A U.S. Army staff sergeant and his wife arrived at his base in Louisiana last week, expecting to begin their life together as newlyweds.

The couple checked in at the visitor center, identification in hand, ready to complete the steps that would allow her to move into his home on the base.

Within hours, that plan had unraveled.

Immigration and Customs Enforcement agents entered the base and detained his wife, an undocumented Honduran immigrant who was brought to the U.S. as a toddler. By nightfall, she was in a detention facility with hundreds of women facing deportation as part of the Trump administration’s immigration crackdown.

The detention came just days after Annie Ramos, 22, a college student with no criminal record, and Matthew Blank, 23, celebrated their marriage with family and friends. Sergeant Blank, who enlisted more than five years ago, is assigned to a brigade at Fort Polk, La. that is set to begin training at the end of the month for deployment.

“Our plan was to drive over, bring her to the office to get her military ID and activate her military spouse benefits,” such as health and life insurance, he said. “She was going to move in after the Easter weekend. Instead, she got ripped away from me.”

When U.S. citizens marry undocumented immigrants, their spouses become eligible for legal permanent residency through marriage, and they can apply for citizenship three years after receiving their green card.

Even those with a prior deportation order, often issued when they were children, are not typically detained and are able to adjust their immigration status, experts said.

Before they were married, Ms. Ramos and Sergeant Blank had hired a lawyer to begin that process.

“I knew she didn’t have status,” he said. “We were doing everything the right way.”

Or so they thought.

Ms. Ramos had been ordered deported in absentia in 2005, when she was 22 months old, after her family failed to appear at a hearing in immigration court, a circumstance that was “very common, “ according to Margaret Stock, who wrote a book called “Immigration Law and the Military.”

“Prior to the Trump administration creating a mass deportation policy, somebody like her would not have been detained,’’ said Ms. Stock, a retired lieutenant colonel in the Army Reserve who practices immigration law and who has handled many similar cases.

She said the military would typically have allowed Ms. Ramos to get her military ID and told the couple to file their immigration papers.

In a statement, the Department of Homeland Security said that Ms. Ramos had been arrested “after she attempted to enter a military base.”

“She has no legal status to be in this country and was issued a final order of removal by a judge,” the statement read. “This administration is not going to ignore the rule of law.”

The U.S. Army did not immediately comment.

Ms. Ramos and Sergeant Blank’s story began early last year. The couple connected through a dating app, then met in person and their romance blossomed. They were engaged on New Year’s Day.

In late March, about 60 guests gathered to celebrate their wedding in Houston, where Ms. Ramos was raised. A mariachi band played as the guests shared a meal of fried chicken, Spanish rice and mashed potatoes.

Sergeant Blank’s mother Jen Rickling, said in an interview that her daughter-in-law was “absolutely a darling, and we adore her” and listed her qualities.

Ms. Ramos is a devout Christian who teaches Sunday school to children at her church, Ms. Rickling said. She was in her high school’s marching band. And she was a few months from finishing a bachelor’s degree in biochemistry.

On April 2, the couple and Sergeant Blank’s parents drove from Houston to the base, arriving early for a 2 p.m. appointment. As instructed, the couple checked in at the visitor center, where they presented Ms. Ramos’s birth certificate, Honduran passport, their marriage license and Sergeant Blank’s military ID.

They never made it to the benefits office.

According to Sergeant Blank and his mother, an attendant scrutinized the documents and asked if Ms. Ramos had a visa or a green card. She did not, they explained, noting that their lawyer had prepared a green card application that would be filed any day.

The attendant made a flurry of calls.

“They told us, ‘We’ll figure it out,’ ’’ recalled Sergeant Blank.

He said that he and his wife were respectful and he kept his composure despite thinking that the officials were “dragging it out.” Tears began to stream down his wife’s face.

A supervisor was called, followed by an officer from the base’s criminal investigation division, who said he would contact Homeland Security and ICE.

After the call, Sergeant Blank said, the officer told them Ms. Ramos would be detained. The family broke down in tears.

Ms. Ramos was separated from them, placed in handcuffs and driven away in a military police vehicle. Sergeant Blank said he and his parents followed in their truck to another building, where she was unshackled and placed in what “looked like an interrogation room.”

Three ICE agents arrived. They first encountered Ms. Rickling and her husband in the lobby.

“They told us that they didn’t have a choice, they said they had to take Annie,” recalled Ms. Rickling. She said they apologized.

“I begged them not to take her,” Ms. Rickling said. “They said the higher-ups made them do it.”

The agents entered the room, offered the couple the same explanation, again shackled Ms. Ramos and took her away.

Their lawyer has asked ICE to release Ms. Ramos on her own recognizance while a motion is prepared to reopen the old deportation order, which would block her removal.

Ms. Stock, the expert on immigration and the military, said ICE could deport Ms. Ramos at any moment.

“It’s fundamentally harmful to national security to be doing this to members of the military, particularly while there is a war going on,” Ms. Stock said. “This is a major crisis for this soldier. His mind can’t be on the job.”

Gaby Pacheco, president of TheDream.US, submitted a letter of support for Ms. Ramos, whose education is being funded by the organization, which provides scholarships to undocumented immigrant youth.

Ms. Ramos had applied in 2020 for Deferred Action for Childhood Arrivals or DACA, a program that shields from deportation undocumented people brought to the U.S. as children. Her application was never processed by the Trump administration because the program was halted for new applicants.

“I grew up here like any American,” Ms. Ramos said during a call from the detention center in Basile, La.

“This is all I know,” she said. “My husband and family are here.”

Sergeant Blank said: “We are going to fight with everything I have. She is going to move in with me. We will start a family.”

Sergeant Blank, who has been previously deployed to the Middle East and Europe, said that his chain of command has been supportive as he works to resolve the situation.

“I am going to be with her and serve my country,” he said

On Saturday, he and his mother traveled to the detention center in Basile, La. with the completed form to apply for Ms. Ramos’s green card. It only needed her signature.

Guards barred them from bringing anything inside.

ICE Agents Detain Newlywed Spouse of Soldier Training to Deploy