As news of Russia’s invasion spread through Ukraine on Feb. 24, 2022, Dr. Natalia Lukina was waiting for a taxi at her home.

It was 6 a.m., and she was eager to get to work at Kherson Children’s Home, a state-run foster home for institutionalized children with special needs, where she served as a doctor.

By the time she arrived, the rumble of artillery fired by Russian troops advancing on Kherson City, the region’s capital, was already reverberating through the hallways. The doctor and her fellow caregivers faced a wrenching dilemma: how to protect the dozens of vulnerable children.

They were all infants and toddlers, and some had serious disabilities, such as cerebral palsy. Some had living parents who retained limited custody over them, while others had been removed from troubled homes or abandoned.

“Who else would have stayed behind to look after them?” Dr. Lukina said about her decision to remain with the children. “Imagine if we all turned our backs and left?”

Olena Korniyenko, the director of the foster home and the children’s legal guardian, had prepared emergency bags for the children two weeks earlier, and she had stocked the home with boxes of food, water and diapers.

But the building was not equipped to withstand gunfire or shelling, and the police had already fled the city. When Ms. Korniyenko called the police chief to ask about using their underground bunker just 300 yards away, he warned her that the station would become a military target.

With limited options, Ms. Korniyenko searched online for a map of nearby bomb shelters and found one within walking distance.

Amid exchanges of fire, the staff carried the children and their mattresses by foot and stroller to a concrete basement, taking with them food, medicine, electric pumps and feeding tubes for the sickest children.

A local pastor got word of their plight later that day and urged the foster home staff to take the children to his church, where he could at least provide heat, electricity and food.

So the staff moved the children again, sweeping them into hiding in the basement of Holhofa Church. They stacked boxes of diapers in the windows to keep anyone from seeing in.

One nurse, Kateryna Sirodchuk, said they were afraid that Russian forces would take the children away. “We feared that they could come and take everything under their control,” she said.

And their fears soon came true: On April 25, 2022, Russian officials found the children and took them under their own authority, eventually moving them 180 miles from home — all while filming them for propaganda.

Evidence shows the transfer was part of the broader, systematic campaign by President Vladimir V. Putin and his political allies to strip the most vulnerable victims of the war of their Ukrainian identity. The New York Times reviewed Russian social media posts; obtained photos, videos, text messages and documents; and interviewed more than 110 caregivers, legal experts and Russian and Ukrainian officials to trace the lives and movement of the children as they were taken into Russian custody.

What happened to them next, legal experts say, may amount to a war crime.

Read our full investigation in the New York Times.