsibilities was dealing with homeless children who were wards of the state. Georgia’s older brother, Rob Roy Tann, may have been one of these children, adopted by the Tann family.

Tann had designs on a career in the law, but her father deemed that profession too “masculine” for his only daughter. Forced to study music, she taught for a time before finding a job in the nascent field of social work in 1916.

Working as a field agent for the Mississippi Children’s Home-Finding Society in Jackson, she may have gotten a taste for the power that she would later wield over so many families. She began placing poor children in adoptive homes, without the consent of both birth parents.

Child welfare laws weren’t as strict as they are today, allowing Tann to wheel and deal in her role. But Tann was not careful with her work, nor with covering up her trail, and at least one birth parent sued for return of her children.

It seemed Mississippi was not the right market for a baby-resale business. Judge Tann had connections in Memphis, and after a brief foray in Texas, his daughter moved there to work for the Tennessee Children’s Home Society in 1924.

Soon after, she launched her adoption racket. To drum up business, Tann placed advertisements aimed at potential adoptive parents in newspapers.

One featured a photo of smiling, towheaded infants with the caption, “Want a Real, Live Christmas Present?” As if children were dolls or puppies.

Foster kids, adopted kids, they’re not blank slates. They’re people. And they have genetic tendencies and . . . talents and abilities that are all their own.

Tann presented herself as a kindly matron and pioneer of a new kind of social work — all the while destroying lives and families. Tann was abetted by the famously corrupt political machine in Memphis, headed by E.H. “Boss” Crump.

Crump, also a transplanted Mississippian, was the sometime mayor and leader of politics in the city for much of the first half of the 20th century.

He developed a cozy patronage relationship with Tann — she paid him off and brought the fame of her society to Memphis. He in turn protected her from prying investigations, while city police ignored the complaints of families who’d lost children to Tann and sometimes even helped Tann seize kids.

Tann’s most useful co-conspirator was Camille Kelley, a juvenile court judge in the city. Like Tann, Kelley pretended to act in the best interests of children. With the stroke of a pen, Kelley would remove parental rights and transfer them to Tann, clearing a path for adoption.

Tann was essentially waging class war. She held to the belief that there were two kinds of people: the poor, whom she viewed as incompetent parents, and the wealthy. She fattened her own coffers in the process.

The big money came from interstate adoptions, especially to New York and LA, for which the agency would charge as much as $5,000. Most of that fee was pocketed by Tann, who was given to traveling in chauffeured Packards.

Among the Tennessee Children’s Home Society’s clientele was that paragon of maternal love Joan Crawford, who adopted her twin daughters, Cathy and Cynthia, through the organization in 1947. Mid-century Hollywood power couple June Allyson and Dick Powell used the agency to adopt their daughter, Pamela. Lana Turner, Pearl S. Buck and New York Gov. Herbert Lehman were also clients. And future pro wrestler Ric Flair was among Tann’s abductees.

As word of Tann’s tactics started to spread, even some adoptive parents started to object. But, in many cases, Tann had these people over a barrel — they were complicit in her operation and feared losing their children. Whispers and accusations went nowhere.

For years, she was too prominent, too well connected, for anything to stick.

In the mid-1940s, Tann was diagnosed with uterine cancer and could sense that her adoption racket was beginning to unravel. Boss Crump’s political influence was starting to wane.

Gordon Browning, a Crump enemy, was elected governor in 1948. When rumors of Tann’s baby racket reached his ears, he saw a way to humiliate his political rival. In the final months of Tann’s life, Browning appointed a special investigator to look into her practices. Such was Tann’s remaining influence that the state’s case against her was announced only days before her death, in September 1950.

Even then, the brunt of the accusations against Tann had to do with her pocketing money from a state-funded enterprise, rather than kidnapping.

Georgia Tann was 59 years old when she died at home of cancer. She never had to face the music for her terrible crimes. She left no money to children’s causes, nor to the Tennessee Children’s Home Society. The home was closed two months later.

Word of Tann’s shocking crimes became national news. “Dead Director Blamed in Baby Black Market,” reported the Chicago Tribune. But though Tann had placed children in every state, few attempts were made at the time to connect birth parents with their children.

A version of the Tann story was told in the 1993 made-for-TV movie “Stolen Babies” (Mary Tyler Moore stars as the infamous baby-dealer), and a more thorough account of her crimes can be found in the non-fiction book “The Baby Thief” by journalist Barbara Bisantz Raymond.

Shortly after Tann’s death, her co-conspirator Kelley announced her retirement from her judgeship. Protected by the Crump machine, she, too, avoided prosecution and died in 1955 from a stroke.

It’s estimated that over 5,000 children were stolen by Tann.

Boss Crump preceded Kelley in death by a year. Memphis drivers honor the former mayor by taking Crump Boulevard across town from Route 55 to Route 78.

Today, the legacy of Georgia Tann and the Tennessee Children’s Home Society is a painful and complicated one.

Ironically, an accidental benefit of her work was popularizing adoption for parents unable to conceive. Before Tann, adoption in the United States was uncommon, but after she became known, the stigma over the practice was largely lifted.

At the same time, a damaging legal procedure was put in place: Tann championed the practice of closed, secretive adoptions. Adoption records were sealed, and adopted children were barred from learning the identities of their birth parents.

This legislation is still in place in many states. Tellingly, Tennessee was the first state to lift these laws, in 1999.

Meanwhile, the emotional cost of Tann’s decades-long scheme is incalculable. Thousands of families were torn apart; parents never saw their children again and siblings were permanently separated. Lisa Wingate’s novel tells of one such family.

“If you’d invented that story, it would seem so far-fetched that you would think, ‘That could never happen. Not in this country,’” Wingate says. “And yet, it did, and it did for a long time.”

For Wingate, this story “still matters today, because there are still so many kids that need that one advocate, that one place to be, that one person who will step in.” She continues, “We do have to still be watching for things that are not aboveboard or are corrupt, where children are being used for profits of one kind or another. That’s on all of us, as a public.”