He looked everywhere for Constantin and asked the officers where his son was, but was not given a clear answer. At the airport, he refused to board without the baby. The immigration officers, he said, told him that Constantin would be handed to him once he had taken his seat. But the plane lifted off and the baby never came.

When Mr. Mutu arrived home, it felt more like walking into a funeral than a celebration.

***

While the months dragged on waiting for his day in immigration court, Constantin settled into a routine with his foster family, in their comfortable brick house on a hilly road in rural Michigan. The family, which had started fostering immigrant children a year earlier after a life-changing experience doing missionary work in Ethiopia, asked not to be identified in this story because it would violate the terms of their contract with the federal government. Their three daughters immediately became enamored with Constantin and would argue over who could pull him out of his crib when he woke up from a nap.

The baby’s foster mother meticulously documented his developments for Ms. Mutu, keeping in mind how hard it would be to miss moments like when he first scooted across the living room floor or developed the belly laugh that shook his whole body. “He would do new sounds or something, and they only do it for a short amount of time, and so you want his mom to be able to hear that,” she said. “And she always wondered if he had teeth yet, and so when he would smile, you could see. So I just wanted her to see that.”

She poured herself into caring for Constantin while she struggled to fathom how he had come into their home. “I can’t imagine being the person who grabs a hold of a child and takes them. I don’t know where you have to go in yourself to be able to do that job,” she said. “If we were in that situation, I would want someone to take care of my child. I would want them in a home, in a bed. I would want someone asking them, ‘What snack do you want before you go to bed at night? Do you want a pink toothbrush or a green toothbrush?’” she said. “Or rocking them in the middle of the night, helping them go back to bed when they have bad dreams.”

Constantin was still in diapers when he appeared in federal immigration court in Detroit, four months to the day after he had arrived in Michigan, on June 14, 2018. During the five-minute proceeding, he babbled on his foster mother’s lap as she sat on the defendant’s bench. His pro bono legal representative requested that he be returned to Romania as soon as possible at government expense.

A lawyer for the Department of Homeland Security argued against the request, stating that as an “arriving alien,” Constantin was not eligible for such help. The judge quickly ruled against her, questioning the idea “that the respondent should be responsible for making his own way back to Romania as an 8-month-old.” The judge granted the request made on behalf of Constantin, giving the government three months to either appeal or send him home.

By the time Constantin’s travel plans were booked for July — a few weeks after President Trump, facing a wave of public outrage, had rescinded the family separation policy — he was 9 months old and had spent the majority of his life in the custody of the United States government.